Prevalence and characterization of Salmonella in bovine lymph nodes potentially destined for use in ground beef

The effect of a direct-fed microbial (10-G) on live animal performance, carcass characteristics, and Salmonella prevalence of fed beef heifers

The objective of this study was to determine the efficacy of the direct-fed microbial 10-G upon cattle growth performance, liver and lung health, carcass quality, and yield outcomes, as well as prevalence and enumeration of Salmonella in feces and lymph nodes. Fed beef heifers (N = 1,400; initial shrunk body weight [BW] 343.3 ± 36.2 kg) were blocked by the day of arrival and randomly allocated to one of two treatments (0 [negative control, CON] or 2 g of a direct-fed microbial [10-G] that provided 1 billion CFUs per animal per day of Lactobacillus acidophilus, Enterococcus faecium, Pediococcus pentosaceus, L. brevis, and L. plantarum) with 10 pens per treatment. Recto-anal mucosal fecal samples (RAMs; n = 477) and subiliac lymph nodes (SLNs; n = 479) were collected longitudinally at harvest from 23 to 25 heifers per pen. Data were analyzed using mixed models; pen served as the experimental unit; block and harvest date were random effects. No differences were detected in dry matter intake (P = 0.78), final BW (P = 0.64), average daily gain (P = 0.51), gain to feed (P = 0.71), hot carcass weight (P = 0.54), dressed carcass yield (P = 0.52), 12th rib fat depth (P = 0.13), longissimus muscle area (P = 0.62), calculated empty body fat (P = 0.26), or marbling score (P = 0.82). Distributions of liver scores (P ≥ 0.34), quality grades (P ≥ 0.23), and yield grades (P ≥ 0.11) were also not different between treatments. A tendency was detected for more normal lungs (P = 0.08; 10-G = 65.96%, CON = 61.12%) and fewer inflated lungs at harvest for cattle fed 10-G (P = 0.10; 10-G = 0.29%, CON = 1.16%); other lung outcomes did not differ (P ≥ 0.54). Salmonella prevalence did not differ for RAM samples (P = 0.41; 10-G = 97.74%, CON = 96.82%) or SLN (P = 0.22; 10-G = 17.92%, CON = 13.66%). Salmonella concentration of RAM samples (P = 0.25; 10-G = 3.87 log CFU/g, CON = 3.32 log CFU/g) or SLN (P = 0.37; 10-G = 1.46 log CFU/g, CON = 1.14 log CFU/g) also did not differ between treatments at harvest. These results do not demonstrate any difference in live animal performance, carcass characteristics, or Salmonella carriage for heifers fed 10-G.

Natural Antimicrobials for Listeria monocytogenes in Ready-to-Eat Meats: Current Challenges and Future Prospects

Listeria monocytogenes, an intra-cellular, Gram-positive, pathogenic bacterium, is one of the leading agents of foodborne illnesses. The morbidity of human listeriosis is low, but it has a high mortality rate of approximately 20% to 30%. L. monocytogenes is a psychotropic organism, making it a significant threat to ready-to-eat (RTE) meat product food safety. Listeria contamination is associated with the food processing environment or post-cooking cross-contamination events. The potential use of antimicrobials in packaging can reduce foodborne disease risk and spoilage. Novel antimicrobials can be advantageous for limiting Listeria and improving the shelf life of RTE meat. This review will discuss the Listeria occurrence in RTE meat products and potential natural antimicrobial additives for controlling Listeria.

Salmonella Outbreaks Linked to Beef, United States, 2012-2019

The Centers for Disease Control and Prevention (CDC) has identified nontyphoidal Salmonella as one of the top five pathogens contributing to foodborne illnesses in the United States. Beef continues to be a common source of Salmonella outbreaks, despite the implementation of interventions at slaughter and processing facilities to reduce contamination of beef. We described Salmonella outbreaks linked to beef in the United States during 2012-2019, examined trends, and identified potential targets for intervention and prevention strategies. We queried CDC's Foodborne Disease Outbreak Surveillance System (FDOSS) for all foodborne nontyphoidal Salmonella outbreaks linked to beef as the single contaminated ingredient or implicated food, with the date of first illness onset from 2012 to 2019. Information on antimicrobial resistance (AR) for outbreak-related isolates was obtained from CDC's National Antimicrobial Resistance Monitoring System (NARMS). We calculated the number of outbreaks, outbreak-related illnesses, hospitalizations, and deaths overall, by beef processing category and Salmonella serotype. During 2012-2019, 27 Salmonella outbreaks were linked to beef consumption, resulting in 1103 illnesses, 254 hospitalizations, and two deaths. The most common category of beef implicated was nonintact raw, ground beef (12 outbreaks, 44%), followed by intact raw (six outbreaks, 22%). Ground beef was responsible for the most illnesses (800, 73%), both of the reported deaths, and was the source of the largest outbreak. AR data were available for 717 isolates from 25 (93%) outbreaks. Nine (36%) of these outbreaks had isolates resistant to one or more of the antibiotics tested by NARMS, of which eight (89%) contained multidrug-resistant isolates. Several outbreaks reported highlight challenges faced during investigations, areas where further research may be warranted, and opportunities to prevent future outbreaks along the farm-to-fork continuum.

Efficacy of Common Antimicrobial Interventions at and above Regulatory Allowable Pick-Up Levels on Pathogen Reduction

The objective of this study was to evaluate the food safety efficacy of common antimicrobial interventions at and above required uptake levels for processing aids on the reduction of Shiga-toxin producing E. coli (STEC) and Salmonella spp. through spray and dip applications. Beef trim was inoculated with specific isolates of STEC or Salmonella strains. Trim was intervened with peracetic or lactic acid through spray or dip application. Meat rinses were serially diluted and plated following the drop dilution method; an enumerable range of 2-30 colonies was used to report results before log transformation. The combination of all treatments exhibits an average reduction rate of 0.16 LogCFU/g for STEC and Salmonella spp., suggesting that for every 1% increase in uptake there is an increase of 0.16 LogCFU/g of reduction rate. There is a statistical significance in the reduction rate of Shiga-toxin producing Escherichia coli in relation to the uptake percentage (p < 0.01). The addition of explanatory variables increases the R² of the regression for STEC, where all the additional explanatory variables are statistically significant for reduction (p < 0.01). The addition of explanatory variables increases the R² of the regression for Salmonella spp., but only trim type is statistically significant for reduction rate (p < 0.01). An increase in uptake percentages showed a significant increase in reduction rate of pathogens on beef trimmings.

Rapid Quantitative Method Development for Beef and Pork Lymph Nodes Using BAX® System Real Time Salmonella Assay

The goal of this study was to develop a rapid RT-PCR enumeration method for Salmonella in pork and beef lymph nodes (LNs) utilizing BAX^®-System-SalQuant^® as well as to assess the performance of the methodology in comparison with existing ones. For study one: PCR curve development, pork, and beef LNs (n = 64) were trimmed, sterilized, pulverized, spiked with 0.00 to 5.00 Log CFU/LN using Salmonella Typhimurium, and then homogenized with BAX-MP media. Samples were incubated at 42 °C and tested at several time points using the BAX^®-System-RT-PCR Assay for Salmonella. Cycle-Threshold values from the BAX^®-System, for each Salmonella concentration were recorded and utilized for statistical analysis. For study two: Method comparison; additional pork and beef LNs (n = 52) were spiked and enumerated by (1) 3M™EB-Petrifilm™ + XLD-replica plate, (2) BAX^®-System-SalQuant^®, and (3) MPN. Linear-fit equations for LNs were estimated with recovery times of 6 h and a limit of quantification (LOQ) of 10 CFU/LN. Slopes and intercepts for LNs using BAX^®-System-SalQuant^® when compared with MPN were not significantly different (p < 0.05), while the same parameters for 3M™EB-Petrifilm™ + XLD-replica plate were significantly different (p > 0.05). The results support the capability of BAX^®-System-SalQuant^® to enumerate Salmonella in pork and beef LNs. This development adds support to the use of PCR-based quantification methodologies for pathogen loads in meat products.

Longitudinal evaluation of Salmonella in environmental components and peripheral lymph nodes of fed cattle from weaning to finish in three distinct feeding locations

Salmonella prevalence in bovine lymph nodes (LNs) varies due to seasonality, geographic location, and feedyard environment. The objectives of this study were to (1) establish prevalence rates of Salmonella in environmental components (trough water, pen soil, individual feed ingredients, prepared rations, and fecal samples) and LNs from weaning to finish in three feeding locations, and (2) characterize recovered salmonellae. Calves (n = 120) were raised at the Texas A&M University McGregor Research Center; in lieu of beginning the backgrounding/stocker phase, thirty weanling calves were harvested. Of the remaining ninety calves, thirty were retained at McGregor and sixty were transported to commercial feeding operations (Location A or B; thirty calves each). Locations A and B have historically produced cattle with relatively "low" and "high" rates of Salmonella-positive LNs, respectively. Ten calves per location were harvested at the conclusion of (1) the backgrounding/stocker phase, (2) 60 d on feed, and (3) 165 d on feed. On each harvest day, peripheral LNs were excised. Environmental samples were obtained from each location before and after each phase, and every 30 d during the feeding period. In line with previous work, no Salmonella-positive LNs were recovered from cattle managed at Location A. Salmonella-positive LNs (30%) and environmental components (41%) were most commonly recovered from Location B. Of 7 and 36 total serovars recovered from Salmonella-positive LN and environmental samples, respectively, Anatum was identified most frequently. Data from this study provide insight into Salmonella prevalence differences among feeding locations and the possible influence of environmental and/or management practices at each. Such information can be used to shape industry best practices to reduce Salmonella prevalence in cattle feeding operations, resulting in a decreased prevalence of Salmonella in LNs, and thus, minimizing risks to human health.

