Molecular ecology of Listeria monocytogenes: evidence for a reservoir in milking equipment on a dairy farm

Prevalence of Listeria monocytogenes, Salmonella spp., Shiga toxin-producing Escherichia coli, and Campylobacter spp. in raw milk in the United States between 2000 and 2019: A systematic review and meta-analysis

Raw (unpasteurized) milk is available for sale and direct human consumption within some states in the United States (US); it cannot be sold or distributed in interstate commerce. Raw milk may contain pathogenic microorganisms that, when consumed, may cause illness and sometimes may result in death. No comprehensive review for prevalence and levels of the major bacterial pathogens in raw milk in the US exists. The objective of the present research was to systematically review the scientific literature published from 2000 to 2019 to estimate the prevalence and levels of Listeria monocytogenes, Salmonella spp., Shiga toxin-producing Escherichia coli (STEC), and Campylobacter spp. in raw milk in the US. Peer-reviewed studies were retrieved systematically from PubMed®, Embase®, and Web of Science^TM. The unique complete nonduplicate references were uploaded into the Health Assessment Work Collaborative (HAWC). Based on the selection criteria, twenty studies were included in the systematic review and meta-analysis. Comprehensive Meta-Analysis (CMA) was used for statistical analyses, specifically, random effects meta-analyses were used to synthesize raw bulk tank milk (BTM) and milk filters (MF) data. Data from studies using culture and non-culture-based detection methods were included. Forest plots generated in CMA (Biostat, Englewood, NJ) were used to visualize the results. The average prevalence (event rate) of L. monocytogenes, Salmonella spp., STEC, and Campylobacter spp. in raw BTM in the US was estimated at 4.3% (95% confidence intervals [CIs], 2.8-6.5%), 3.6% (95% CIs, 2.0-6.2%), 4.3% (95% CIs, 2.4-7.4%), and 6.0% (95% CIs, 3.2-10.9%), respectively. Estimated prevalence was generally larger in MF than in BTM. There was not enough data to perform a meta-analysis for the prevalence or levels of pathogens in raw milk from retail establishments or other milk categories.

Biofilms in hoses utilized to divert colostrum and milk on dairy farms: A report exploring their potential role in herd health, milk quality, and public health

Biofilms in milking equipment on dairy farms have been associated with failures in cleaning and sanitizing protocols. These biofilms on milking equipment can be a source of contamination for bulk tank milk and a concern for animal and public health, as biofilms can become on-farm reservoirs for pathogenic bacteria that cause disease in cows and humans. This report describes a cross-sectional study on 3 dairy farms, where hoses used to divert waste milk, transition milk, and colostrum were analyzed by culture methods and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) to assess the presence of pathogenic bacteria such as Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella spp. In addition, the presence of biofilms was analyzed using scanning electron microscopy and confocal spectral microscopy. Biofilms composed of multispecies microbial communities were observed on the surfaces of all milk hoses. In two dairy farms, S. aureus, P. aeruginosa, Klebsiella pneumoniae, and Klebsiella oxytoca were isolated from the milk hose samples collected. Cleaning and sanitation protocols of all surfaces in contact with milk or colostrum are crucial. Hoses used to collect waste milk, colostrum, and transition milk can be a source of biofilms and hence pathogenic bacteria. Waste milk used to feed calves can constitute a biosecurity issue and a source of pathogens, therefore an increased exposure and threat for the whole herd health and, potentially, for human health.

WGS analysis of Listeria monocytogenes from rural, urban, and farm environments in Norway: Genetic diversity, persistence, and relation to clinical and food isolates

Listeria monocytogenes is a ubiquitous environmental bacterium associated with a wide variety of natural and human-made environments, such as soil, vegetation, livestock, food processing environments, and urban areas. It is also among the deadliest foodborne pathogens, and knowledge about its presence and diversity in potential sources is crucial to effectively track and control it in the food chain. Isolation of L. monocytogenes from various rural and urban environments showed higher prevalence in agricultural and urban developments than in forest or mountain areas, and that detection was positively associated with rainfall. Whole-genome sequencing (WGS) was performed for the collected isolates and for L. monocytogenes from Norwegian dairy farms and slugs (218 isolates in total). The data were compared to available data sets from clinical and food-associated sources in Norway collected within the last decade. Multiple examples of clusters of isolates with 0 to 8 whole-genome multilocus sequence typing (wgMLST) allelic differences were collected over time in the same location, demonstrating persistence of L. monocytogenes in natural, urban, and farm environments. Furthermore, several clusters with 6 to 20 wgMLST allelic differences containing isolates collected across different locations, times, and habitats were identified, including nine clusters harboring clinical isolates. The most ubiquitous clones found in soil and other natural and animal ecosystems (CC91, CC11, and CC37) were distinct from clones predominating among both clinical (CC7, CC121, and CC1) and food (CC9, CC121, CC7, and CC8) isolates. The analyses indicated that ST91 was more prevalent in Norway than other countries and revealed a high proportion of the hypovirulent ST121 among Norwegian clinical cases.

IMPORTANCEListeria monocytogenes is a deadly foodborne pathogen that is widespread in the environment. For effective management, both public health authorities and food producers need reliable tools for source tracking, surveillance, and risk assessment. For this, whole-genome sequencing (WGS) is regarded as the present and future gold standard. In the current study, we use WGS to show that L. monocytogenes can persist for months and years in natural, urban, and dairy farm environments. Notably, clusters of almost identical isolates, with genetic distances within the thresholds often suggested for defining an outbreak cluster, can be collected from geographically and temporally unrelated sources. The work highlights the need for a greater knowledge of the genetic relationships between clinical isolates and isolates of L. monocytogenes from a wide range of environments, including natural, urban, agricultural, livestock, food production, and food processing environments, to correctly interpret and use results from WGS analyses.

Ruminant-associated Listeria monocytogenes isolates belong preferentially to dairy-associated hypervirulent clones: a longitudinal study in 19 farms

Studies have shown that ruminants constitute reservoirs of Listeria monocytogenes, but little is known about the epidemiology and genetic diversity of this pathogen within farms. Here we conducted a large-scale longitudinal study to monitor Listeria spp. in 19 dairy farms during three consecutive seasons (N = 3251 samples). L. innocua was the most prevalent species, followed by L. monocytogenes. Listeria monocytogenes was detected in 52.6% of farms and more frequently in cattle (4.1%) and sheep (4.5%) than in goat farms (0.2%). Lineage I accounted for 69% of L. monocytogenes isolates. Among animal samples, the most prevalent sublineages (SL) and clonal complexes (CC) were SL1/CC1, SL219/CC4, SL26/CC26 and SL87/CC87, whereas SL666/CC666 was most prevalent in environmental samples. Sixty-one different L. monocytogenes cgMLST types were found, 28% common to different animals and/or surfaces within the same farm and 21% previously reported elsewhere in the context of food and human surveillance. Listeria monocytogenes prevalence was not affected by farm hygiene but by season: higher prevalence was observed during winter in cattle, and during winter and spring in sheep farms. Cows in their second lactation had a higher probability of L. monocytogenes faecal shedding. This study highlights dairy farms as a reservoir for hypervirulent L. monocytogenes.

