Salmonella and Enterohemorrhagic Escherichia coli Serogroups O45, O121, O145 in Wheat Flour: Effects of Long-Term Storage and Thermal Treatments

Systematic risk ranking of microbiological hazards in infant foods

Ensuring food safety, particularly for vulnerable groups, like infants and young children, requires identifying and prioritizing potential hazards in food chains. We previously developed a web-based decision support system (DSS) to identify specific microbiological hazards (MHs) in infant and toddler foods through a structured five-step process. This study takes the framework further by introducing systematic risk ranking (RR) steps to rank MH risks with seven criteria: process survival, recontamination, growth opportunity, meal preparation, hazard-food association evidence, food consumption habits of infants and toddlers in the EU, and MH severity. Each criterion is given a semi-quantitative or quantitative score or risk value, contributing to the final MH risk calculation via three aggregation methods: semi-quantitative risk scoring, semi-quantitative risk value, and outranking multi-criteria decision analysis (MCDA). To validate the criteria and ranking approaches, we conducted a case study to rank MH risks in infant formula, compared the results of the three risk ranking methods, and additionally evaluated the ranking results against expert opinions to ensure their accuracy. The results showed strong agreement among the three methods, consistently ranking Salmonella non-Typhi and Cronobacter spp. and Shiga-toxin-producing Escherichia coli as the top MH risks in infant formulae, with minor deviations. When MHs were ranked after an initial hazard identification step, all three methods produced nearly identical MH rankings, reinforcing the reliability of the ranking steps and the selected criteria. Notably, the risk value and MCDA methods provided more informative MH rankings compared to the risk scoring method. The risk value and risk scoring methods were implemented into an online tool, called the MIcrobiological hazards risk RAnking decision support system (Mira-DSS), available at https://foodmicrobiologywur.shinyapps.io/MIcrobial_hazards_RAnking/. In conclusion, our framework enables the ranking of MH risks, facilitating intervention comparisons and resource allocations to mitigate MH risks in infant foods, with potential applicability to broader food categories.

Quantitative detection and survival analysis of VBNC Salmonella Typhimurium in flour using droplet digital PCR and DNA-intercalating dyes

The difficulty in detecting viable but non-culturable (VBNC) Salmonella by culture-dependent methods poses a risk to food safety. In our study, we applied a viability test to Salmonella following a lethal treatment and to flour samples inoculated with Salmonella to evaluate the effectiveness of viability polymerase chain reaction (PCR). Our findings revealed that the combination of both ddPCR and qPCR with those DNA-intercalating dyes could quantify viable cells at low concentrations when the plate counting method failed to detect them post-inactivation. Prolonged UV exposure did not induce cell membrane disruption, as confirmed with PMA-ddPCR, with insignificant differences in gene copies. However, samples exposed to DyeTox13 and DyeTox13 + EMA showed lower gene copy numbers, implying that enzymatic activity was decreased by UV exposure duration. In addition, temperature-dependent survival in flour revealed uniform decay rates and D values (time required for a 1 log reduction) of DNA in untreated samples across various temperatures. By contrast, different decay rates were observed with DNA-intercalating dyes (DyeTox13 and DyeTox13 + EMA), showing faster metabolic activity loss at higher temperatures in flour. The decay rates and D values, determined through plate counting and those DNA-intercalating dyes, indicated the potential presence of VBNC Salmonella. A strong correlation between DyeTox13 dyes and the plate counting method suggested DyeTox13 as a rapid alternative for detecting Salmonella in flour. The ddPCR with DNA-intercalating dyes could effectively evaluate Salmonella viability, facilitating more precise monitoring of VBNC in food.

IMPORTANCE

Salmonella, a major foodborne pathogen, poses significant risks, particularly to vulnerable groups like infants, older people, and the immunocompromised. Accurate detection is vital for public health and food safety, given its potential to cause severe and life-threatening symptoms. Our study demonstrated digital polymerase chain reaction (ddPCR) with DNA-intercalating dyes for identifying the different physiological statuses of Salmonella. Also, the application of ddPCR with DNA-intercalating dyes offers quantification of viable cells post-disinfection as an alternative method in food. Utilizing ddPCR and DNA-intercalating dyes, we enhanced the detection of VBNC Salmonella, a form often undetectable by conventional methods. This innovative approach could significantly improve the precision and efficiency of detection for viable Salmonella. By providing deeper insights into its transmission potential, our method is a critical tool in preventing outbreaks and ensuring the safety of food products. This research contributes substantially to global efforts in controlling foodborne illnesses and safeguarding public health.