Plasmid Composition, Antimicrobial Resistance and Virulence Genes Profiles of Ciprofloxacin- and Third-Generation Cephalosporin-Resistant Foodborne Salmonella enterica Isolates from Russia

Salmonella enterica is an important foodborne pathogen worldwide. Ciprofloxacin and extended-spectrum cephalosporins are the common first-line antimicrobial drugs for the treatment of salmonellosis, antimicrobial resistance genes for which are mostly transferred via plasmids. The goal of this work was to perform genomic analysis of plasmids from foodborne S. enterica isolates obtained in Russia based on whole-genome sequencing. In the current study, 11 multidrug-resistant samples isolated in 2021 from 8 regions of Russia were selected based on their resistance to ciprofloxacin and third-generation cephalosporins (CIP-3rd). Whole-genome short-read sequencing (WGS) was performed for all isolates; the samples belonged to five different sequence types (ST32, ST469, ST11, ST142, and ST548) which had different profiles of antimicrobial resistance (AMR) and virulence genes. We have performed additional long-read sequencing of four representative S. enterica isolates, which showed that they carried pESI-like megaplasmids of 202-280 kb length harboring extended-spectrum β-lactamase genes, fluoroquinolone, tetracycline, and aminoglycosides resistance genes, as well as several virulence determinants. We believe that the WGS data obtained will greatly facilitate further studies of foodborne S. enterica isolates epidemiology in terms of their self-transmissible plasmid composition that mediated antimicrobial resistance and virulence determinants conferring selective advantages of this important bacterial pathogen.

Survival of bovine-associated serotypes of Salmonella enterica in bedding sand

Cattle persistently infected with bovine-adapted serotypes of Salmonella enterica are an important animal health and food safety issue. One possible mechanism by which infection is sustained in a dairy herd is by survival of Salmonella in sand used as bedding material. In this study we assessed the survival of 10⁷ to 10⁸ cfu bovine-associated serotypes of Salmonella enterica (sv. Cerro, Dublin, and Heidelberg) in sterile sand, recycled bedding sand, and gray water collected from a Wisconsin dairy farm. All 3 serotypes persisted at relatively high numbers (>10⁶ cfu/g) for at least 28 d in sterile sand, with Salmonella sv. Dublin decreasing less than 1 log₁₀ over 70 d. To our surprise, when low numbers of Salmonella sv. Dublin (10³ cfu) were inoculated into sterile sand, the organism multiplied within 3 d to approximately 10⁶ cfu/g sand and persisted at that level for 28 d. When we inoculated Salmonella sv. Dublin into recycled bedding sand or sand taken directly from cow pens, we observed a significant decrease in colony-forming units by d 7. In contrast, we observed a significant increase in colony-forming units when Salmonella sv. Dublin was inoculated into gray water from the sand recycling system. These data demonstrate that Salmonella can persist for extended periods of time in bedding sand, although this is limited to some extent by the native microbiota in recycled bedding sand.

Reduction of Pathogens in Feces and Lymph Nodes Collected from Beef Cattle Fed Lactobacillus salivarius (L28), Lactobacillus acidophilus (NP51) and Propionibacterium freudenreichii (NP28), Commercially Available Direct-Fed Microbials

The purpose of the study was to evaluate the prevalence and concentration of foodborne pathogens in the feces and peripheral lymph nodes (PLNs) of beef cattle when supplemented with direct-fed microbials (DFMs) in feedlots. Fecal samples were collected from the pen floors over a 5-month period at three different feedlots in a similar geographical location in Nebraska, where each feed yard represented a treatment group: (i.) control: no supplement, (ii.) Bovamine Defend: supplemented with NP51 and NP24 at a target dose of 9 log₁₀CFU/g/head/day, and (iii.) Probicon: supplemented with L28 at a target dose of 6 log₁₀CFU/g/head/day. Each fecal sample was tested for the prevalence of E. coli O157:H7 and Salmonella, and concentration of E. coli O157:H7, Enterobacteriaceae and Clostridium perfringens. Cattle were harvested and PLNs were collected on the harvest floor. Real-time Salmonella PCR assays were performed for each PLN sample to determine Salmonella presence. The cattle supplemented with both DFMs had reduced foodborne pathogens in fecal samples, but feces collected from the pens housing the cattle supplemented with Probicon consistently had significantly less E. coli O157:H7 and Salmonella prevalence as well as a lower C. perfringens concentration. While DFMs do not eliminate foodborne pathogens in fecal shedding and PLNs, the use of DFMs as a pre-harvest intervention allows for an effective way to target multiple pathogens reducing the public health risks and environmental dissemination from cattle.

The effect of supplementing CLOSTAT 500 (Bacillus subtilis PB6) to yearling steers in a commercial feedyard on health, Salmonella spp. prevalence, feedlot growth performance and carcass characteristics

British and British × Continental crossbred beef steers, n = 2,100; 313 ± 38 kg of initial body weight (BW) were used to evaluate the effects of Bacillus subtilis PB6 supplementation to yearling steers in a commercial feedyard on health, prevalence of Salmonella spp., growth performance, and carcass characteristics. Steers were blocked by arrival date and assigned randomly to pens within the block; pens were randomly assigned to 1 of 2 dietary treatments within block. Treatments, replicated in 15 pens/treatment with 70 steers/pen, included: 1) control (CON), diets containing no supplemental direct-fed microbials; 2) CLOSTAT (CLO), diets supplemented with 0.5 g/steer/d Bacillus subtilis PB6 (CLOSTAT 500, Kemin Industries, Des Moines, IA) to provide 6.6 × 109 CFU/g of the active ingredient. Supplementing CLO decreased the overall incidence of morbidity (P = 0.03), 10.38% (CLO) vs. 13.43% (CON), decreased the percentage of steers treated once for bovine respiratory disease (BRD; P < 0.01), 9.14% (CLO) vs. 12.76% (CON), and decreased the incidence of BRD retreatment (P = 0.03) compared with CON. Mortality did not differ among treatments (P = 0.23); however, overall deads and removals tended to be less for CLO than CON (53 heads vs. 73 heads respectively, P = 0.06). Prevalence of fecal Salmonella did not differ among treatments, (P ≥ 0.35); overall fecal Salmonella counts tended to be less for CLO (1.59 log (10) CFU/g) than CON (2.04 log (10) CFU/g; P = 0.07). Salmonella concentration in subiliac lymph nodes (n =150/treatment) was not different (P = 0.62) between CON (0.22 log (10) CFU/g) or CLO (0.19 log (10) CFU/g); however, there was a 46% reduction in the overall mean prevalence of lymph node Salmonella (P = 0.46; 15.48% vs. 28.66%) for CLO and CON, respectively. With deads and removals included, final BW was heavier for CLO steers than CON, (654 kg vs. 641 kg, respectively, P = 0.05), and average daily gain (ADG; P = 0.08) and gain efficiency (G:F; P = 0.06) tended to be greater for CLO than CON. With deads and removals excluded, final BW, ADG, and G:F did not differ among treatments (P ≥ 0.30). Carcass traits were not different between treatments (P ≥ 0.15). Supplementing CLO throughout the feeding period in a commercial feedyard improved the health outcomes of yearling steers by decreasing BRD and overall treatment rates, reducing the overall abundance of Salmonella, and resulting in fewer steers removed from the study compared with CON.

Metaphylactic antimicrobial effects on occurrences of antimicrobial resistance in Salmonella, Escherichia coli, and Enterococcus spp. measured longitudinally from feedlot arrival to harvest in high-risk beef cattle

Aims

Our objective was to determine how injectable antimicrobials affected populations of Salmonella enterica, Escherichia coli and Enterococcus spp. in feedlot cattle.

Methods and Results

Two arrival date blocks of high‐risk crossbred beef cattle (n = 249; mean BW = 244 kg) were randomly assigned one of four antimicrobial treatments administered on day 0: sterile saline control (CON), tulathromycin (TUL), ceftiofur (CEF) or florfenicol (FLR). Faecal samples were collected on days 0, 28, 56, 112, 182 and study end (day 252 for block 1 and day 242 for block 2). Hide swabs and subiliac lymph nodes were collected the day before and the day of harvest. Samples were cultured for antimicrobial‐resistant Salmonella, Escherichia coli and Enterococcus spp. The effect of treatment varied by day across all targeted bacterial populations (p ≤ 0.01) except total E. coli. Total E. coli counts were greatest on days 112, 182 and study end (p ≤ 0.01). Tulathromycin resulted in greater counts and prevalence of Salmonella from faeces than CON at study end (p ≤ 0.01). Tulathromycin and CEF yielded greater Salmonella hide prevalence and greater counts of 128ERY^RE. coli at study end than CON (p ≤ 0.01). No faecal Salmonella resistant to tetracyclines or third‐generation cephalosporins were detected. Ceftiofur was associated with greater counts of 8ERY^REnterococcus spp. at study end (p ≤ 0.03). By the day before harvest, antimicrobial use did not increase prevalence or counts for all other bacterial populations compared with CON (p ≥ 0.13).

Conclusions

Antimicrobial resistance (AMR) in feedlot cattle is not caused solely by using a metaphylactic antimicrobial on arrival, but more likely a multitude of environmental and management factors.

A Commentary on Salmonella From a Pre-Harvest Perspective

Salmonella occurs in all the major meat producing livestock species (ruminants, swine and poultry), most often residing within the gastrointestinal tract asymptomatically. While considerable success has been achieved post-harvest, the design of effective pre-harvest interventions to control Salmonella has lagged. A simplistic view of the extremely complex host/pathogen interaction suggests that the pathogen has a vested interest in not causing illness or death to the host. The former would initiate an immune response from the host and/or the application of therapeutic antibacterial agents, while the latter would require finding another suitable host. Due to the widespread prevalence of Salmonella within livestock and poultry, and the relatively few salmonellosis cases in comparison, it appears, and is supported by new research, that Salmonella has developed methods to avoid detection by the animal’s immune system and live essentially as a commensal organism within the gastrointestinal tract of the animal. Yet, for reasons that are not fully understood, this “commensal” Salmonella does on occasion become virulent, in young and mature animals alike. Indeed, these researchers have documented Salmonella carriage throughout the year in cattle, but only rarely, if at all, was salmonellosis observed. Further, evaluation of Salmonella isolates (serotype and antimicrobial resistance patterns) from sick and healthy cattle failed to explain that while Salmonella was present in the majority of cattle sampled on that farm, only a few developed salmonellosis. Virulence, as well as multi-drug resistance, in both livestock and humans appears to cluster within a few serotypes. As a result, petitions are circulating calling for the labeling of some Salmonella serotypes as adulterants, as was done with Escherichia coli O157:H7 and other enterohemorrhagic E. coli strains. Regulators are considering approaching the Salmonella problem by serotype, such as focusing specifically on the top 10 reported serotypes causing human illness. Herein, the authors will discuss the many challenges of controlling Salmonella pre-harvest, reflecting on the significant research portfolio that has been generated over the last 25 years, as well as challenging existing paradigms surrounding this pathogen and the experimental methods used to further our understanding of Salmonella and/or evaluate methods of control.