Molecular diversity of Staphylococcus aureus and the role of milking equipment adherences or biofilm as a source for bulk tank milk contamination

Staphylococcus aureus is one of the most frequent pathogens causing intramammary infections in dairy herds. Consequently, virulence factors, pathobiology, and epidemiology of Staphylococcus aureus strains have been widely assessed through the years. Nevertheless, not much has been described about the epidemiology of Staph. aureus strains from bulk tank milk (BTM) and adherences on milking equipment (AMES), even when these strains may play a role in the quality of milk that is intended for human consumption. The objective of this study was to assess the strain diversity of 166 Staph. aureus isolates collected from 3 consecutive BTM samples, and from AMES in contact with milk from 23 Chilean dairy farms. Isolates were analyzed and typed using pulsed-field gel electrophoresis. Diversity of strains, both within and among farms, was assessed using Simpson's index of diversity (SID). On farms where Staph. aureus was isolated from both AMES and BTM (n = 8), pulsotypes were further analyzed to evaluate the role of AMES as a potential source of Staph. aureus strains in BTM. Among all Staph. aureus analyzed by pulsed-field gel electrophoresis, a total of 42 pulsotypes (19 main pulsotypes and 23 subtypes) were identified. Among dairy farms, strain diversity was highly heterogeneous (SID = 0.99). Within dairy farms, Staph. aureus strain diversity was variable (SID = 0 to 1), and 18 dairy operations (81.8%) had one pulsotype that was shared between at least 2 successive BTM samples. In those farms where Staph. aureus was isolated in both AMES and BTM (n = 8), 7 (87.5%) showed a clonal distribution of Staph. aureus strains between these 2 types of samples. The overlapping of certain Staph. aureus strains among dairy farms may point out common sources of Staph. aureus among otherwise epidemiologically unrelated farms. Indistinguishable Staph. aureus strains between AMES and BTM across dairy farms suggest that Staph. aureus-containing AMES may represent a source for BTM contamination, thus affecting milk quality. Our study highlights the role of viable Staph. aureus in AMES as a source for BTM contamination on dairy farms, and also describes the overlapping and presence of specific BTM and AMES pulsotypes among farms.

LisRK is required for optimal fitness of Listeria monocytogenes in soil

Listeria monocytogenes is a food-borne pathogen responsible for the disease listeriosis. It is ubiquitously found in the environment and soil is one of its natural habitats. Listeria monocytogenes is highly capable of coping with various stressful conditions. We hypothesized that stress-responsive two-component systems such as LisRK might contribute to the adaptation of L. monocytogenes to the soil environment. Indeed, investigations of the population dynamics of wild-type and mutant strains suggest an important role of LisRK for optimal fitness of L. monocytogenes in sterile soil. Results from non-sterile soil showed that the parental strain was capable of surviving longer than mutant strains lacking lisRK or genes encoding the LisRK-regulated LhrC small RNAs (sRNAs), suggesting that LisRK as well as the LhrC sRNAs were important for survival. Transcription of five LisRK-regulated genes was assessed after 1 h incubation in sterile soil. We observed that LisRK and the LhrC sRNAs contribute to the upregulation of lmo2522 in the soil environment. Notably, lmo2522 encodes an equivalent of the resuscitation promoting factors, Rpfs, in actinobacteria. Collectively, our study demonstrates that LisRK is important for growth and survival in sterile and non-sterile soil and suggests a role for LisRK-regulation of Lmo2522 in resuscitation from dormancy in the soil environment.

Occurrence of Listeria spp. and Listeria monocytogenes Isolated from PDO Taleggio Production Plants

The present study evaluated the presence of Listeria spp. and L. monocytogenes in four plants producing PDO Taleggio cheese. A total of 360 environmental samples were collected from different areas during production. The sampling points were identified as Food Contact Surfaces (FCS), transfer-Non Food Contact Surfaces (tr-NFCS), and non-transfer-NFCS (non-tr-NFCS). Fifty-nine ingredients/products were also analyzed. Listeria spp. was found in all the plants with a mean prevalence of 23.1%; plants that included a ripening area showed significantly higher prevalence if compared to the other plants. The positivity rate detected on FCS was moderate (~12%), but significantly lower if compared to NFCS (about 1/4 of the samples, p < 0.01). Among the FCS, higher prevalence was revealed on ripening equipment. Listeria spp. was never detected in the ingredients or products. A total of 125 Listeria spp. isolates were identified, mostly as L. innocua (almost 80%). L. monocytogenes was detected only from two FCS samples, in an area dedicated to the cutting of ripened blue cheeses; strain characterization by whole genome sequencing (WGS) evidenced a low virulence of the isolates. The results of the present study stress the importance of Listeria spp. management in the dairy plants producing PDO Taleggio and similar cheeses, mainly by the application of strict hygienic practices.

Enhanced Antibacterial Activity of Silver Nanoparticles Combined with Hydrogen Peroxide Against Multidrug-Resistant Pathogens Isolated from Dairy Farms and Beef Slaughterhouses in Egypt

Purpose

The last few decades have witnessed a rapid and global increase in multidrug-resistant bacteria (MDR) emergence.

Methods

The aim of the current study is to isolate the most common MDR bacteria from dairy farms and beef slaughterhouses followed by evaluation of their antimicrobial resistance pattern and assessment of the antibacterial activity of AgNPs-H₂O₂ as an alternative to conventional antibiotics. In this regard, 200 samples were collected from two dairy farms and one beef slaughterhouse located in Dakhliya Governorate, Egypt.

Results

Interestingly, out of 120 collected samples from dairy farms, the prevalence of the isolated strains was 26.7, 23.3, 21.7, 16.7, and 11.7% for S. typhimurium, E. coli O157:H7, L. monocytogenes, K. pneumoniae and P. aeruginosa, respectively. Meanwhile, the overall prevalence was 30, 25, 22.5, 17.5, and 5% for E. coli O157:H7, L. monocytogenes, S. typhimurium, P. aeruginosa, and K. pneumoniae, respectively, for the 80 samples collected from a beef slaughterhouse. The antimicrobial susceptibility pattern elucidated that all isolated strains exhibited resistance to at least four of the tested antimicrobials, with multiple-antibiotic resistance index values (MAR) ranging between 0.44 and 0.88. Furthermore, the commercial AgNPs-H₂O₂ product was characterized by transmission electron microscopy (TEM) and zeta potential that showed spherical particles with a surface charge of -0.192 mV. The antimicrobial activity of synergized nano-silver (AgNP) with H₂O₂ product toward MDR strains was assessed via measuring minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and time-kill curve.