Evaluating the Safety of Sous-Vide Cooking for Beef Products Inoculated with Single Strains of Salmonella enterica and Escherichia coli O157

Sous-videcooking is a growing trend among retailers and consumers. Foodborne pathogens may survive the cooking if nonvalidated parameters are used or if pathogens have enhanced thermalresistance. Pathogen inactivation from sous-vide cooking was determined when introduced directly to beef products or via contaminated spices, and with or without a finishing step. Beef products (ground beef, tenderized, and nontenderized steaks) were inoculated with pathogens (Salmonella Montevideo and Escherichia coli O157:NM) in three ways: 1) directly onto the meat 2) ground black pepper incorporated into the recipe 3) ground pepper equilibrated at 30% RH (4 d) prior to incorporation. Beef samples were vacuum-packaged and submerged in a 62.5°C water bath for 120 min. Samples were sampled at 5, 10, 20, and 120 min (recommended from a partner quality study), and a duplicate was grilled to a specific internal temperature (74°C for ground beef, 57°C for steaks) and sampled. Sous-vide cooking reduced pathogen populations by >5 log CFU/g after most treatment times, but less than grilled counterparts (ca. 1-2 log CFU/g difference; p < 0.05).There were no statistically significant differences between inoculation methods, but the tenderization of steaks resulted in significantly lower reductions of pathogens from sous-vide cooking (p < 0.05). Thisresearch challenged sous-vide cooking parameters (120 min, 62.5°C). It showed sous-vide alone lowered pathogens by >4 log CFU/g after most 20-min treatments, but 120-min sous-vide treatments or grilling would be needed for >5-log reductions.Contaminated pepper led to less consistent reductions during the cooking process, yet 2-h sous-vide still achieved a 5-log reduction. Sous-vide cooking instructions must be validated as more products and recipes are marketed.

Persistence comparison of two Shiga-toxin producing Escherichia coli (STEC) serovars during long-term storage and thermal inactivation in various wheat flours

Foodborne outbreaks associated with Shiga toxin-producing Escherichia coli (STEC) contaminated wheat flour have been an increasing food safety concern in recent decades. However, there is little literature aimed at investigating the impact of different flour types on the persistence of STEC during storage and thermal inactivation. Therefore, two serovars of STEC, O121 and O157, were selected to inoculate each of five different types of common wheat flours: whole wheat, bleached, unbleached, bread, and self-rising. Inoculated flours were examined for the stability of STEC during storage for up to 42 days at room temperature (RT) and aw ~0.56. Additionally, the thermal resistance of O121 and O157 under isothermal conditions at 60, 70, 80, and 90°C was analyzed for the inoculated flours. STEC storage persistence at RT was generally not affected by flour type, however, decreases of 1.2 and 2.4 log CFU/day within whole wheat flour for O121 and O157, respectively, were significantly lower than other flours. Though few differences were identified in relation to flour type, O121 exhibited significantly better survival rates than O157 during both equilibrium and storage periods. Compared to an approximate 6 log reduction in the population of O157, O121 population levels were reduced by a significantly lower amount (~3 log) during the entire storage period at RT. At each isothermal temperature, the impact of flour type on the thermal resistance capabilities of O121 or O157 was not a significant factor and resulted in similar survival curves regardless of serovar. Instead of exhibiting linear survival curves, both O121 and O157 displayed nonlinear curves with some shoulder/tail effect. Similar for both O121 and O157, the predicted decimal reduction time (D-value) decreased from approximately 25 min to around 8 min as the isothermal temperature increased from 60°C to 90°C. Results reported here can contribute to risk assessment models concerning contamination of STEC in wheat flour and add to our understanding of the impacts of flour type and STEC serovar on desiccation stability during storage and isothermal inactivation during thermal treatment.