Prevalence and Sources of Salmonella Lymph Node Infection in Special-Fed Veal Calves

ABSTRACT

Peripheral lymph nodes (LNs) have been implicated as potential contaminants of ground beef, yet the source and timing of Salmonella LN infection in cattle is still unclear, limiting targeted intervention. The aim of this study was to leverage the vertical integration of special-fed veal production to identify preharvest environmental exposures, specifically in livestock trailers and harvest facility holding pens where calves spend 30 min to 4 h, that result in Salmonella LN infection. Ten cohorts of 80 to 82 veal calves were followed through the harvest process, and environmental samples were collected in barns, trailers, and holding pens. Mesenteric LNs from 35 calves were collected at harvest, and 25 prefemoral LNs per cohort were pooled. Within the same cohort, for 12 samples for which the serovar of the environmental and calf LN Salmonella isolates matched, the isolates were submitted for whole genome sequencing to determine whether environmental exposure resulted in LN infection. Cohort-level Salmonella mesenteric LN prevalence ranged from 0% (0 of 35 samples) to 80% (28 of 35 samples), and pooled prefemoral LNs were positive for Salmonella in 3 of the 10 cohorts. Salmonella prevalence in samples from barns, livestock trailers, and harvest facility holding pens was 22% (13 of 60 samples), 74% (59 of 80 samples), and 93% (74 of 80 samples), respectively. Some environmental and LN isolates were multidrug resistant. Four instances of Salmonella transmission from trailers and/or holding pens to calf LNs were supported by sequence data. Salmonella serovars Agona, Give, and Muenster were identified in transmission events. One instance of transmission from the livestock trailer, two instances from holding pens, and one instance from either trailer or holding pens were observed. Further research is needed to evaluate the extent of environmental Salmonella transmission in cattle and to determine whether targeted interventions in trailers or holding pens could reduce novel Salmonella LN infection in veal calves before harvest.

HIGHLIGHTS

Effect of a direct-fed microbial (10-G Armor) on feedlot performance, carcass characteristics, and prevalence of Salmonella in fed-beef heifers

Crossbred beef heifers [N = 1,394; initial shrunk body weight (BW) 291 ± 9.9 kg] were used to investigate the efficacy of 10-G Armor (Life Products, Inc., Norfolk, NE; 10-G) upon feedlot performance, carcass characteristics, and fecal and subiliac lymph nodes Salmonella prevalence. Heifers were blocked by day of arrival and allocated to 1 of 20 pens (N = 70 heifers/pen) and assigned one of two treatments (10 pens/treatment) : no direct fed microbial (CON) or 2g/heifer/d of L. acidophilus, E. faecium, P. pentosaceus, L. brevis and L. plantarum respectively (Life Products, Inc., Norfolk, NE; 10-G). Twenty four animals were randomly selected from each pen for Salmonella sampling. Recto-anal mucosal swab samples (RAMS) were obtained at initial processing and harvest; subiliac lymph nodes were collected at harvest. In addition, pen surface fecal pats were collected and composited by pen (10 pats per composite, 5 composites per pen) on day 0, 52, 120 and 192. Data were analyzed as a generalized complete block design and pen served as the experimental unit. No differences were observed in live growth performance metrics (P ≥ 0.55). Yield grade distributions did not differ between treatments (P ≥ 0.62), however cattle fed 10-G tended (P = 0.06; 14.6 vs 18.9%) to have fewer USDA Select carcasses and more (P = 0.09; 73.6 vs 78.0%) USDA Choice carcasses. Cattle fed 10-G tended (P = 0.10; 9.2% vs 12.3%) to have fewer liver abscesses and had fewer (P = 0.04; 5.3 vs 8.5%) severe liver abscesses. Salmonella prevalence of RAMS did not differ between treatments at initial processing (P = 0.97; CON = 11.6%, 10-G = 11.5%) or at harvest (P = 0.91; CON = 99.0%, 10-G = 98.6%), however RAMS differed (P &lt; 0.01) in Salmonella prevalence between the two collection times. Cattle fed 10-G had a lower frequency of Salmonella positive lymph nodes (P = 0.01; CON = 15.8%, 10-G = 7.4%) than CON. However, Salmonella log (mpn/g) of lymph nodes did not differ between treatments at harvest (P = 0.34; CON = 0.73, 10-G = 0.34). These data indicate that cattle fed 10-G have decreased rates of severe liver abscesses without altering live animal performance or carcass characteristics. Supplementation of 10-G significantly reduced prevalence rate of Salmonella recovered from the subiliac lymph nodes. The factors responsible for the observed difference in the effects of 10-G on Salmonella warrants further investigation

Salmonella Prevalence Varies Over Time and Space in Three Large, Adjacent Cattle Operations in the Southwestern United States

We set out to describe the prevalence of Salmonella enterica in three large, adjacent cattle operations in the southern High Plains of the United States. Operations included two dairies (one of which routinely administers a commercially available Salmonella vaccine) and one feedlot. Samples were collected monthly for 12 months. At each sample collection, 25 freshly voided fecal pats and a sample from each of the water troughs were collected from each of five pens of cattle within an operation. Each monthly collection included a total of 375 fecal and ~32 water samples for a yearly total of 4,500 and 379 samples, respectively (note that the number of water troughs per pen varied within an operation). Salmonella was commonly recovered from fecal (71.3%) and water (28.5%) samples and tended to follow somewhat similar temporal patterns over time. However, its prevalence varied among operations despite being adjacent properties in that Salmonella was recovered from 61.3, 80.1, and 75% of fecal samples from dairy 1, dairy 2 and the feedlot, respectively. Salmonella prevalence in water samples across collection times averaged 36.1, 70.2, and 46.1% for dairy 1, dairy 2, and the feedlot, respectively. While it is uncertain why the Salmonella prevalence varied from operation to operation, the higher observed prevalence of Salmonella in water on dairy 2 and/or the use of a commercial Salmonella vaccine by dairy 1 may offer a partial explanation.

Evaluation of a Preharvest Bacteriophage Therapy for Control of Salmonella within Bovine Peripheral Lymph Nodes

ABSTRACT

A series of proof-of-concept studies were developed to determine whether a commercial bacteriophage cocktail could be utilized for the mitigation of Salmonella in bovine peripheral lymph nodes (LNs). The first objective sought to determine whether exogenous phage could be isolated from the LNs following administration. If isolation were successful, the second objective was to determine whether the phage in the LNs could effectively reduce Salmonella. Salmonella Montevideo was inoculated intradermally at multiple sites and multiple times, followed by delivery of the phage cocktail subcutaneously in two injections around each of the right and left prescapular and subiliac LNs. At the conclusion of each study, animals were euthanized, and the popliteal, prescapular, and subiliac LNs were examined. The inoculated phage was successfully isolated from the LNs; transmission electron microscopy revealed phages in the LNs of the treated cattle, and these phages were identical to those in the cocktail. Levels of phage were higher (P < 0.01) in the prescapular and subiliac LNs in the phage-treated than in the control cattle. In subsequent studies, the protocols were modified to increase Salmonella and phage levels within the LNs. Compared with the first study, overall Salmonella levels were increased in the LNs, and phage treatment decreased (P < 0.01) Salmonella in the some of the LNs. Phage levels were numerically but not significantly increased (P = 0.12) in the treated cattle. The final study was modified, hypothesizing that a 48-h postmortem period before LN removal would facilitate phage-Salmonella interaction; however, no differences (P > 0.10) in Salmonella levels were found among treatments. Salmonella-specific phages administered to live cattle can translocate to the LNs; however, these phages had limited to no effect on Salmonella in these LNs under these experimental conditions.

HIGHLIGHTS

Prevalence, serovar distribution, and antibiotic resistance of Salmonella spp. isolated from pork in China: A systematic review and meta-analysis

The epidemiological characteristics of Salmonella spp. in pork have been widely studied in China, but the results remain inconsistent. This study aimed to summarize the epidemiological characteristics of Salmonella spp. isolated from pork, including its prevalence, serovar distribution, and antibiotic resistance rate. We systematically reviewed published studies on Salmonella spp. isolated from pork in China between 2000 and 2020 in two Chinese and three English databases and quantitatively summarized its prevalence, serovar distribution, and antibiotic resistance using meta-analysis methods. Furthermore, we conducted subgroup analysis and meta-regression to explore the source of the heterogeneity from historical changes and regional difference perspectives. Ninety-one eligible studies published between 2000 and 2020 were included. The meta-analysis showed that the pooled prevalence of Salmonella isolated from pork was 0.17 (95% CI: 0.14, 0.20), with a detected growing trend over time. For the proportions of serovars, Derby (0.32, 95% CI: 0.26, 0.38), Typhimurium (0.10, 95% CI: 0.07, 0.15) and London (0.05, 95% CI: 0.03, 0.08) were dominant in these studies. The antibiotic resistance rates were high for tetracycline (0.68, 95% CI: 0.59, 0.77), sulfisoxazole (0.65, 95% CI: 0.45, 0.83), ampicillin (0.43, 95% CI: 0.34, 0.53), streptomycin (0.42, 95% CI: 0.29, 0.56), and sulfamethoxazole (0.42, 95% CI: 0.25, 0.60). The results of this study revealed a high prevalence, the regional characteristics of serovar distribution, and the severe challenges of antibiotic resistance of Salmonella originating from pork in China, suggesting the potential increasing risk and disease burden. Therefore, it is necessary to improve the prevention and control strategies of Salmonella in pork.