Conclusion

The present data report high prevalence rates of MDR pathogens in dairy farms and abattoirs. More importantly, AgNPs-H₂O₂ exerted broad-spectrum bactericidal activity toward MDR bacterial strains, suggesting their promising usage as safe, ecofriendly, cost-effective antibacterial agents. To our knowledge, this study is a pioneer in investigating the potential alternative antimicrobial role of silver nanoparticles for control of multiple drug-resistant pathogens in Egypt.

On-Farm Surfaces in Contact with Milk: The Role of Staphylococcus aureus-Containing Biofilms for Udder Health and Milk Quality

Staphylococcus aureus is a Gram-positive bacterium that causes intramammary infections and bulk tank milk (BTM) contamination in dairy operations around the world in spite of on-farm application of preventive measures. The study was conducted on a 30-cow dairy farm in the Ñuble Region of Chile. For BTM culture and somatic cell count (SCC) analysis, three consecutive BTM samples were collected. Samples for bacterial culture (n = 16) were collected from macroscopic adherence on previously washed, sanitized, and dry milking equipment surfaces in direct contact with milk during milking or cooling. A total of 48 S. aureus isolates from BTM, milking equipment, and cows' quarters with intramammary infections were analyzed by pulsed-field gel electrophoresis (PFGE). Selected milking equipment pieces were removed for biofilm visualization using scanning electron microscopy (SEM). S. aureus was isolated from all three BTM samples; the average SCC for the three BTM samples was 1,429,333 cells/mL. Fourteen of the 16 samples of milking equipment (87.5%) were culture positive for S. aureus. Biofilms were visualized by SEM in all four removed milking equipment pieces. Microorganisms observed by SEM in those biofilms were mainly coccus-shaped bacteria, and microbiological culture of these biofilms yielded viable S. aureus isolates in all samples. All pulsotypes observed among S. aureus isolates from BTM were indistinguishable from those in milking equipment surfaces. All PFGE pulsotypes observed among S. aureus isolates from biofilms on rubber liners were indistinguishable from isolates from intramammary infections in cows. Our findings suggest that milking equipment films may act as source of S. aureus contamination for BTM and cows during milking, thus compromising the microbiological quality of milk used for manufacturing dairy products.

Bacterial community composition of biofilms in milking machines of two dairy farms assessed by a combination of culture-dependent and -independent methods

Dairy biofilms as a source of contamination of milk and its products are of great concern in the dairy industry. For a reliable risk assessment, knowledge about the microbial community composition of biofilms in the milking systems of dairy farms must be improved. In this work, swab samples of milking machine biofilms of two dairy farms were investigated by a combination of culture-dependent and -independent methods. Spots in the milking system with enhanced microbial colonization were identified by quantification on selective and non-selective media. In addition, stainless steel coupons were placed into the piping system of a milking machine, removed after several milking intervals, and investigated for colonization by cultivation and culture-independently. Isolates were differentiated and identified by a combination of chemotaxonomical methods and 16S rRNA sequencing. The culture-independent approach involved treatment of the samples with the viability dye propidium monoazide prior to direct DNA-extraction by enzymatic cell lysis and cloning to exclude bias from dead biomass. The milking equipment retainers and the outlet of the milk bulk tank were identified as highly colonized spots on both farms. A high bacterial diversity was detected covering the phyla Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. Presence of biofilms was demonstrated on several materials including stainless steel and plastic, which are frequently used in milking machines, but also in dairy processing plants. Growth of mainly Gram-positive bacteria with high percentages of the phylum Actinobacteria was detected on the stainless steel coupons after exposition in the milking system for two to three days. Knowledge about the heterogenic microbial load on different parts of the milking machines and the stainless steel coupons will help to identify primary colonizers of the milking system, to assess the risk potential of biofilms for raw milk, to improve sanitation processes and to identify parts of the milking machine, which should be improved by hygienic design.

Source tracking on a dairy farm reveals a high occurrence of subclinical mastitis due to hypervirulent Listeria monocytogenes clonal complexes

AIMS

An extensive source investigation was conducted on a dairy farm with neurolisteriosis and subclinical mastitis cases to identify infection source and potential transmission routes of Listeria monocytogenes.

METHODS AND RESULTS

A total of 36 L. monocytogenes isolates were obtained from animal clinical cases (neurolisteriosis and udder infection) and the farm environment (silage, faeces, water). Isolates were typed using pulsed-field gel electrophoresis (PFGE) and whole-genome sequencing (WGS). Their virulence potential was assessed using the gentamicin protection assay and WGS-based identification of virulence genes. PFGE and WGS revealed a high genetic diversity of L. monocytogenes. An epidemiological link was confirmed for isolates from (i) several subclinical mastitis cases, (ii) silage and subclinical mastitis cases and (iii) different water sources. The neurolisteriosis isolate belonged to clonal complex (CC) 1, but infection source was not identified. A high occurrence (9/47 cows; 19·1%) of subclinical mastitis was observed with isolates belonging to CC2, CC4 and CC11.

CONCLUSIONS

The dairy farm environment was contaminated with diverse L. monocytogenes strains, including genotypes associated with human disease. Several isolates harboured genetic determinants associated with increased infectious potential in humans.

SIGNIFICANCE AND IMPACT OF THE STUDY

Results suggest that subclinical listerial mastitis should not be neglected as a potential source of milk contamination. The presence of hypervirulent CCs in subclinical mastitis cases calls for the implementation of improved mastitis detection.

Listeria monocytogenes in Brazilian foods: occurrence, risks to human health and their prevention

Listeria monocytogenes is a foodborne pathogen which occurs mainly in ready-to-eat food products, especially in artisanal products manufactured from raw milk such as some types of cheese, meat products and leafy vegetables. L. monocytogenes requires special attention in the food industry because of its ability to survive under adverse conditions and form biofilms on different surfaces in food processing environments. The potential for product contamination by L. monocytogenes strains in the industrial environment emphasizes the importance of preventive measures in the food industry. This review presents an overview on the main characteristics, pathogenicity and occurrence data of L. monocytogenes in Brazilian foods. The main prevention measures to avoid contamination by L. monocytogenes in foods are also highlighted, especially the adoption of quality assurance programs by the food industry.

Listeria monocytogenes in Milk: Occurrence and Recent Advances in Methods for Inactivation

Milk is one of the most important food items consumed by humans worldwide. In addition to its nutritional importance, milk is an excellent culture medium for microorganisms, which may include pathogens such as Listeria monocytogenes (L. monocytogenes). Traditional processing of milk for direct consumption is based on thermal treatments that efficiently eliminate pathogens, including pasteurization or sterilization. However, the occurrence of L. monocytogenes in milk as a consequence of failures in the pasteurization process or postpasteurization contamination is still a matter of concern. In recent years, consumer demand for minimally processed milk has increased due to the perception of better sensory and nutritional qualities of the products. This review deals with the occurrence of L. monocytogenes in milk in the last 10 years, including regulatory aspects, and recent advances in technologies for the inactivation of this pathogen in milk. The results from studies on nonthermal technologies, such as high hydrostatic pressure, pulsed electric fields, ultrasounds, and ultraviolet irradiation, are discussed, considering their potential application in milk processing plants.