Heat Resistance, Virulence, and Gene Expression of Desiccation-Adapted Salmonella Enteritidis During Long-Term Storage in Low-Water Activity Foods

Desiccation stress could induce crossprotection and even affect virulence of Salmonella enterica. However, the influence of food matrices with low-water activity on desiccation adaptation of Salmonella still remains unclear. This study investigated the survival and adaptation of Salmonella Enteritidis in skim milk powder, ginger powder, and chocolate powder under desiccation storage conditions for a total of 12 weeks. High survival rates of Salmonella Enteritidis in all food matrices maintained over the long-term desiccation storage. Desiccation-adapted Salmonella Enteritidis enhanced heat resistance (p < 0.05) with the increase of storage time. Food composition plays an important role in the induction of crossresistance of desiccation-adapted Salmonella. After desiccation storage, Salmonella Enteritidis in ginger powder was most tolerant to heat treatment. Salmonella Enteritidis in skim milk powder was most resistant to the gastrointestinal simulation environment, and had strongest adhesion to Caco-2 cells. The effects of food composition on gene expression (rpoS, proV, otsA, otsB, grpE, dnaK, rpoH, and sigDE) in desiccation-adapted Salmonella Enteritidis were not significant (p > 0.05). At initial desiccation storage, osmotic protection-related genes (fadA, proV, otsA, and otsB), stress response regulator (rpoS), and heat-resistance-related genes (grpE, dnaK, and rpoH) were all significantly upregulated (p < 0.05). However, after 4-week storage, the expression level of desiccation-related genes, proV, otsA, otsB, grpE, dnaK, and rpoH, significantly decreased (p < 0.05). This study enables a better understanding of Salmonella's responses to long-term desiccation stress in different kinds of low-water activity foods.

Inactivation kinetics of a surrogate yield conservative predictions of foodborne pathogen reductions from low water activity foods of varying size and composition during low-temperature steam processing

There is a growing interest in using models to predict foodborne pathogen inactivation as a way to validate or verify preventive controls. Unlike liquid foods, solid, low water activity foods (LWAF) are heterogenous in composition and structure and do not transfer heat uniformly. Using models constructed from one food to predict pathogen inactivation on another LWAF is complex and may not always be possible, even if the foods have similar composition. Using models constructed from inactivation kinetics of three foodborne pathogens and a surrogate from vacuum-steam-pasteurized (72 and 82 °C) whole macadamia nuts and dried apricot halves, 3-log reductions were predicted for the same pathogens and foods of reduced size. Model fits (First-order, Weibull, and Gompertz) were significantly impacted by the food type regardless of particle size. Despite the foods being identical in composition with particle size as the only altered characteristic, best-fit models accurately predicted the 3-log reductions only 50% of the time, but the surrogate inactivation models provided conservative predictions for pathogen reductions, highlighting that a surrogate's model may be a suitable tool for predicting pathogen reduction on LWAFs.

Growth Kinetics of Listeria monocytogenes and Salmonella enterica on Dehydrated Vegetables during Rehydration and Subsequent Storage

Dehydrated vegetables have low water activities and do not support the proliferation of pathogenic bacteria. Once rehydrated, vegetables can be incorporated into other foods or held for later use. The aim of this study was to examine the survival and proliferation of Listeria monocytogenes and Salmonella enterica on dehydrated vegetables during rehydration and subsequent storage. Carrots, corn, onion, bell peppers, and potatoes were heat dehydrated, inoculated at 4 log CFU/g, and rehydrated at either 5 or 25 °C for 24 h. Following rehydration, vegetables were stored at 5, 10, or 25 °C for 7 d. Both L. monocytogenes and S. enterica survived on all vegetables under all conditions examined. After 24 h of rehydration at 5 °C, pathogen populations on the vegetables were generally <1.70 log CFU/g, whereas rehydration at 25 °C resulted in populations of 2.28 to 6.25 log CFU/g. The highest growth rates during storage were observed by L. monocytogenes on potatoes and S. enterica on carrots (2.37 ± 0.61 and 1.63 ± 0.18 log CFU/g/d, respectively) at 25 °C when rehydration occurred at 5 °C. Results indicate that pathogen proliferation on the vegetables is both rehydration temperature and matrix dependent and highlight the importance of holding rehydrated vegetables at refrigeration temperatures to hinder pathogen proliferation. Results from this study inform time and temperature controls for the safety of these food products.