Kinome Analysis of Cattle Peripheral Lymph Nodes to Elucidate Differential Response to Salmonella spp

Salmonella spp., contained within the peripheral lymph nodes (PLNs) of cattle, represents a significant source of contamination of ground beef. Herein is the first report where species-specific kinome peptide arrays designed for bovine biology were used to further the understanding of Salmonella spp. within these PLNs. For the purpose of this research, multiple comparisons of sub-iliac lymph nodes were made to include nodes from feedlot cattle that were infected with Salmonella spp. to those that were non-infected; seasonal differences in feedlot cattle harvested in either August or January; cull dairy cows compared to feedlot cattle; and PLNs from cattle experimentally inoculated with Salmonella spp. versus naturally infected animals. The first comparison of Salmonella-positive and -negative PLNs found that considering the kinotypes for these animals, the major distinguishing difference was not the presence or absence of Salmonella spp. in the PLNs but the concentration. Further, the majority of pathways activated were directly related to immune responses including innate immunity, thus Salmonella spp. within the PLNs activates the immune system in that node. Results from the comparison of feedlot cattle and cull dairy cows suggests that a Salmonella spp.-negative animal, regardless of type, has a more consistent kinome profile than that of a Salmonella spp.-positive animal and that the differences between feedlot and cull dairy cattle are only pronounced when the PLNs are Salmonella spp. positive. PLNs collected in the winter showed a much more consistent kinome profile, regardless of Salmonella status, suggesting that in the winter these cattle are similar, and this is not affected by the presence of Salmonella spp., whereas significant variability among kinotypes was observed for PLNs collected in the summer. The most distinct clustering of kinotypes observed in this study was related to how the animal was infected with Salmonella spp. There were significant differences in the phosphorylation state of the immune response peptides between experimentally and naturally infected animals, suggesting that the immune system is activated in a significantly different manner when comparing these routes of infection. Increasing our understanding of Salmonella spp. within cattle, and specifically within the PLNs, will ultimately help design effective pre-harvest intervention strategies as well as appropriate experimentation to validate those technologies.

Serotyping, MLST, and Core Genome MLST Analysis of Salmonella enterica From Different Sources in China During 2004-2019

Salmonella enterica (S. enterica) is an important foodborne pathogen, causing food poisoning and human infection, and critically threatening food safety and public health. Salmonella typing is essential for bacterial identification, tracing, epidemiological investigation, and monitoring. Serotyping and multilocus sequence typing (MLST) analysis are standard bacterial typing methods despite the low resolution. Core genome MLST (cgMLST) is a high-resolution molecular typing method based on whole genomic sequencing for accurate bacterial tracing. We investigated 250 S. enterica isolates from poultry, livestock, food, and human sources in nine provinces of China from 2004 to 2019 using serotyping, MLST, and cgMLST analysis. All S. enterica isolates were divided into 36 serovars using slide agglutination. The major serovars in order were Enteritidis (31 isolates), Typhimurium (29 isolates), Mbandaka (23 isolates), and Indiana (22 isolates). All strains were assigned into 43 sequence types (STs) by MLST. Among them, ST11 (31 isolates) was the primary ST. Besides this, a novel ST, ST8016, was identified, and it was different from ST40 by position 317 C → T in dnaN. Furthermore, these 250 isolates were grouped into 185 cgMLST sequence types (cgSTs) by cgMLST. The major cgST was cgST235530 (11 isolates), and only three cgSTs contained isolates from human and other sources, indicating a possibility of cross-species infection. Phylogenetic analysis indicated that most of the same serovar strains were putatively homologous except Saintpaul and Derby due to their multilineage characteristics. In addition, serovar I 4,[5],12:i:- and Typhimurium isolates have similar genomic relatedness on the phylogenetic tree. In conclusion, we sorted out the phenotyping and genotyping diversity of S. enterica isolates in China during 2004-2019 and clarified the temporal and spatial distribution characteristics of Salmonella from different hosts in China in the recent 16 years. These results greatly supplement Salmonella strain resources, genetic information, and traceability typing data; facilitate the typing, traceability, identification, and genetic evolution analysis of Salmonella; and therefore, improve the level of analysis, monitoring, and controlling of foodborne microorganisms in China.

High-Resolution Genomic Comparisons within Salmonella enterica Serotypes Derived from Beef Feedlot Cattle: Parsing the Roles of Cattle Source, Pen, Animal, Sample Type, and Production Period

Salmonella enterica is a major foodborne pathogen, and contaminated beef products have been identified as one of the primary sources of Salmonella-related outbreaks. Pathogenicity and antibiotic resistance of Salmonella are highly serotype and subpopulation specific, which makes it essential to understand high-resolution Salmonella population dynamics in cattle. Time of year, source of cattle, pen, and sample type (i.e., feces, hide, or lymph nodes) have previously been identified as important factors influencing the serotype distribution of Salmonella (e.g., Anatum, Lubbock, Cerro, Montevideo, Kentucky, Newport, and Norwich) that were isolated from a longitudinal sampling design in a research feedlot. In this study, we performed high-resolution genomic comparisons of Salmonella isolates within each serotype using both single-nucleotide polymorphism-based maximum-likelihood phylogeny and hierarchical clustering of core-genome multilocus sequence typing. The importance of the aforementioned features in clonal Salmonella expansion was further explored using a supervised machine learning algorithm. In addition, we identified and compared the resistance genes, plasmids, and pathogenicity island profiles of the isolates within each subpopulation. Our findings indicate that clonal expansion of Salmonella strains in cattle was mainly influenced by the randomization of block and pen, as well as the origin/source of the cattle, i.e., regardless of sampling time and sample type (i.e., feces, lymph node, or hide). Further research is needed concerning the role of the feedlot pen environment prior to cattle placement to better understand carryover contributions of existing strains of Salmonella and their bacteriophages. IMPORTANCE Salmonella serotypes isolated from outbreaks in humans can also be found in beef cattle and feedlots. Virulence factors and antibiotic resistance are among the primary defense mechanisms of Salmonella, and are often associated with clonal expansion. This makes understanding the subpopulation dynamics of Salmonella in cattle critical for effective mitigation. There remains a gap in the literature concerning subpopulation dynamics within Salmonella serotypes in feedlot cattle from the beginning of feeding up until slaughter. Here, we explore Salmonella population dynamics within each serotype using core-genome phylogeny and hierarchical classifications. We used machine learning to quantitatively parse the relative importance of both hierarchical and longitudinal clustering among cattle host samples. Our results reveal that Salmonella populations in cattle are highly clonal over a 6-month study period and that clonal dissemination of Salmonella in cattle is mainly influenced spatially by experimental block and pen, as well by the geographical origin of the cattle.

Presence of Presumptive Shiga Toxin–Producing Escherichia coli and Salmonella on Sheep during Harvest in Honduras

ABSTRACT

Shiga toxin–producing Escherichia coli (STEC) and Salmonella are foodborne pathogens commonly harbored in the gastrointestinal tract of sheep. These pathogens can be on the hide of sheep and transferred to the carcass, causing a foodborne hazard. Salmonella can also be found in the lymph nodes of sheep, creating a biological hazard during harvest and processing. Developing countries lack baseline data on the presence of these foodborne pathogens on meat products, specifically sheep meat. Therefore, the objective of this study was to evaluate the presence of STEC and Salmonella on sheep hides, preevisceration carcasses, and final carcasses and Salmonella in subiliac lymph nodes from two small Honduran harvest facilities, plants A and B. Sponge swabs from the foreshank region of hides and carcasses and subiliac lymph node samples were collected from 96 sheep (86 at plant A; 10 at plant B). Microbial detection of STEC and Salmonella was performed by using the BAX System to screen for prevalence, and suspect samples were subjected to conventional culture isolation for confirmation. Overall baseline contamination on hides were 34.4 and 10.4% present for STEC and Salmonella; however, through implementation of sanitary procedures, preevisceration and final carcass samples were significantly decreased from the hide for both pathogens (P = 0.05). Moreover, overall plant A had significantly higher rates (P = 0.05) of STEC and Salmonella at each carcass sampling site compared with plant B. After each sampling was performed, recommendations were provided to each facility on the basis of pathogen presence, performance techniques, and contamination risks. Through recommendations and implementation of hazard analysis and critical control points, good manufacturing practices, and sanitation standard operating procedures in each facility, the meat supply in Honduras will become safer and more wholesome. Therefore, the development of a baseline with continued sampling is crucial to understand the risk of foodborne pathogens to consumers in the Honduran sheep meat supply.

HIGHLIGHTS

Evaluation of two commercially-available Salmonella vaccines on Salmonella in the peripheral lymph nodes of experimentally-infected cattle

Background

Salmonella is a common inhabitant of the ruminant gastrointestinal tract, where it often resides asymptomatically and may be shed into the feces. More recently it was discovered that Salmonella may be contained within the peripheral, non-mesenteric lymph nodes, where it is impervious to in-plant pathogen control interventions and may serve as a source of Salmonella-contamination of ground beef. Over the past 10 years considerable research effort has been expended at understanding how this pathogen gets to these lymph nodes, the duration of infection, and, most importantly, screening and developing potential intervention strategies that may be employed on farm prior to the animal being presented for slaughter.

Methods

Utilizing an experimental model of Salmonella inoculation of bovine peripheral lymph nodes (PLNs), two pilot vaccine experiments were conducted to evaluate two Salmonella vaccines: Salmonella Newport Bacterial Extract (Experiment I) and Endovac-Bovi^® (Experiment II) on preventing Salmonella acquisition by these nodes. In Experiment I, 4 months following the booster vaccination, 30 steers were inoculated with three Salmonella serotypes intradermally: Newport, Montevideo, and Anatum administered to the right legs, left legs, and to the caudal thorax and abdomen, respectively. Cattle were inoculated every other day over the course of five days (three total inoculation events) and 6 and 12 days following the final Salmonella inoculation, 16 and 14 head in each treatment were euthanized, respectively. In Experiment II, 12 head of Holstein steers were utilized. Seven days following the booster and weekly thereafter for 3 weeks (four total inoculation events), cattle were inoculated as above and euthanized 7 days following final inoculation. Right and left sub-iliac, popliteal and pre-scapular lymph nodes were collected in each experiment, weighed and cultured for Salmonella.

Results

In Experiment I, no treatment differences were observed in Salmonella prevalence 6 days post-inoculation (necropsy 1). However, in vaccinated cattle at the second necropsy, a reduction (p = 0.05) in Salmonella prevalence was observed in the sub-iliac and pre-scapular lymph nodes as well as when all nodes were evaluated collectively (p = 0.04). In Experiment II, the vaccine reduced (p = 0.03) Salmonella prevalence in the right popliteal and tended (p = 0.09) to decrease prevalence in both popliteal lymph nodes.

Conclusion

Under these experimental conditions, the data generated provide evidence of a partial vaccine effect on Salmonella within PLNs and indicate that further research may be warranted.