Detection of Listeria spp. in cattle and environment of pasture-based dairy farms

ABSTRACT: The aim of the study was to detect Listeria spp., particularly Listeria monocytogenes, in cattle and environment of pasture based dairy farms in Paysandú, Uruguay. A two-stage sampling was conducted, 10 farms were selected by probability proportional to size. A single visit was made to each farm. Samples from bovine faeces, feedstuffs, bulk tank milk, drinking water and soil from the entry and exit pens of the milking parlour were collected for bacteriological studies. PCR assays were used to confirm species and determine the serotype profile of L. monocytogenes isolates. AscI-pulsed-field gel electrophoresis was done to genetically compare them. Listeria spp. were isolated from eight of ten dairy farms, whereas L. monocytogenes in three of them. Serotype distribution in L. monocytogenes was as follows: 1/2a, three isolates; 4b, one isolate. L. monocytogenes or L. innocua excreted from clinically healthy milking cows was detected via faeces. In feedstuffs, only one L. monocytogenes 1/2a isolate from a pasture was obtained. The strain was identical by PFGE to an isolate 1/2a obtained from a pool of milking cow feces that grazed on this farm. No isolation of Listeria spp. was retrieved from the bulk tank milk or drinking water from any of the farms. Listeria innocua was detected in 13 feedstuffs and seven samples of soil from the entry and exit pens of the milking parlour. This is a first local study that confirms the presence of Listeria spp. including L. monocytogenes in healthy cattle and environment of pasture-based dairy farms. These results suggest the potential role that healthy cattle and their sub-products would play as a source of these agents for humans and/or others animals. More detailed studies that include genetic comparison of human and animal isolates are required in order to clearly establish the epidemiological relationship.

Occurence of antibiotic resistant bacteria in raw cow milk from vending machines

The prevalence of antibiotic resistant coliform bacteria and enterococci in fresh raw cow milk obtained from four vending machines in Slovakia was assessed. Also, differences in the number of antibiotic-resistant bacteria in milk according to season (winter and summer) were compared. Number of total coliform bacteria ranged from 2.45 to 4.18 log CFU/mL. Majority of them were resistant to ampicillin. This was expected due to their intrinsic resistance apart from Escherichia coli which is sensitive to ampicillin. In addition, we observed also tetracycline and gentamicin resistance. Each of our samples contained E.coli. Number of total enterococci ranged from 1.95 to 3.78 log CFU/mL. We have observed predominantly vancomycin resistance in all tested samples. In samples taken during winter we have found higher number of total and resistant coliforms as well as total enterococci compared to those taken during summer.

Foodborne pathogens in unpasteurized milk in Sweden

Raw milk may be a risk for public health if it is contaminated with zoonotic pathogens. To study the prevalence in unpasteurized milk from Swedish farms, bovine and small ruminant dairy farms were sampled. Since the sampling method and transport conditions may influence the outcome of analyses, efforts were made to optimize the methodology. Culturing of bacteria was done from in-line milk filters collected from the milk pipe at the point where it enters the milk bulk tank at the farms and this way of sampling was compared to sampling bulk tank milk (BTM) directly. Analysing milk filters were found to be superior to analysing BTM directly. Conditions for transport of milk filter samples were further improved by the addition of Cary Blair transport medium, which significantly increased the number of positive samples for pathogenic bacteria. The isolation of several foodborne pathogens from milk filters was demonstrated. The prevalence of samples with Staphylococcus aureus was 71% and 64%, and Listeria spp. 21% and 29% from dairy cow and goat/sheep farms, respectively. Campylobacter jejuni, Yersinia enterocolitica and verotoxigenic Escherichia coli (VTEC) O157 were detected in 9%, 2% and 2% of samples from bovine milk, respectively. We conclude that the choice of sampling method and sample handling influence the results of bacterial culturing. From the results of this study, we strongly recommend to sample in-line milk filters instead of BTM directly and to use Cary Blair medium during transport, especially if the samples are to be analysed for Campylobacter spp. and/or Listeria spp. The findings also show that unpasteurized milk from Swedish farms occasionally contain bacteria with zoonotic potential.

Genetic diversity and virulence profiles of Listeria monocytogenes recovered from bulk tank milk, milk filters, and milking equipment from dairies in the United States (2002 to 2014)

Unpasteurized dairy products are known to occasionally harbor Listeria monocytogenes and have been implicated in recent listeriosis outbreaks and numerous sporadic cases of listeriosis. However, the diversity and virulence profiles of L. monocytogenes isolates recovered from these products have not been fully described. Here we report a genomic analysis of 121 L. monocytogenes isolates recovered from milk, milk filters, and milking equipment collected from bovine dairy farms in 19 states over a 12-year period. In a multi-virulence-locus sequence typing (MVLST) analysis, 59 Virulence Types (VT) were identified, of which 25% were Epidemic Clones I, II, V, VI, VII, VIII, IX, or X, and 31 were novel VT. In a multi-locus sequence typing (MLST) analysis, 60 Sequence Types (ST) of 56 Clonal Complexes (CC) were identified. Within lineage I, CC5 and CC1 were among the most abundant, and within lineage II, CC7 and CC37 were the most abundant. Multiple CCs previously associated with central nervous system and maternal-neonatal infections were identified. A genomic analysis identified variable distribution of virulence markers, Listeria pathogenicity islands (LIPI) -1, -3, and -4, and stress survival island-1 (SSI-1). Of these, 14 virulence markers, including LIPI-3 and -4 were more frequently detected in one lineage (I or II) than the other. LIPI-3 and LIPI-4 were identified in 68% and 28% of lineage I CCs, respectively. Results of this analysis indicate that there is a high level of genetic diversity among the L. monocytogenes present in bulk tank milk in the United States with some strains being more frequently detected than others, and some being similar to those that have been isolated from previous non-dairy related outbreaks. Results of this study also demonstrate significant number of strains isolated from dairy farms encode virulence markers associated with severe human disease.