Fate of salmonella and shiga-toxin producing Escherichia coli on wheat grain during tempering

Outbreaks of Salmonella and Shiga-toxin producing Escherichia coli (STEC) linked to wheat flour led to increased interest in characterizing the fate of Salmonella and STEC on wheat during processing. Tempering is the stage of wheat processing where water is added to toughen the bran prior to milling, which has the potential to influence pathogen behavior on the kernels. This study aimed to quantify changes in the numbers of STEC and Salmonella inoculated onto soft red winter wheat, and to observe potential changes in the distribution of the pathogens on the surface of kernels during tempering. Lab-scale tempering experiments were conducted to quantify the water activity of and bacterial populations on wheat grain at various time points during 16 h of tempering. The highest water activity observed throughout 16 h of tempering was 0.88. There was no significant change (p > 0.05) in numbers of Salmonella, STEC, or native mesophiles. Using confocal microscopy, observation of Salmonella and STEC cells expressing mCherry on wheat kernels showed an even distribution of inoculated cells, though the localization of cells on kernels did not change significantly after tempering. Even though the environment was not favorable for pathogen replication on grain, the population remained stable, suggesting that disinfection of the kernels prior to milling could reduce food safety concerns in flour.

Practice and Progress: Updates on Outbreaks, Advances in Research, and Processing Technologies for Low-moisture Food Safety

Large, renowned outbreaks associated with low-moisture foods (LMFs) bring to light some of the potential, inherent risks that accompany foods with long shelf lives if pathogen contamination occurs. Subsequently, in 2013, Beuchat et al. (2013) noted the increased concern regarding these foods, specifically noting examples of persistence and resistance of pathogens in low-water activity foods (LWAFs), prevalence of pathogens in LWAF processing environments, and sources of and preventive measures for contamination of LWAFs. For the last decade, the body of knowledge related to LMF safety has exponentially expanded. This growing field and interest in LMF safety have led researchers to delve into survival and persistence studies, revealing that some foodborne pathogens can survive in LWAFs for months to years. Research has also uncovered many complications of working with foodborne pathogens in desiccated states, such as inoculation methods and molecular mechanisms that can impact pathogen survival and persistence. Moreover, outbreaks, recalls, and developments in LMF safety research have created a cascading feedback loop of pushing the field forward, which has also led to increased attention on how industry can improve LMF safety and raise safety standards. Scientists across academia, government agencies, and industry have partnered to develop and evaluate innovate thermal and nonthermal technologies to use on LMFs, which are described in the presented review. The objective of this review was to describe aspects of the extensive progress made by researchers and industry members in LMF safety, including lessons-learned about outbreaks and recalls, expansion of knowledge base about pathogens that contaminate LMFs, and mitigation strategies currently employed or in development to reduce food safety risks associated with LMFs.

Validation of baking as a kill-step for controlling Shiga toxin-producing Escherichia coli during traditional crust pizza baking process

A study was conducted to validate a simulated traditional crust pepperoni pizza baking process to control Shiga toxin-producing Escherichia coli (STEC) and to determine the heat resistance characteristics of STEC in pizza dough. Pizza dough and pepperoni slices were inoculated with 7 strains STEC cocktail and baked at 500°F (260°C) for 12 min using a conventional kitchen oven followed by 15 min of ambient air cooling. The mean internal temperature of the pizza reached 209.32 ± 1.94°F by the end of 12 min of baking and dropped to 137.90 ± 2.88°F after the 15 min ambient air cooling. The a_w and pH of the traditional crust pizza did not alter significantly during the baking process. The STEC population decreased by >5 log CFU/g in traditional crust pizza after 12 min of baking. Where pepperoni slices were used as a source of STEC introduction, a reduction of >6.5 log CFU/g was observed. The D-values of STEC cocktail in pizza dough at 55, 58, 61°C were 49.5 ± 4.10, 15.3 ± 0.68, and 2.8 ± 0.31 min, respectively. The z-value of STEC was 4.8 ± 0.16°C. This study validated that a typical traditional crust pizza baking process with ~209°F internal temperature for at least 12 min will result in 5 log reductions in STEC population.

Thermal Inactivation of Salmonella enterica and Nonpathogenic Bacterial Surrogates in Wheat Flour by Baking in a Household Oven