A Preliminary Study on the Presence of Salmonella in Lymph Nodes of Sows at Processing Plants in the United States

Salmonella-contaminated lymph nodes (LN), when included into edible meat products, are a potential source of Salmonella foodborne disease. In this survey, ventral superficial cervical and mandibular LN were tested for the presence of Salmonella from two sow processing plants in the midwestern United States. Results indicate that both LN can be contaminated with Salmonella; mandibular LN have higher prevalence (p < 0.05) of Salmonella than cervical LN (16% vs. 0.91%), and the majority (>90%) of Salmonella isolates are pan-susceptible or resistant to one antimicrobial, while 9.78% of isolates were multi-drug-resistant (MDR-resistant to three or more classes of antimicrobials). Intervention methods to prevent foodborne disease could include elimination of these LN from pork products or inclusion of LN only into products that are destined for cooking. Integrated multi-faceted intervention methods need to be developed to reduce Salmonella in the food chain.

Bovine lymph nodes as a source of Escherichia coli contamination of the meat

Ground beef contamination with Escherichia coli is usually a result of carcass faecal contamination during the slaughter process. Carcasses are contaminated when they come into contact with soiled hides or intestinal leakage content during dressing and the evisceration processes. A more recent and compelling hypothesis is that, when lymph nodes are present in manufacturing beef trimmings, they can be a potential source of Enterobacteriaceae contamination of ground beef. The aim of this study was to investigate the occurrence of E. coli in lymph nodes from beef carcasses used for ground meat production, in six slaughter plants situated in central Italy A total of 597 subiliac (precrural) lymph nodes were obtained from 597 cattle carcasses and screened for E. coli by culture. Furthermore, E. coli isolates (one per positive carcass) were tested for stx1, stx2 eaeA and hlyA genes that are commonly used to identify and characterise shiga toxin-producing E. coli (STEC). In addition, the E. coli isolates were profiled for antimicrobial susceptibility. A proportion of 34.2% (204/597) carcasses were positive for E. coli. PCR revealed that 29% (59/204) of E. coli possessed stx1 or stx2 which corresponded to 9.9% of the cattle sampled. Moreover, a combination of stx1 or stx2 and eaeA was found in in 4 isolates (2% among E. coli positive samples and 1% among cattle sampled) and a combination of stx1 or stx2 and eaeA and hly in 1 isolate (0.5% and 0.2%). More than 95% of isolates were susceptible to gentamicin, ceftriaxone, cyprofloxacin and cefotaxime while high rates of resistance were recorded for cephalotin, ampicillin, tetracycline, tripe sulfa and streptomycin. The multivariate analysis identified "age" as the factor most closely related to E. coli positivity (either generic E. coli or STEC) in bovine lymph nodes. In conclusion, subiliac lymph nodes represent a source of E. coli for ground beef. These results are of major importance for risk assessment and improving good manufacturing practices during animal slaughter and ground meat production.

Translocation of Orally Inoculated Salmonella Following Mild Immunosuppression in Holstein Calves and the Presence of the Salmonella in Ground Beef Samples

The objective of this study was to determine if immunosuppression through daily dexamethasone (DEX) infusion altered Salmonella translocation from the gastrointestinal tract. Weaned Holstein steers (n = 20; body weight [BW] = 102 ± 2.7 kg) received DEX (n = 10; 0.5 mg/kg BW) or saline (control [CON]; n = 10;) for 4 days (from day -1 to 2) before oral inoculation of naldixic acid-resistant Salmonella enterica Typhimurium (SAL; 3.4 × 10⁶ colony-forming units [CFU]/animal) on day 0. Fecal swabs were obtained daily, and blood was collected daily for hematology. At harvest (day 5), ileum, cecal fluid, lymph nodes (ileocecal, mandibular, popliteal, and subscapular), and synovial (stifle, coxofemoral, and shoulder) samples were collected for isolation of the inoculated strain of SAL. White blood cell (WBC) and neutrophil concentrations were elevated (p < 0.01) in DEX calves following each administration event. Following inoculation, 100% of DEX calves shed the experimental strain of SAL for all 5 days, 90% of CON calves shed from day 1 to 3, and 100% of CON calves shed from day 4 to 5. Greater (p < 0.01) concentrations of SAL were quantified from the cecum of DEX calves (3.86 ± 0.37 log CFU/g) compared with CON calves (1.37 ± 0.37 log CFU/g). There was no difference in SAL concentrations between DEX and CON calves in ileal tissue (p = 0.07) or ileocecal (p = 0.57), mandibular (p = 0.12), popliteal (p = 0.99), or subscapular (p = 0.83) lymph nodes. Of the stifle samples collected, 3.3% were positive for SAL, highlighting a contamination opportunity during hindquarter breakdown. While more research is needed to elucidate the interactions of immunosuppression and pathogen migration patterns, these data confirm that orally inoculated SAL can translocate from the gastrointestinal tract and be harbored in atypical locations representing a food safety risk.

Bacillus subtilis PB6 Supplementation in Weaned Holstein Steers During an Experimental Salmonella Challenge

To evaluate the effects of a patented Bacillus subtilis probiotic, weaned Holstein steers, not shedding Salmonella (n = 40; ∼90 kg), were supplemented (CLO) or not (CON) with CLOSTAT^® (13 g/hd per day; Kemin Industries, Des Moines, IA) in a starter ration for 35 d. The calves were assigned to one of four treatments in a 2 × 2 factorial design with CLO and CON calves that were orally administered Salmonella (STM) or not (NoSTM). Calves were challenged with 1.6 × 10⁶ colony-forming unit (CFU) Salmonella Typhimurium (resistant to 50 μg/mL nalidixic acid) in 1 L of milk replacer on day 0. Blood samples were collected through jugular catheters every 6 h for 96 h, and body temperature was measured every 5 min through indwelling rectal temperature recording devices. Five calves from each treatment were harvested 48 h postchallenge, and the remaining calves were harvested 96 h postchallenge. During necropsy, tissues were collected for the isolation and quantification of the inoculated STM from various tissues. The CLOSTM group had reduced STM concentrations in the jejunum, ileum, and transverse colon 48 h after the challenge (p ≤ 0.03), but were not different 96 h postchallenge (p > 0.05). Decreased (p < 0.01) pyrexia was observed after the challenge in CLOSTM calves when compared with CONSTM calves. White blood cells and lymphocyte counts were increased (p ≤ 0.05) in CLOSTM calves after the challenge in comparison with other treatments. In calves given STM, the CLO group had greater feed intake before and after the challenge (p < 0.01) compared with the CON group. Increased serum IL-6 and IFN-γ concentrations were observed in the CONSTM group compared with other treatments. Overall, CLO reduced Salmonella presence and concentrations in gastrointestinal tissues while simultaneously reducing the severity of the challenge as indicated by blood parameters and the reduced febrile response.

Evaluation of the Efficacy of Three Direct Fed Microbial Cocktails To Reduce Fecal Shedding of Escherichia coli O157:H7 in Naturally Colonized Cattle and Fecal Shedding and Peripheral Lymph Node Carriage of Salmonella in Experimentally Infected Cattle

Two experiments were conducted to evaluate the feeding of direct fed microbials (DFMs) on fecal shedding of Escherichia coli O157:H7 in naturally infected cattle (experiment I) and on Salmonella in the feces and peripheral lymph nodes (PLNs) of experimentally infected cattle (experiment II). Thirty cattle, 10 per treatment, were used in each experiment. Treatments in experiment I consisted of a control (lactose carrier only); DFM1, a 1:1 ratio of Enterococcus faecium and Lactobacillus animalis; and DFM2, a 1:1 ratio of Lactobacillus acidophilus and Pediococcus acidilactici. In Experiment II, DFM1 was replaced with DFM3, a 1:2 ratio of Lactobacillus reuteri and other Lactobacillus strains. Additives were mixed in water and applied as a top-dressing to each pen's daily ration for 50 days. Approximately half-way through each experiment, the DFM concentration was doubled for the remainder of the study. Fecal samples were collected throughout experiment I and cultured for E. coli O157:H7. Cattle in experiment II were inoculated intradermally with Salmonella Montevideo on days 32, 37, and 42 and then necropsied on days 49 and 50 (five cattle per treatment on each day). Innate immune function was assessed on days 29, 49, and 50. In experiment I, fecal concentration and prevalence of E. coli O157:H7 were not different (P > 0.10) nor was there an effect (P = 0.95) on the percentage of super shedders (cattle shedding ≥3.0 log CFU/g of feces). In experiment II, no treatment differences (P > 0.05) were observed for Salmonella in the PLNs except for the inguinal nodes, which had a significantly lower Salmonella prevalence in DFM-supplemented cattle than in the controls. Immune function, as measured by monocyte nitric oxide production and neutrophil oxidative burst, was decreased (P < 0.05) in the DFM treatment groups. Although results of this research indicate little to no effect of these DFMs on E. coli O157:H7 or Salmonella in cattle, an increase in the duration of administration to that similar to what is used for commercial cattle might elicit treatment differences.

Nature and consequences of interactions between Salmonella enterica serovar Dublin and host cells in cattle

Salmonella enterica is a veterinary and zoonotic pathogen of global importance. While murine and cell-based models of infection have provided considerable knowledge about the molecular basis of virulence of Salmonella, relatively little is known about salmonellosis in naturally-affected large animal hosts such as cattle, which are a reservoir of human salmonellosis. As in humans, Salmonella causes bovine disease ranging from self-limiting enteritis to systemic typhoid-like disease and exerts significant economic and welfare costs. Understanding the nature and consequences of Salmonella interactions with bovine cells will inform the design of effective vaccines and interventions to control animal and zoonotic infections. In calves challenged orally with S. Dublin expressing green fluorescent protein (GFP) we observed that the bacteria were predominantly extracellular in the distal ileal mucosa and within gut-associated lymph nodes 48 h post-infection. Intracellular bacteria, identified by flow cytometry using the GFP signal, were predominantly within MHCII⁺ macrophage-like cells. In contrast to observations from murine models, these S. Dublin-infected cells had elevated levels of MHCII and CD40 compared to both uninfected cells from the same tissue and cells from the cognate tissue of uninfected animals. Moreover, no gross changes of the architecture of infected lymph nodes were observed as was described previously in a mouse model. In order to further investigate Salmonella-macrophage interactions, net replication of S. enterica serovars that differ in virulence in cattle was measured in bovine blood-derived macrophages by enumeration of gentamicin-protected bacteria and fluorescence dilution, but did not correlate with host-specificity.