Occurrence, Persistence, and Contamination Routes of Listeria monocytogenes Genotypes on Three Finnish Dairy Cattle Farms: a Longitudinal Study

The molecular epidemiology of Listeria monocytogenes was investigated in a longitudinal study of three Finnish dairy farms during 2013 to 2016. A total of 186 bulk tank milk (BTM), 224 milk filter sock (MFS), and 1,702 barn environment samples were analyzed, and isolates of L. monocytogenes were genotyped using pulsed-field gel electrophoresis. L. monocytogenes occurred throughout the year in all sample types, and the prevalence in MFS increased significantly during the indoor season. L. monocytogenes was more prevalent in MFS (29%) than in BTM (13%) samples. However, the prevalence of L. monocytogenes varied more between farms in samples of MFS (13 to 48%) than in BTM (10 to 16%). For each farm, the L. monocytogenes genotypes detected were classified by persistence (defined as persistent if isolated from ≥3 samples during ≥6 months) and predominance (defined as predominant if >5% prevalence on at least one farm visit). The prevalence of sporadic genotypes was 4 to 5% on all three farms. In contrast, the prevalence of persistent predominant genotypes varied between farms by 4% to 16%. The highest prevalence of persistent predominant genotypes was observed on the farm with the poorest production hygiene. Persistent predominant genotypes were most prevalent on feeding surfaces, water troughs, and floors. Genotypes isolated from the milking system or from cow udders had a greater relative risk of occurring in BTM and MFS than genotypes that only occurred elsewhere in the farm, supporting the hypothesis that L. monocytogenes is transmitted to milk from contamination on the udder surface or in the milking equipment.IMPORTANCEListeria monocytogenes is a ubiquitous environmental bacterium and the causative agent of a serious foodborne illness, listeriosis. Dairy products are common vehicles of listeriosis, and dairy cattle farms harbor L. monocytogenes genotypes associated with human listeriosis outbreaks. Indeed, dairy cattle farms act as a reservoir of L. monocytogenes, and the organism is frequently detected in bulk tank milk (BTM) and in the feces of clinically healthy cows. The ecology of L. monocytogenes in the farm environment is complex and poorly understood. Isolates of the same L. monocytogenes genotype can occur in the farm for years, but the factors contributing to the persistence of genotypes on dairy farms are unknown. Knowledge of the persistence patterns and contamination routes of L. monocytogenes on dairy farms can improve management of the contamination pressure in the farm environment and aid in the development of focused control strategies to reduce BTM contamination.

Occurrence and growth of Listeria monocytogenes in packaged raw milk

The increased availability of packaged raw drinking milk necessitates the investigation of the occurrence and growth of Listeria monocytogenes in raw milk during distribution and storage. The occurrence of L. monocytogenes in 105 retailed raw milk bottles, 115 bulk tank milk samples, 23 in-line milk filter socks and in 50 environmental samples collected from an on-farm dairy establishment were investigated. Growth of inoculated low-level L. monocytogenes contamination was also investigated in two types of raw milk packaging, namely in 1-litre plastic bottles and 3-litre bag-in-boxes, both stored at three different storage temperatures of 6, 8 and 10°C. The occurrence of L. monocytogenes was higher (4.8%) in bottled raw milk stored until the use-by-date of the package compared to fresh bulk tank milk (1.7%). L. monocytogenes counts were ≤13CFU/ml in bottled raw milk and ≤1CFU/ml in bulk tank milk. L. monocytogenes was not detected in the packaging facility, but occurred very frequently (39%) in the milk filter socks. Subtyping of L. monocytogenes isolates using pulsed-field gel-electrophoresis revealed seven pulsotypes, of which two occurred in multiple samples. Targeted inoculum levels of 1-2CFU/ml yielded L. monocytogenes counts≥100CFU/ml within seven days of storage in 22% of the raw milk packages stored at 6°C, and in all of the raw milk packages stored at 8°C. The frequent occurrence of L. monocytogenes in raw milk and the ability of a low-level L. monocytogenes contamination to grow at refrigeration temperatures highlight the importance of consumer education regarding the appropriate raw milk storage and handling.

Rhombencephalitis caused by Listeria monocytogenes in a pastured bull

A pastured 2-y-old cross-breed bull developed brainstem encephalitis (rhombencephalitis); Listeria monocytogenes was isolated from the brain. In the brainstem, there was perivascular cuffing, multiple microabscesses, and positive immunostaining for L. monocytogenes. Samples of bovine feces, water, feedstuffs, milking parlor soil, and bulk tank milk were collected from the dairy farm. Seven isolates of the genus Listeria were obtained, 6 of L. innocua and 1 of L. monocytogenes, which was found in the pasture where the bull grazed. Both isolates belonged to serotype 4b and were positive for internalins A, C, and J. According to the DNA fragment patterns of pulsed-field gel electrophoresis, the isolates were closely related. The source of infection was the pasture, implying that listeriosis should not be discounted in cases with compatible clinical signs but the absence of silage feeding.

Determination of virulence and antibiotic resistance pattern of biofilm producing Listeria species isolated from retail raw milk

BACKGROUND

One of the foodborne pathogens is Listeria monocytogenes, which causes serious invasive illness in elderly and immunocompromised patients, pregnant women, newborns and infants ranking second after salmonellosis because of its high case fatality rate. Listerial cow mastitis marked by abnormal milk, increased cell counts and reduced production has not been reported. Therefore, apparently healthy cows can be reservoirs of L. monocytogenes. A number of 203 udder milk samples from apparently healthy animals (buffalo, n = 100; cow, n = 103) were collected and tested for Listeria. Isolated colonies on the PALCAM agar were Listeria species confirmed according to their biochemical and the Christie-Atkins-Munch-Petersen (CAMP) reactions. The Listeria species pathogenicity of was tested by phosphatidylinositol-specific phospholipase C, DL-alanine-β-naphthylamide HCl, Dalanine-p-nitroanilide tests, chick embryo, mice inoculation tests, Vero cell cytotoxicity and biofilm formation. The virulence-associated genes, hlyA, plcB, actA and iap associated with Listeria were molecularly assayed.

RESULTS

The 17 isolated Listeria spp. represented a prevalence rate of 8.4 %. Of these 3 (1.4 %), 2 (1 %), 5 (2.5 %), 4 (2 %) and 3 (1.5 %) were confirmed as L. monocytogenes, L. innocua, L. welshimeri, L. seelegeri, respectively. While the L. monocytogenes isolate displayed all the four virulence-associated genes, L. seelegeri carried the hlyA gene only. The L. monocytogenes had a strong in vitro affinity to form a biofilm, in particular serotype 4 which is associated with human infections. L. monocytogenes showed resistance for 9/27 antibiotics.

CONCLUSIONS

The biofilm forming capability of the Listeria spps. makes them particularly successful in colonizing surfaces within the host thus being responsible for persistence infections and due to their antimicrobial resistant phenotype that this structure confers. In addition, strains belonging to serotypes associated with human infections and characterized by pathogenic potential (serotype 4) are capable to persist within the processing plants forming biofilm and thus being a medical hazard.

Estimating Bacterial Pathogen Levels in New Zealand Bulk Tank Milk

Zoonotic bacteria such as Campylobacter, Listeria, and Shiga toxin-producing Escherichia coli have been found in bulk tank milk in many countries, and the consumption of raw milk has been implicated in outbreaks of disease in New Zealand. Fecal contamination at milking is probably the most common source of pathogenic bacteria in bulk tank milk. Raw milk was collected from 80 New Zealand dairy farms during 2011 and 2012 and tested periodically for Campylobacter, E. coli O157, Listeria monocytogenes, and Salmonella. Milk quality data such as coliform counts, total bacterial counts, and somatic cell counts also were collected. By treating the total bacterial count as a proxy for fecal contamination of milk and utilizing farm and animal level prevalence and shedding rates of each pathogen, a predictive model for the level of pathogenic bacteria in bulk tank raw milk was developed. The model utilizes a mixture distribution to combine the low level of contamination inherent in the milking process with isolated contamination events associated with significantly higher pathogen levels. By simulating the sampling and testing process, the predictive model was validated against the observed prevalence of each pathogen in the survey. The predicted prevalence was similar to the observed prevalence for E. coli O157 and Salmonella, although the predicted prevalence was higher than that observed in samples tested for Campylobacter.