ABSTRACT

Wheat flour has been implicated in recalls and outbreaks linked to Salmonella and pathogenic Escherichia coli. An instructional online video posted on a popular YouTube channel with over 20 million subscribers claimed that safe raw cookie dough could be made from flour baked in a household oven at 177°C (350°F) for 5 min, but no evidence in support of that claim was provided. This study was conducted to assess thermal inactivation of two Salmonella strains, as well as Enterobacter aerogenes and Pantoea dispersa in wheat flour during home oven baking. Wheat flour was inoculated with Salmonella Enteritidis PT 30, Salmonella Typhimurium PT 42, or their potential surrogates at high concentrations (4.8 to 6.1 log CFU/g) before baking in a consumer-style convection oven (toaster oven) at 149, 177, and 204°C (300, 350, or 400°F) for up to 7 min. Flour was heated in an aluminum tray, with a maximum depth of ∼2 cm. Heated wheat flour samples (5 g each) were enumerated in triplicate, and the microbial concentration was expressed in log CFU per gram. Thermal profiles of the geometric center of the wheat flour pile and air in the oven during the baking were recorded. Water activity of wheat flour samples was also measured before and after baking. The water activity of wheat flour decreased, as baking temperature and time increased. Water activity values ranged from 0.30 to 0.06 after 7 min, as oven temperature increased from 149 to 204°C. Thermal inactivation kinetics were linear until counts approached the limit of detection for all microorganisms. D-values for Salmonella and potential surrogate strains ranged from 1.86 to 2.13 min at 149°C air temperature, 1.66 to 1.92 min at 177°C air temperature, and 1.12 to 1.38 min at 204°C air temperature. Both Salmonella strains and surrogates showed similar inactivation patterns. Baking of wheat flour in household toaster ovens has potential as an inactivation treatment of pathogenic bacteria in consumer homes, despite its low water activity.

HIGHLIGHTS

Microbiological Profile, Prevalence, and Characterization of Salmonella enterica in Peanuts, Pecans, Raisins, Sun-Dried Tomatoes, and Chocolate Sprinkles Sold in Bulk in Markets in Querétaro, Mexico

ABSTRACT

In Mexico, the prevalence of Salmonella enterica in low-water-activity foods and its link to outbreaks are unknown. The aim of this study was to determine the microbiological profile and the prevalence of S. enterica in several low-water-activity foods, including peanuts, pecans, raisins, sun-dried tomatoes, and chocolate sprinkles, purchased in retail establishments in Querétaro, Mexico. Seventy samples of each food item sold in bulk were purchased. Aerobic plate count, molds, yeasts, total coliforms, Escherichia coli, and Staphylococcus aureus were quantified in 10-g samples. The prevalence of S. enterica in 25-g samples was determined. From positive samples, S. enterica isolates (60) were characterized based on their antimicrobial susceptibility to 14 antibiotics, the presence-absence of 13 virulence genes, and serotype. The concentration of aerobic plate count, molds, yeasts, total coliforms, and E. coli ranged from 3.1 to 5.2 log CFU g-1, from 2.0 to 2.4 log CFU g-1, from 2.0 to 3.0 log CFU g-1, from 0.6 to 1.1 log most probable number (MPN) g-1, and from 0.5 to 0.9 log MPN g-1, respectively. S. aureus was not detected in any sample (<10 CFU g-1). The prevalence of S. enterica in chocolate sprinkles, raisins, peanuts, pecans, and sun-dried tomatoes was 26, 29, 31, 40, and 52%, respectively. Most isolates (68.3%) were resistant to at least one antibiotic. Chromosome-associated virulence genes were found in all isolates, and only one strain had sopE, and 98.3% of the isolates were grouped in the same virulotype. Among the isolates, the most frequent serotype was Tennessee (51 of 60). According to the characteristics evaluated, we grouped the isolates into 24 clusters. The elevated prevalence of S. enterica highlights the role of low-water-activity food items sold in bulk at markets as potential vehicles for pathogen transmission. Regardless of the low variability among S. enterica isolates, their characterization could be helpful to elucidate which strains are circulating in these foods for improving epidemiological surveillance.

HIGHLIGHTS

Impact of Chlorinated Water on Pathogen Inactivation during Wheat Tempering and Resulting Flour Quality

ABSTRACT

Outbreaks of enteric pathogens linked to wheat flour have led the wheat milling industry to seek solutions addressing this food safety concern. Chlorinated water at 400 to 700 ppm has been used in the flour milling industry as a tempering aid to control growth of yeast and mold in tempering bins. However, the effectiveness of chlorinated water for inactivating enteric pathogens on wheat kernels was unknown. Five strains of Shiga toxin-producing Escherichia coli and two strains of Salmonella were inoculated onto hard red spring wheat at 7 log CFU/g and stored at room temperature for 1 month. Inoculated wheat was tempered with four concentrations (0, 400, 800, and 1,200 ppm) of chlorinated water (pH 6.5). The reduction due to chlorine was determined by calculating change in microbial loads at each chlorine level by using the response at 0 ppm as a reference. Uninoculated wheat tempered with chlorinated water was used to measure flour quality parameters. Changes in pathogen population over 18 h ranged from -2.35 to -0.30 log CFU/g with 800 ppm of chlorinated water and were not significantly different from changes at 400 and 1,200 ppm. Significant (P < 0.05) differences in the extent of reduction were observed among strains. However, the effect of chlorinated water at reducing native microbes on wheat kernels was minimal, with an average reduction of 0.39 log CFU/g for all concentrations. No significant (P > 0.05) changes occurred in flour quality and gluten functionality or during bread making for grains tempered at 400 and 800 ppm of chlorinated water. There were small but significant (P < 0.05) changes in flour protein content, final viscosity, and water absorption when tempered with 1,200 ppm of chlorinated water. The data showed that the level of chlorinated water currently used in industry for tempering could reduce enteric pathogen numbers by 1.22 log CFU/g for Shiga toxin-producing Escherichia coli and 2.29 log CFU/g for Salmonella, with no significant effects on flour quality and gluten functionality.