Population Dynamics of Salmonella enterica within Beef Cattle Cohorts Followed from Single-Dose Metaphylactic Antibiotic Treatment until Slaughter

Antibiotic use in cattle can select for multidrug-resistant Salmonella enterica, which is considered a serious threat by the U.S. Centers for Disease Control and Prevention. A randomized controlled longitudinal field trial was designed to determine the long-term effects of a single dose of ceftiofur or tulathromycin on Salmonella population characteristics in cattle feces and peripheral lymph nodes and on hides. A total of 134 beef cattle from two sources were divided among 12 pens, with cattle in each of the 3-pen blocks receiving a single dose of either ceftiofur or tulathromycin or neither (control) on day 0. Fecal samples were collected before treatment (day 0) and repeatedly following treatment until slaughter (day 99+). Hide and lymph node samples were collected at slaughter age. Salmonella prevalence, phenotypic antimicrobial resistance, serotype, and phylogenetic relationships were examined. Multilevel mixed logistic regression models indicated no significant effects (P ≥ 0.218) of metaphylactic antibiotics on the prevalence of Salmonella across sample types. However, there was a significant time effect observed, with prevalence increasing from spring through the midsummer months (P < 0.0001) in feces. The majority of Salmonella isolates were pansusceptible to a panel of 14 antibiotics both before and after treatment. Highly prevalent Salmonella serotypes were Salmonella enterica serovar Montevideo, Salmonella enterica serovar Anatum, Salmonella enterica serovar Cerro, and Salmonella enterica serovar Lubbock across all sample types. Strong pen and cattle source serotype clustering effects were observed among Salmonella isolates originating from fecal, lymph node, and hide samples; however, the potential role of Salmonella isolates from the pen environment prior to animal placement was not assessed in this study.IMPORTANCE Salmonella is a leading bacterial foodborne pathogen, causing a significant number of human infections and deaths every year in the United States. Macrolides and 3rd-generation cephalosporins play critical roles in the treatment of human salmonellosis. Use of these antibiotics in beef cattle can select for resistant bacteria that may enter the food chain or spread from the farm via manure. There is a lack of longitudinal research concerning the long-term effects of metaphylactic antibiotic administration. Here, we assessed Salmonella population dynamics during the feeding period until slaughter following single-dose antibiotic treatment. We found no long-term effects of antibiotic use early in the cattle-feeding period on Salmonella prevalence and antimicrobial resistance at slaughter. We identified the pens in which cattle were housed as the factor that contributed most to Salmonella serotypes being shared; importantly, the dominant strain in each pen changed repeatedly over the entire feeding period.

Worldwide Epidemiology of Salmonella Serovars in Animal-Based Foods: a Meta-analysis

Salmonella spp. are among the most important foodborne pathogens and the third leading cause of human death among diarrheal diseases worldwide. Animals are the primary source of this pathogen, and animal-based foods are the main transmission route to humans. Thus, understanding the global epidemiology of Salmonella serovars is key to controlling and monitoring this bacterium. In this context, this study aimed to evaluate the prevalence and diversity of Salmonella enterica serovars in animal-based foods (beef, pork, poultry, and seafood) throughout the five continents (Africa, the Americas [North and Latin America], Asia, Europe, and Oceania). The meta-analysis consisted of a chemometric assessment (hierarchical cluster analysis and principal component analysis) to identify the main epidemiological findings, including the prevalence and diversity of the Salmonella serovars in each matrix. Regarding the serovar distribution, S Typhimurium presented a cosmopolitan distribution, reported in all four assessed matrices and continents; poultry continues to play a central role in the dissemination of the Enteritidis serovar to humans, and Anatum and Weltevreden were the most frequently found in beef and seafood, respectively. Additionally, we recommended careful monitoring of certain serovars, such as Derby, Agona, Infantis, and Kentucky. Finally, given the scientific data regarding the most frequently reported serovars and which matrices constitute the main vehicles for the transmission of this pathogen, control programs may be improved, and specific interventions may be implemented in an attempt to reduce the risk of this pathogen reaching humans.IMPORTANCE Salmonellosis is caused by Salmonella spp. and is the third leading cause of death among food-transmitted diseases. This pathogen is commonly disseminated in domestic and wild animals, and the infection's symptoms are characterized by acute fever, nausea, abdominal pain, and diarrhea. The animals are the primary source of salmonellae, and animal-based foods are the main transmission route to humans. Therefore, data collected from these sources could contribute to future global interventions for effective control and surveillance of Salmonella along the food chain. In light of this, the importance of our research is in identifying the prevalence of Salmonella serovars in four animal-based food matrices (pork, poultry, beef, and seafood) and to evaluate the importance that each matrix has as the primary source of this pathogen to humans.

Assessment of Salmonella Prevalence in Lymph Nodes of U.S. and Mexican Cattle Presented for Slaughter in Texas

Foodborne salmonellosis has been traced to undercooked ground beef and other beef products in the past, and peripheral lymph node (LN) presence in the fatty tissues of beef carcasses is one possible source of Salmonella contamination. Researchers have previously reported higher rates of Salmonella prevalence in LNs from cattle raised and harvested in Mexico compared with rates typically observed from cattle harvested in the United States. With cattle of Mexican origin comprising the majority of U.S. live cattle imports, the objectives of this study were designed to determine whether Salmonella prevalence in LNs differed (i) between cattle of Mexican and U.S. origins when exposed to the same South Texas feeding operation and (ii) between warm and cool seasons. To meet these objectives, paired (left and right sides) subiliac LNs ( n = 800 LNs; n = 400 pooled samples) were collected from 100 carcasses per origin (Mexico and United States) per season (cool, December to January; warm, July to September). Overall, Salmonella prevalence in LN samples was 52.0% (208 of 400). No difference ( P = 0.4836) was seen in Salmonella prevalence as a function of origin, with 54.0% (108 of 200) and 50.0% (100 of 200) of LN samples returning Salmonella-positive results from cattle of Mexican and U.S. origin, respectively. Salmonella prevalence differed ( P = 0.0354) between seasons, with 46.5% (93 of 200) of cool and 57.5% (115 of 200) of warm season samples returning Salmonella-positive results. Serotyping of PCR-confirmed positive samples resulted in 14 different serovars being identified, with Cerro (21.6%), Anatum (19.7%), Muenchen (17.8%), Montevideo (14.4%), and Kentucky (12.0%) comprising the majority of serovars. These results suggest that factors other than cattle origin may be impacting Salmonella prevalence rates in bovine LNs and that additional research is needed to better understand the role of environment and management-related factors on Salmonella prevalence in bovine LNs.

Retrospective application of transposon-directed insertion-site sequencing to investigate niche-specific virulence of Salmonella Typhimurium in cattle

Background

Salmonella enterica subspecies enterica is an animal and zoonotic pathogen of global importance. Cattle are a significant reservoir of human non-typhoidal salmonellosis and can suffer enteric and systemic disease owing to the ability of Salmonella to survive within the bovine lymphatic system and intestines. Contamination of food can occur due to the incorporation of contaminated peripheral lymph nodes or by direct contamination of carcasses with gut contents. It is essential to understand the mechanisms used by Salmonella to enter and persist within the bovine lymphatic system and how they differ from those required for intestinal colonization to minimize zoonotic infections.

Results

Transposon-directed insertion site sequencing (TraDIS) was applied to pools of mutants recovered from mesenteric lymph nodes (MLNs) draining the distal ileum of calves after oral inoculation with a library of 8550 random S. Typhimurium mini-Tn5Km2 mutants in pools of 475 mutants per calf. A total of 8315 mutants representing 2852 different genes were detected in MLNs and their in vivo fitness was calculated. Using the same improved algorithm for analysis of transposon-flanking sequences, the identity and phenotype of mutants recovered from the distal ileal mucosa of the same calves was also defined, enabling comparison with previously published data and of mutant phenotypes across the tissues. Phenotypes observed for the majority of mutants were highly significantly correlated in the two tissues. However, 32 genes were identified in which transposon insertions consistently resulted in differential fitness in the ileal wall and MLNs, suggesting niche-specific roles for these genes in pathogenesis. Defined null mutations affecting ptsN and spvC were confirmed to result in tissue-specific phenotypes in calves, thus validating the TraDIS dataset.

Conclusions

This validation of the role of thousands of Salmonella genes and identification of genes with niche-specific roles in a key target species will inform the design of control strategies for bovine salmonellosis and zoonotic infections, for which efficacious and cross-protective vaccines are currently lacking.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5319-0) contains supplementary material, which is available to authorized users.

Neutral barcoding of genomes reveals the dynamics of Salmonella colonization in cattle and their peripheral lymph nodes

Feedlot cattle often contain Salmonella. The number of bacteria that initiate colonization of different cattle organs and the bacterial migration within these large animals are poorly understood. To investigate these questions, we constructed wild-type isogenic tagged strains (WITS) of Salmonella by inserting 21-base barcodes flanked by Illumina sequencing primers into a neutral genome location. We then delivered several different pools of uniquely barcoded clones orally and into multiple intradermal sites, in individual Holstein steers, and subsequently performed Salmonella-directed sequence tag-based analysis of microbial populations (STAMP). Using high-throughput sequencing of the barcodes of Salmonella grown from steer lymph nodes, organs and feces, we monitored how individual barcoded clones travel from different entry sites within animals. Data showed that gastrointestinal colonization was established by up to hundreds of Salmonella founder cells, whereas peripheral lymph nodes were usually colonized by very low numbers of founding bacteria, often originating from the nearest draining intradermal delivery site. Transmission of Salmonella from the gastrointestinal tract to the lymphatic system was frequently observed, whereas entry of intradermally delivered bacteria into the gut was rare. Bacteria undergo limited extraintestinal proliferation within or prior to arrival at peripheral lymph nodes. Overall, the application of the STAMP technique facilitated characterization of the migration routes and founder population size of Salmonella within feedlot cattle and their organs and lymph nodes in unprecedented detail.