Diversity of Listeria monocytogenes within a U.S. dairy herd, 2004-2010

Listeria monocytogenes, the causative agent of listeriosis, is frequently isolated from the environment. Dairy cows and dairy farm environments are reservoirs of this pathogen, where fecal shedding contributes to its environmental dispersal and contamination of milk, dairy products, and meat. The molecular diversity of 40 L. monocytogenes isolates representing 3 serogroups (1/2a, 1/2b, and 4b) collected between 2004 and 2010 from the feces of dairy cattle on a single dairy farm was assessed using a multivirulence locus sequence typing (MVLST) assay. The dairy farm L. monocytogenes MVLST patterns were compared to those from 138 strains isolated globally from clinical cases, foods, and the environment. Results of the study demonstrated that several distantly related L. monocytogenes strains persisted among members of the herd over the course of the study while other strains were transient. Furthermore, some strains isolated during this study appear to be distantly related to previously isolated L. monocytogenes while others are closely related to Epidemic Clones associated with human illness. This work demonstrates that dairy cows can be reservoirs of a diverse population of potentially human pathogenic L. monocytogenes that represents a risk to consumers of milk, dairy products, and meat.

Survival of Listeria monocytogenes in Soil Requires AgrA-Mediated Regulation

In a recent paper, we demonstrated that inactivation of the Agr system affects the patterns of survival of Listeria monocytogenes (A.-L. Vivant, D. Garmyn, L. Gal, and P. Piveteau, Front Cell Infect Microbiol 4:160, http://dx.doi.org/10.3389/fcimb.2014.00160). In this study, we investigated whether the Agr-mediated response is triggered during adaptation in soil, and we compared survival patterns in a set of 10 soils. The fate of the parental strain L. monocytogenes L9 (a rifampin-resistant mutant of L. monocytogenes EGD-e) and that of a ΔagrA deletion mutant were compared in a collection of 10 soil microcosms. The ΔagrA mutant displayed significantly reduced survival in these biotic soil microcosms, and differential transcriptome analyses showed large alterations of the transcriptome when AgrA was not functional, while the variations in the transcriptomes between the wild type and the ΔagrA deletion mutant were modest under abiotic conditions. Indeed, in biotic soil environments, 578 protein-coding genes and an extensive repertoire of noncoding RNAs (ncRNAs) were differentially transcribed. The transcription of genes coding for proteins involved in cell envelope and cellular processes, including the phosphotransferase system and ABC transporters, and proteins involved in resistance to antimicrobial peptides was affected. Under sterilized soil conditions, the differences were limited to 86 genes and 29 ncRNAs. These results suggest that the response regulator AgrA of the Agr communication system plays important roles during the saprophytic life of L. monocytogenes in soil.

Farm to Fork Quantitative Risk Assessment of Listeria monocytogenes Contamination in Raw and Pasteurized Milk Cheese in Ireland

The objective of this study was to model and quantify the level of Listeria monocytogenes in raw milk cheese (RMc) and pasteurized milk cheese (PMc) from farm to fork using a Bayesian inference approach combined with a quantitative risk assessment. The modeling approach included a prediction of contamination arising from the farm environment as well from cross-contamination within the cheese-processing facility through storage and subsequent human exposure. The model predicted a high concentration of L. monocytogenes in contaminated RMc (mean 2.19 log10 CFU/g) compared to PMc (mean -1.73 log10 CFU/g). The mean probability of illness (P1 for low-risk population, LR) and (P2 for high-risk population, HR, e.g., immunocompromised) adult Irish consumers following exposure to contaminated cheese was 7 × 10(-8) (P1 ) and 9 × 10(-4) (P2 ) for RMc and 7 × 10(-10) (P1 ) and 8 × 10(-6) (P2 ) for PMc, respectively. In addition, the model was used to evaluate performance objectives at various stages, namely, the cheese making and ripening stages, and to set a food safety objective at the time of consumption. A scenario analysis predicted various probabilities of L. monocytogenes contamination along the cheese-processing chain for both RMc and PMc. The sensitivity analysis showed the critical factors for both cheeses were the serving size of the cheese, storage time, and temperature at the distribution stage. The developed model will allow food processors and policymakers to identify the possible routes of contamination along the cheese-processing chain and to reduce the risk posed to human health.

Occurrence and antimicrobial resistance of Staphylococcus aureus in bulk tank milk and milk filters

This work is focused on the monitoring of Staphylococcus aureus prevalence in raw milk and milk filters, its antibiotic resistance and detection of methicillin resistant Staphylococcus aureus (MRSA). Samples of raw cow´s milk and milk filters were collected in the period from 2012 till 2014, from 50 dairy farms in the Czech Republic. The total of 261 samples (164 samples of raw milk and 97 milk filters) were cultivated on Baird-Parker agar. Both the typical and atypical colonies were examined by plasmacoagulase test and PCR method was used for detection of species specific fragment SA442 and mecA gene. Standard disk diffusion method was used to determinate resistance to antimicrobial agents. The bacterium Staphylococcus aureus was detected on 25 farms (50%). The antimicrobial resistance showed differences between the farms. Total of 58 samples were positive for Staphylococcus aureus, of which were 37 (14.2%) isolated from raw milk samples and 21 (8.1%) from milk filters. From these samples we isolated 62 Staphylococcus aureus strains, 41 isolates bacteria S. aureus from raw milk (66.1%) and 21 isolates S. aureus from milk filters (33.9%). The presence of antibiotic resistance in Staphylococcus aureus isolates was low, most of them were resistant to amoxicilin. According to the results obtained by the PCR method for the methicillin - resistant S. aureus (MRSA), the mecA gene was present in 6 strains (9.7%), 4 isolates obtained from milk samples (6.5%) and 2 isolates from milk filters (3.2%).  These isolates can be considered as a possible source of resistance genes, which can be spread through the food chain. Nowadays, a globally unfavourable increasing trend of prevalence of methicillin resistant staphylococci strains especially Staphylococcus aureus is being observed worldwide. The improper hygiene and poor farm management practices contributed to the presence of S. aureus in the milk. This may have contributed to the high level of S. aureus isolated. Improving the hygienic conditions of the milking environment and utensils may reduce the prevalence of S. aureus in milk. Objective of this study was monitoring of Staphylococcus aureus prevalence and determine the prevalence rate of antimicrobial resistance of S. aureus isolated from raw milk and milk filters in the Czech Republic.

Prevalence and characteristics of Listeria monocytogenes in feces of black beef cattle reared in three geographically distant areas in Japan

This study was conducted to determine the prevalence and characteristics of Listeria monocytogenes in the feces of black beef cattle reared in geographically distant areas in Japan. We surveyed 130 farms in the following three areas: northern (Hokkaido prefecture), central (Gifu and Mie prefectures), and southern (Oita, Miyazaki, and Kagoshima prefectures) areas and collected 1738 fecal samples. Our data showed the following isolation rate for each area: northern, 11.4% of 651; central, 2.8% of 572; and southern, 2.9% of 515, indicating that the isolation rate in the northern area was significantly higher than that in the central or southern areas (p<0.01). Moreover, serotyping of 996 isolates identified 1/2b as the most prevalent serotype (40.5%), followed by 1/2a (36.9%), 4b (21.6%), and 4ab (1.0%). In the northern area, multiple serotypes were isolated from 60% of L. monocytogenes-positive farms. In addition, multiple serotypes were isolated from individual fecal samples from 18 cattle. Pulsed-field gel electrophoresis (PFGE) characterization of 239 isolates detected 48 different PFGE types. We found that isolates from northern farms were genetically diverse compared to those from central and southern farms. Five isolates from human clinical cases and three isolates from animal clinical cases were identical to isolates from black beef cattle. Furthermore, the isolates from northern and central farms were characterized to possess epidemic clone II or III markers. We next showed that the isolates were susceptible to penicillin, ampicillin, amoxicillin, gentamicin, kanamycin, streptomycin, erythromycin, vancomycin, tetracycline, chloramphenicol, ciprofloxacin, and trimethoprim/sulfamethoxazole. Taken together, our survey provides crucial data regarding the prevalence and characteristics of L. monocytogenes in black beef cattle farms throughout Japan.

Four-year monitoring of foodborne pathogens in raw milk sold by vending machines in Italy

Prevalence data were collected from official microbiological records monitoring four selected foodborne pathogens (Salmonella, Listeria monocytogenes, Escherichia coli O157:H7, and Campylobacter jejuni) in raw milk sold by self-service vending machines in seven Italian regions (60,907 samples from 1,239 vending machines) from 2008 to 2011. Data from samples analyzed by both culture-based and real-time PCR methods were collected in one region. One hundred raw milk consumers in four regions were interviewed while purchasing raw milk from vending machines. One hundred seventy-eight of 60,907 samples were positive for one of the four foodborne pathogens investigated: 18 samples were positive for Salmonella, 83 for L. monocytogenes, 24 for E. coli O157:H7, and 53 for C. jejuni in the seven regions investigated. No significant differences in prevalence were found among regions, but a significant increase in C. jejuni prevalence was observed over the years of the study. A comparison of the two analysis methods revealed that real-time PCR was 2.71 to 9.40 times more sensitive than the culture-based method. Data on consumer habits revealed that some behaviors may enhance the risk of infection linked to raw milk consumption: 37% of consumers did not boil milk before consumption, 93% never used an insulated bag to transport raw milk home, and raw milk was consumed by children younger than 5 years of age. These results emphasize that end-product controls alone are not sufficient to guarantee an adequate level of consumer protection. The beta distribution of positive samples in this study and the data on raw milk consumer habits will be useful for the development of a national quantitative risk assessment of Salmonella, L. monocytogenes, E. coli O157, and C. jejuni infection associated with raw milk consumption.

Microbial diversity and structure are drivers of the biological barrier effect against Listeria monocytogenes in soil

Understanding the ecology of pathogenic organisms is important in order to monitor their transmission in the environment and the related health hazards. We investigated the relationship between soil microbial diversity and the barrier effect against Listeria monocytogenes invasion. By using a dilution-to-extinction approach, we analysed the consequence of eroding microbial diversity on L. monocytogenes population dynamics under standardised conditions of abiotic parameters and microbial abundance in soil microcosms. We demonstrated that highly diverse soil microbial communities act as a biological barrier against L. monocytogenes invasion and that phylogenetic composition of the community also has to be considered. This suggests that erosion of diversity may have damaging effects regarding circulation of pathogenic microorganisms in the environment.

Incorporation of Listeria monocytogenes strains in raw milk biofilms

Biofilms develop successively on devices of milk production without sufficient cleaning and originate from the microbial community of raw milk. The established biofilm matrices enable incorporation of pathogens like Listeria monocytogenes, which can cause a continuous contamination of food processing plants. L. monocytogenes is frequently found in raw milk and non-pasteurized raw milk products and as part of a biofilm community in milk meters and bulk milk tanks. The aim of this study was to analyze whether different L. monocytogenes strains are interacting with the microbial community of raw milk in terms of biofilm formation in the same manner, and to identify at which stage of biofilm formation a selected L. monocytogenes strain settles best. Bacterial community structure and composition of biofilms were analyzed by a cloning and sequencing approach and terminal restriction fragment length polymorphism analysis (T-RFLP) based on the bacterial 16S rRNA gene. The chemical composition of biofilms was analyzed by Fourier transform infrared spectroscopy (FTIR), while settled L. monocytogenes cells were quantified by fluorescence in situ hybridization (FISH). Addition of individual L. monocytogenes strains to raw milk caused significant shifts in the biofilm biomass, in the chemical as well as in the bacterial community composition. Biofilm formation and attachment of L. monocytogenes cells were not serotype but strain specific. However, the added L. monocytogenes strains were not abundant since mainly members of the genera Citrobacter and Lactococcus dominated the bacterial biofilm community. Overall, added L. monocytogenes strains led to a highly competitive interaction with the raw milk community and triggered alterations in biofilm formation.

Quantitative risk assessment of verocytotoxin-producing Escherichia coli O157 and Campylobacter jejuni related to consumption of raw milk in a province in Northern Italy

A quantitative risk assessment was developed to describe the risk of campylobacteriosis and hemolytic uremic syndrome (HUS) linked to consumption of raw milk sold in vending machines in Northern Italy. Exposure assessment considered the microbiological status of dairy farms, expected milk contamination, storage conditions from bulk tank to home storage, microbial growth during storage, destruction experiments, consumption frequency of raw milk, age of consumers, serving size, and consumption preference. The differential risk between milk handled under regulation conditions (4°C throughout all phases) and the worst field handling conditions was considered. The probability of Campylobacter jejuni infection was modeled with a single-hit dose-response beta-Poisson model, whereas for HUS an exponential dose-response model was chosen and two probabilities were used to model the higher susceptibility of children younger than 5 years old. For every 10,000 to 20,000 consumers each year, the models predicted for the best and worst storage conditions, respectively, 2.12 and 1.14 campylobacteriosis cases and 0.02 and 0.09 HUS cases in the 0- to 5-year age group and 0.1 and 0.5 HUS cases in the >5-year age group. The expected pediatric HUS cases do not differ considerably from those reported in Italy by the Minister of Health. The model developed may be a useful tool for extending the assessment of the risk of campylobacteriosis and HUS due to raw milk consumption at the national level in Italy. Considering the epidemiological implications of this study, the risk of illness linked to raw milk consumption should not be ignored and could be reduced by the use of simple measures. Boiling milk before consumption and strict control of temperatures by farmers during raw milk distribution have significant effects on campylobacteriosis and HUS and are essential measures for risk management.