Collection and Processing of Lymph Nodes from Large Animals for RNA Analysis: Preparing for Lymph Node Transcriptomic Studies of Large Animal Species

Large animals (both livestock and wildlife) serve as important reservoirs of zoonotic pathogens, including Brucella, Mycobacterium bovis, Salmonella, and E. coli, and are useful for the study of pathogenesis and/or spread of the bacteria in natural hosts. With the key function of lymph nodes in the host immune response, lymph node tissues serve as a potential source of RNA for downstream transcriptomic analyses, in order to assess the temporal changes in gene expression in cells over the course of an infection. This article presents an overview of the process of lymph node collection, tissue sampling, and downstream RNA processing in livestock, using cattle (Bos taurus) as a model, with additional examples provided from the American bison (Bison bison). The protocol includes information about the location, identification, and removal of lymph nodes from multiple key sites in the body. Additionally, a biopsy sampling methodology is presented that allows for a consistency of sampling across multiple animals. Several considerations for sample preservation are discussed, including the generation of RNA suitable for downstream methodologies like RNA-sequencing and RT-PCR. Due to the long delays inherent in large animal vs. mouse time course studies, representative results from bison and bovine lymph node tissues are presented to describe the time course of the degradation in this tissue type, in the context of a review of previous methodological work on RNA degradation in other tissues. Overall, this protocol will be useful to both veterinary researchers beginning transcriptome projects on large animal samples and to molecular biologists interested in learning techniques for in vivo tissue sampling and in vitro processing.

A multiplex real-time PCR assay, based on invA and pagC genes, for the detection and quantification of Salmonella enterica from cattle lymph nodes

Cattle lymph nodes can harbor Salmonella and potentially contaminate beef products. We have developed and validated a new real-time PCR (qPCR) assay for the detection and quantification of Salmonella enterica in cattle lymph nodes. The assay targets both the invA and pagC genes, the most conserved molecular targets in Salmonella enterica. An 18S rRNA gene assay that amplifies from cattle and other animal species was also included as an internal control. Available DNA sequences for invA, pagC and 18S rRNA genes were used for primer and probe selections. Three Salmonella serotypes, S. Typhimurium, S. Anatum, and S. Montevideo, were used to assess the assay's analytical sensitivity. Correlation coefficients of standard curves generated for each target and for all three serotypes were >99% and qPCR amplification efficiencies were between 93% and 110%. Assay sensitivity was also determined using standard curve data generated from Salmonella-negative cattle lymph nodes spiked with 10-fold dilutions of the three Salmonella serotypes. Assay specificity was determined using Salmonella culture method, and qPCR testing on 36 Salmonella strains representing 33 serotypes, 38 Salmonella strains of unknown serotypes, 252 E. coli strains representing 40 serogroups, and 31 other bacterial strains representing 18 different species. A collection of 647 cattle lymph node samples from steers procured from the Midwest region of the US were tested by the qPCR, and compared to culture-method of detection. Salmonella prevalence by qPCR for pre-enriched and enriched lymph nodes was 19.8% (128/647) and 94.9% (614/647), respectively. A majority of qPCR positive pre-enriched samples (105/128) were at concentrations between 10⁴ and 10⁵ CFU/mL. Culture method detected Salmonella in 7.7% (50/647) and 80.7% (522/647) of pre- and post-enriched samples, respectively; 96.0% (48/50) of pre-enriched and 99.4% (519/522) of post-enriched culture-positive samples were also positive by qPCR. More samples tested positive by qPCR than by culture method, indicating that the real-time PCR assay was more sensitive. Our data indicate that this triplex qPCR can be used to accurately detect and quantify Salmonella enterica strains from cattle lymph node samples. The assay may serve as a useful tool to monitor the prevalence of Salmonella in beef production systems.

Comparison of Salmonella Prevalence Rates in Bovine Lymph Nodes across Feeding Stages

Peripheral lymph nodes (LNs) located in the fatty tissues of beef carcasses have been shown to harbor Salmonella and, thus, potentially contaminate ground beef. Salmonella prevalence within LNs is known to differ among feedlots. Two South Texas feeding operations (identified as locations A and B) known to harbor salmonellae in the feedlot environment, while historically producing cattle with opposing rates (one "high" and one "low") of Salmonella prevalence in LNs, were used in this study. To determine whether this difference was due to cattle source or factors associated with different stages of feeding, weanling steers of common and known origin were followed through normal feeding stages at both operations. Eighty Angus-sired beef steers were harvested at each of four feeding stages: 1, postweaning; 2, background or stocker; 3, 60 days on feed; and 4, 120 days on feed. Left and right subiliac and superficial cervical LNs ( n = 304) were collected from each carcass, and similar node types were pooled by animal ( n = 152). Results showed a difference ( P < 0.05) in prevalence of Salmonella in bovine lymph nodes between location A and location B and among feeding stages in location B. Salmonella was not isolated from any feeding stage 1 (postweaning) or location A LN samples. Within location B, there was an increase in Salmonella prevalence as cattle moved into later stages of feeding: at 22.2% (4 of 18), 77.8% (14 of 18), and 94.4% (17 of 18) for feeding stages 2, 3, and 4, respectively. Although the reasons for the differences seen between feeding operations and for increased Salmonella prevalence in LNs at later feeding stages remain unexplained, these results indicate that factors other than cattle source are likely influencing Salmonella prevalence in LNs.

Quantifying the Survival of Multiple Salmonella enterica Serovars In Vivo via Massively Parallel Whole-Genome Sequencing To Predict Zoonotic Risk

Salmonella enterica is an animal and zoonotic pathogen of worldwide importance. Salmonella serovars that differ in their host and tissue tropisms exist. Cattle are an important reservoir of human nontyphoidal salmonellosis, and contaminated bovine peripheral lymph nodes enter the food chain via ground beef. The relative abilities of different serovars to survive within the bovine lymphatic system are poorly understood and constrain the development of control strategies. This problem was addressed by developing a massively parallel whole-genome sequencing method to study mixed-serovar infections in vivo. Salmonella serovars differ genetically by naturally occurring single nucleotide polymorphisms (SNPs) in certain genes. It was hypothesized that these SNPs could be used as markers to simultaneously identify serovars in mixed populations and quantify the abundance of each member in a population. The performance of the method was validated in vitro using simulated pools containing up to 11 serovars in various proportions. It was then applied to study serovar survival in vivo in cattle challenged orally with the same 11 serovars. All the serovars successfully colonized the bovine lymphatic system, including the peripheral lymph nodes, and thus pose similar risks of zoonosis. This method enables the fates of multiple genetically unmodified strains to be evaluated simultaneously in a single animal. It could be useful in reducing the number of animals required to study mixed-strain infections and in testing the cross-protective efficacy of vaccines and treatments. It also has the potential to be applied to diverse bacterial species which possess shared but polymorphic alleles.

IMPORTANCE While some Salmonella serovars are more frequently isolated from lymph nodes rather than the feces and environment of cattle, the relative abilities of serovars to survive within the lymphatic system of cattle remain ill defined. A sequencing-based method which used available information from sequenced Salmonella genomes to study the dynamics of mixed-serovar infections in vivo was developed. The main advantages of the method include the simultaneous identification and quantification of multiple strains without any genetic modification and minimal animal use. This approach could be used in vaccination trials or in epidemiological surveys where an understanding of the dynamics of closely related strains of a pathogen in mixed populations could inform the prediction of zoonotic risk and the development of intervention strategies.

Low Prevalence of Salmonella and Shiga Toxin-Producing Escherichia coli in Lymph Nodes of Australian Beef Cattle

Salmonella contamination of ground beef has been viewed as originating from the surface of carcasses. Recent studies have identified lymph nodes as a potential source of Salmonella contamination because these tissues play an active role in containment of pathogens in the live animal and because some lymph nodes are unavoidably present in manufacturing beef trimmings or primal cuts that may be incorporated into ground beef. A survey was conducted of the microbiological status of lymph nodes from Australian cattle at the time of slaughter to determine the prevalence of microbiological contamination. Sets of lymph nodes (n = 197), consisting of the superficial cervical (prescapular), prepectoral, axillary, presternal, popliteal, ischiatic, subiliac (precrural), coxalis, and iliofemoralis (deep inguinal), were collected from five geographically separated Australian abattoirs over a period of 14 months. Samples were tested for the presence of Salmonella spp. and Shiga toxin-producing Escherichia coli by BAX PCR assay. Aerobic plate count, E. coli, and coliforms were enumerated with a lower limit of detection of 80 CFU per node. The observed prevalence of Salmonella within peripheral lymph nodes was 0.48% (7 of 1,464). Two of the seven lymph nodes in which Salmonella organisms were detected came from the same animal. Grass-fed, grain-fed, and cull dairy cattle were all found to have detectable Salmonella in lymph nodes. All Salmonella detections occurred during cooler months of the year. No Shiga toxin-producing E. coli were detected. Aerobic microorganisms were detected above the limit of quantification in 3.2% of nodes (median count 2.24 log per node), and E. coli was detected in 0.8% of nodes (median count 3.05 log per node). The low prevalence of Salmonella and low concentration of aerobic microorganisms in Salmonella-positive lymph nodes of Australian cattle at the time of slaughter suggest that the likelihood of lymph nodes contributing significantly to the presence of Salmonella in ground beef is low.

Phage Therapy Approaches to Reducing Pathogen Persistence and Transmission in Animal Production Environments: Opportunities and Challenges

The era of genomics has allowed for characterization of phages for use as antimicrobials to treat animal infections with a level of precision never before realized. As more research in phage therapy has been conducted, several advantages of phage therapy have been realized, including the ubiquitous nature, specificity, prevalence in the biosphere, and low inherent toxicity of phages, which makes them a safe and sustainable technology for control of animal diseases. These unique qualities of phages have led to several opportunities with respect to emerging trends in infectious disease treatment. However, the opportunities are tempered by several challenges to the successful implementation of phage therapy, such as the fact that an individual phage can only infect one or a few bacterial strains, meaning that large numbers of different phages will likely be needed to treat infections caused by multiple species of bacteria. In addition, phages are only effective if enough of them can reach the site of bacterial colonization, but clearance by the immune system upon introduction to the animal is a reality that must be overcome. Finally, bacterial resistance to the phages may develop, resulting in treatment failure. Even a successful phage infection and lysis of its host has consequences, because large amounts of endotoxin are released upon lysis of Gram-negative bacteria, which can lead to local and systemic complications. Overcoming these challenges will require careful design and development of phage cocktails, including comprehensive characterization of phage host range and assessment of immunological risks associated with phage treatment.