Foodborne pathogens in in-line milk filters and associated on-farm risk factors in dairy farms authorized to produce and sell raw milk in northern Italy

All dairy farms authorized to produce and sell raw milk in a province of Northern Italy were investigated to determine the presence of Campylobacter spp., verocytotoxin-producing Escherichia coli (VTEC), Listeria monocytogenes, and Salmonella spp. in in-line milk filters and to assess their association with suspected risk factors on farms. A logistic regression model was used to analyze data collected describing the characteristics and management practices of 27 farms and the microbiological status of 378 in-line milk filters by both culture-based and molecular methods. Thermotolerant Campylobacter, VTEC, and L. monocytogenes were detected in 24 (6.45%), 32 (8.4%), and 2 (0.5%) samples, respectively. No Salmonella spp. were detected. For risk analysis, data of L. monocytogenes and Salmonella spp. were not included in the model because of the low prevalence or absence of these organisms. The univariate analysis disclosed that the presence of VTEC and/or Campylobacter spp. in milk filters was associated with lack of cleanliness of bedding, water trough, and feed trough; nonevaluation of water hardness; lack of cleanliness of milk tank; and nonapplication of forestripping. After multivariate analysis, an association was observed with inadequate cleanliness of bedding and milk tank and the nonapplication of forestripping. PCR analysis of milk filters was a rapid and sensitive method for the microbiological evaluation of herd contamination status and should be included among the registration requirements for the authorization to produce and sell raw milk. Specific control actions must be incorporated into the farmer's daily practices to ensure the low-risk production of raw milk.

Quantitative risk assessment of listeriosis due to consumption of raw milk

The objectives of this study were to estimate the risk of illness for raw milk consumers due to Listeria monocytogenes in raw milk sold by permitted dealers, and the risk for people on farms who consume raw milk. Three scenarios were evaluated for raw milk sold by dealers: raw milk purchased directly from bulk tanks, from on-farm stores, and from retail. To assess the effect of mandatory testing of raw milk by regulatory agencies, the number of listeriosis cases per year was compared where no raw milk testing was done, only a screening test to issue a permit was conducted, and routine testing was conducted and milk was recalled if it was L. monocytogenes positive. The median number of listeriosis cases associated with consumption of raw milk from bulk tanks, farm stores, and retail for an intermediate-age population was 6.6 × 10(-7), 3.8 × 10(-5), and 5.1 × 10(-5) cases per year, respective ly. In populations with high susceptibility, the estimated median number of cases per year was 2.7 × 10(-7) (perinatal, i.e., pregnant women and their fetuses or newborns) and 1.4 × 10(-6) (elderly) for milk purchased from bulk tanks, 1.5 × 10(-5 ) (perinatal) and 7.8 × 10(-5) (elderly) for milk from farm stores, and 2.1 × 10(-5) (perinatal) and 1.0 × 10(-4) (elderly) for milk from retail. For raw milk consumed on farms, the median number of listeriosis cases was 1.4 × 10(-7) cases per year. A greater risk of listeriosis was associated with consumption of raw milk obtained from retail and farm stores as compared with milk obtained from bulk tanks. This was likely due to additional time-temperature combination steps in the retail and farm store models, which increased the chances for growth of L. monocytogenes in raw milk. A close relationship between prevalence of L. monocytogenes in raw milk and the values of disease incidence was observed. Hence, a reduction in the number of cases per year in all populations was observed when a raw milk-testing program was in place, especially when routine testing and recalling of milk was conducted.

Increased in vitro adherence and on-farm persistence of predominant and persistent Listeria monocytogenes strains in the milking system

Dairy farms are a reservoir for Listeria monocytogenes, and the reduction of this pathogen at the farm level is important for reducing human exposure. The objectives of this research were to study the diversity of L. monocytogenes strains on a single dairy farm, assess strain dynamics within the farm, identify potential sources of L. monocytogenes in bulk tank milk and milk filters, and assess the adherence abilities of representative strains. A total of 248 L. monocytogenes isolates were analyzed by pulsed-field gel electrophoresis (PFGE). Combined AscI and ApaI restriction analysis yielded 40 PFGE types (strains). The most predominant strains were T (28.6%), D (22.6%), and F (14.9%). A high level of heterogeneity of strains among isolates from fecal (Simpson's index of diversity [SID] = 0.96) and environmental (SID = 0.96) samples was observed. A higher homogeneity of strains was observed among isolates from milk filters (SID = 0.71) and bulk tank milk (SID = 0.65). Six of 17 L. monocytogenes isolates (35.3%) were classified in an in vitro assay as having a "low adherence ability," 9 (52.9%) were classified as having a "medium adherence ability," and 2 (11.8%) were classified as having a "high adherence ability." The L. monocytogenes strains that were predominant and persistent showed significantly better adherence than did strains that were only sporadic, predominant, or persistent (P = 0.0006). Our results suggest that the milking system was exposed to several L. monocytogenes strains from different sources. Only 3 strains, however, were successful in persisting within the milking system, suggesting that some strains are more suitable to that particular ecological environment than others.

Prevalence of Salmonella enterica, Listeria monocytogenes, and Escherichia coli virulence factors in bulk tank milk and in-line filters from U.S. dairies

The zoonotic bacteria Salmonella enterica, Listeria monocytogenes, and Escherichia coli are known to infect dairy cows while not always causing clinical signs of disease. These pathogens are sometimes found in raw milk, and human disease outbreaks due to these organisms have been associated with the consumption of raw milk or raw milk products. Bulk tank milk (BTM) samples (536) and in-line milk filters (519) collected from dairy farms across the United States during the National Animal Health Monitoring System's Dairy 2007 study were analyzed by real-time PCR for the presence of S. enterica and pathogenic forms of E. coli and by culture techniques for the presence of L. monocytogenes. S. enterica was detected in samples from 28.1% of the dairy operations, primarily in milk filters. Salmonella was isolated from 36 of 75 PCR-positive BTM samples and 105 of 174 PCR-positive filter samples, and the isolates were serotyped. Cerro, Kentucky, Muenster, Anatum, and Newport were the most common serotypes. L. monocytogenes was isolated from 7.1% of the dairy operations, and the 1/2a complex was the most common serotype, followed by 1/2b and 4b (lineage 1). Shiga toxin genes were detected in enrichments from 15.2% of the BTM samples and from 51.0% of the filters by real-time PCR. In most cases, the cycle threshold values for the PCR indicated that toxigenic strains were not a major part of the enrichment populations. These data confirm those from earlier studies showing significant contamination of BTM by zoonotic bacterial pathogens and that the consumption of raw milk and raw milk products presents a health risk.