Salmonella in Peripheral Lymph Nodes of Healthy Cattle at Slaughter

To more fully characterize the burden of Salmonella enterica in bovine peripheral lymph nodes (PLN), PLN (n = 5,450) were collected from healthy cattle at slaughter in 12 commercial abattoirs that slaughtered feedlot-fattened (FF) cattle exclusively (n = 7), cattle removed (or culled) from breeding herds (n = 3), or both FF and cull cattle (n = 2). Qualitative and quantitative methods were used to estimate prevalence and concentration of Salmonella in PLN. Isolates were subjected to a variety of phenotypic, serological, and molecular assays. Overall, Salmonella prevalence in PLN from FF and cull cattle was 7.1 and 1.8%. However, burden varied by season in that observed prevalence in PLN collected in cooler or warmer seasons was 2.4 and 8.2%, respectively. Prevalence in PLN from cull cattle in the southwest region of the US was 2.1 and 1.1% for cool and warm seasons, respectively; however, prevalence in FF PLN was far greater in that it was 6.5 and 31.1%, respectively. Salmonella was recovered from 289 (5.6%) PLN and 2.9% (n = 160) of all PLN tested had quantifiable concentrations that varied from 1.6 to 4.9 log₁₀ colony forming units/PLN. The most common serotypes isolated from PLN were Montevideo (26.9%), Lille (14.9%), Cerro (13.0%), Anatum (12.8%), and Dublin (6.9%). In all, 376 unique isolates were collected from the 289 Salmonella-positive PLN. Antimicrobial susceptibility testing revealed the majority (80.6%) of these isolates were pansusceptible; however, 10.7% of isolates were found to be resistant to two or more antimicrobial classes. We were able to document an observed increased in prevalence of Salmonella in PLN during the warmer season, particularly in FF cattle from the southwest region of the US. The mechanisms underlying the observed association between season, region, and production source have yet to be elucidated. Nevertheless, these findings increase our understanding of the sources of contamination of beef products and shed light on transmission dynamics that may be useful in targeting these sources.

Rapid Detection and Classification of Salmonella enterica Shedding in Feedlot Cattle Utilizing the Roka Bioscience Atlas Salmonella Detection Assay for the Analysis of Rectoanal Mucosal Swabs

With an increasing focus on preharvest food safety, rapid methods are required for the detection and quantification of foodborne pathogens such as Salmonella enterica in beef cattle. We validated the Atlas Salmonella Detection Assay (SEN), a nucleic acid amplification technology that targets Salmonella rRNA, for the qualitative detection of S. enterica with sample enrichment using immunomagnetic separation as a reference test, and we further evaluated its accuracy to predict pathogen load using SEN signal-to-cutoff (SCO) values from unenriched samples to classify animals as high or nonhigh shedders. Rectoanal mucosal swabs (RAMS) were collected from 238 beef cattle from five cohorts located in the Midwest or southern High Plains of the United States between July 2015 and April 2016. Unenriched RAMS samples were used for the enumeration and SEN SCO analyses. Enriched samples were tested using SEN and immunomagnetic separation methods for the detection of Salmonella. The SEN method was 100% sensitive and specific for the detection of Salmonella from the enriched RAMS samples. A SEN SCO value of 8, with a sensitivity of 93.5% and specificity of 94.3%, was found to be an optimum cutoff value for classifying animals as high or nonhigh shedders from the unenriched RAMS samples. The SEN assay is a rapid and reliable method for the qualitative detection and categorization of the shedding load of Salmonella from RAMS in feedlot cattle.

Impact of Clinical Salmonellosis in Veal Calves on the Recovery of Salmonella in Lymph Nodes at Harvest

The objective of this study was to determine the prevalence, serotypes, antimicrobial resistance phenotypes, and pulsed-field gel electrophoresis (PFGE) patterns of Salmonella recovered in feces and mesenteric and prefemoral lymph nodes (LNs) from cohorts of calves with and without a confirmed outbreak of salmonellosis. In a prospective cohort study, 160 calves from four farms without a reported outbreak (nonoutbreak farms) were sampled at farm and harvest. In addition, harvest samples from 80 calves of two farms with a confirmed outbreak (outbreak farms) were collected. A culture protocol for Salmonella isolation was applied for all samples and recovered isolates were further characterized by serotyping, antimicrobial susceptibility testing, and PFGE. Among nonoutbreak farms, Salmonella was recovered from 0% (0/160) farm fecal samples, 3.7% (6/160) harvest fecal swabs, 21.9% (35/160) mesenteric LNs, and 0.6% (1/160) prefemoral LNs. Serotypes identified in nonoutbreak herds included Salmonella Typhimurium, Cerro, Hartford, and Newport. Most isolates (64.3%, 27/42) exhibited a unique multidrug-resistant (MDR) phenotype, including resistance to extended-spectrum cephalosporins. Salmonella prevalence in harvest fecal samples and prefemoral LNs among calves from outbreak farms was numerically higher, but not significantly different than those without an outbreak. Serotypes recovered from outbreak farms included Salmonella Heidelberg and Typhimurium, and the monophasic Salmonella Typhimurium strains 4,5,12:i:- and 4,12:i:-, which have been also reported as highly pathogenic in humans. All isolates (33/33) exhibited an MDR phenotype. Salmonella strains recovered from ill calves in two outbreaks had indistinguishable PFGE patterns, suggesting between-farm transmission. In addition, the genotype of Salmonella Heidelberg causing an outbreak among calves was recovered from three prefemoral LNs of surviving members of the cohort at harvest. Implementation of preharvest biosecurity measures (limited personnel and visitor traffic, vehicle, footwear, and utensils disinfection) should be highly recommended to decrease the prevalence of Salmonella on farms and safeguard the food safety.

Reducing Foodborne Pathogen Persistence and Transmission in Animal Production Environments: Challenges and Opportunities

Preharvest strategies to reduce zoonotic pathogens in food animals are important components of the farm-to-table food safety continuum. The problem is complex; there are multiple pathogens of concern, multiple animal species under different production and management systems, and a variety of sources of pathogens, including other livestock and domestic animals, wild animals and birds, insects, water, and feed. Preharvest food safety research has identified a number of intervention strategies, including probiotics, direct-fed microbials, competitive exclusion cultures, vaccines, and bacteriophages, in addition to factors that can impact pathogens on-farm, such as seasonality, production systems, diet, and dietary additives. Moreover, this work has revealed both challenges and opportunities for reducing pathogens in food animals. Animals that shed high levels of pathogens and predominant pathogen strains that exhibit long-term persistence appear to play significant roles in maintaining the prevalence of pathogens in animals and their production environment. Continued investigation and advancements in sequencing and other technologies are expected to reveal the mechanisms that result in super-shedding and persistence, in addition to increasing the prospects for selection of pathogen-resistant food animals and understanding of the microbial ecology of the gastrointestinal tract with regard to zoonotic pathogen colonization. It is likely that this continued research will reveal other challenges, which may further indicate potential targets or critical control points for pathogen reduction in livestock. Additional benefits of the preharvest reduction of pathogens in food animals are the reduction of produce, water, and environmental contamination, and thereby lower risk for human illnesses linked to these sources.

Estimating correlation of prevalence at two locations in the farm-to-table continuum using qualitative test data

The presence or absence of contaminants in food samples changes as a commodity moves along the farm-to-table continuum. Interest lies in the degree to which the prevalence (i.e., infected animals or contaminated sample units) at one location in the continuum, as measured by the proportion of test-positive samples, is correlated with the prevalence at a location later in the continuum. If prevalence of a contaminant at one location in the continuum is strongly correlated with the prevalence of the contaminant later in the continuum, then the effect of changes in contamination on overall food safety can be better understood. Pearson's correlation coefficient is one of the simplest metrics of association between two measurements of prevalence but it is biased when data consisting of presence/absence testing results are used to directly estimate the correlation. This study demonstrates the potential magnitude of this bias and explores the utility of three methods for unbiased estimation of the degree of correlation in prevalence. An example, based on testing broiler chicken carcasses for Salmonella at re-hang and post-chill, is used to demonstrate the methods.

Salmonella Presence in Mandibular, Mesenteric, and Subiliac Lymph Nodes Collected from Sheep and Goats in the United States

Even though often underappreciated in the United States, meat derived from goats and lambs is a valuable global commodity. Although extensive studies have been conducted examining pathogen prevalence in beef, pork, and poultry species, less research is available about pathogen presence in small ruminants. Understanding the presence of Salmonella in small ruminants can influence processing method improvements for these species to minimize food safety risks. The objective of this study was to establish the Salmonella presence in lymph nodes from sheep (n = 311) and goats (n =357) in the United States. Mandibular, mesenteric, and subiliac lymph nodes were collected from animals at abattoirs located in California, New Mexico, and Texas over a 14-month period. Lymph nodes were analyzed for Salmonella presence by using standard isolation methods, and presumptive isolates were confirmed with latex agglutination. Salmonella was detected in 3.15% of mandibular lymph nodes (n = 222), 5.83% of mesenteric lymph nodes (n = 223), and 7.62% of subiliac lymph nodes (n = 223). A total of 30 animals (13.39%) were found to have one or more Salmonella -positive lymph nodes. Our data indicate Salmonella is harbored within the lymph nodes of small ruminants. With this knowledge, further work can target strategies and interventions to minimize the risk associated with this pathogen during the processing of small ruminants.

The Growth Kinetics of Salmonella Enteritidis in Raw Ground Beef

The growth kinetics of Salmonella Enteritidis in raw beef has been little studied so far. Thus, this study aimed to clarify the growth kinetics of the pathogen in ground beef using a growth model. When Salmonella cells inoculated at various initial doses into ground beef were incubated at a given temperature (24℃), the maximum population (Nmax) of the microbe at the stationary phase varied with the doses. This relationship was expressed with a polynomialequation for Nmax using the initial dose. The combination of the growth model and the polynomial equation successfully predicted Salmonella growth at a given initial dose. When Salmonella cells inoculated in ground beef were incubated at various constant temperatures, the growth curves of the pathogen and natural microflora (NM) were well described with the growth model. The rate constant of growth and the Nmax values for Salmonella and NM were then analyzed kinetically. From these results, growth curves of Salmonella and NM in ground beef stored at dynamic temperatures were successfully predicted. Competition between Salmonella and NM in ground beef was also found during the storage. This study could give usable information on the growth of Salmonella and NM in ground beef at various temperatures.