HIGHLIGHTS

Metabolic Responses of "Big Six" Escherichia coli in Wheat Flour to Thermal Treatment Revealed by Nuclear Magnetic Resonance Spectroscopy

Escherichia coli outbreaks linked to wheat flour consumption have kept emerging in recent years, which necessitated an antimicrobial step being incorporated into the flour production process. The objectives of this in vivo study were to holistically evaluate the sanitizing efficacy of thermal treatment at 60 and 70°C against the "big six" E. coli strains (O26:H11, O45:H2, O103:H11, O111, O121:H19, and O145) in wheat flour and to assess the strain-specific metabolic responses using nuclear magnetic resonance (NMR) spectroscopy. The 70°C treatment temperature indiscriminatingly inactivated all strains by over 4.3-log CFU/g within 20 min, suggesting the high sanitization effectiveness of this treatment temperature, whereas the treatment at 60°C inactivated the strains to various degrees during the 1-h process. The most resistant strains at 60°C, O26 and O45, were characterized by amino acid and sugar depletion, and their high resistance was attributed to the dual effects of activated heat shock protein (HSP) synthesis and promoted glycolysis. O121 also demonstrated these metabolic changes, yet its thermal resistance was largely impaired by the weakened membrane structure and diminished osmotic protection due to phosphorylcholine exhaustion. In contrast, O111, O145, and O103 presented a substantial elevation of metabolites after stress at 60°C; their moderate thermal resistance was mainly explained by the accumulation of amino acids as osmolytes. Overall, the study enhanced our understanding of the metabolic responses of big six E. coli to heat stress and provided a model for conducting NMR-based metabolomic studies in powdered food matrices. IMPORTANCE "Big six" Escherichia coli strains have caused several outbreaks linked to wheat flour consumption in the last decade, revealing the vital importance of adopting an antimicrobial treatment during the flour production process. Therefore, the present study was carried out to evaluate the efficacy of a typical sanitizing approach, thermal treatment, against the big six strains in wheat flour along with the underlying antimicrobial mechanisms. Findings showed that thermal treatment at 60 and 70°C could markedly mitigate the loads of all strains in wheat flour. Moreover, activated heat shock protein synthesis combined with expedited glycolysis and enhanced osmotic protection were identified as two major metabolic alteration patterns in the E. coli strains to cope with the heat stress. With the responses of big six in wheat flour to thermal treatment elucidated, scientific basis for incorporating a thermal inactivation step in wheat flour production was provided.

Fate of Salmonella and Enterohemorrhagic Escherichia coli on Wheat Grain

ABSTRACT

Wheat flour has been connected to outbreaks of foodborne illnesses with increased frequency in recent years, specifically, outbreaks involving Salmonella enterica and enterohemorrhagic Escherichia coli (EHEC). However, there is little information regarding the survival of these pathogens on wheat grain during long-term storage in a low-moisture environment. This study aims to evaluate the long-term survival of these enteric pathogens on wheat grain over the course of a year. Hard red spring wheat was inoculated with strains of four serovars of Salmonella (Enteritidis, Agona, Tennessee, and Montevideo) and six serotypes of EHEC (O157:H7, O26:H11, O121:H19, O45:NM, O111:H8, and O103:H2) in triplicate, sealed in Mylar bags to maintain the water activity, and stored at room temperature (22 ± 1°C). The survival of each pathogen was evaluated by plating onto differential media. Viable counts of strains from all four serovars of Salmonella (Enteritidis, Agona, Tennessee, and Montevideo) were detected on wheat grain stored at room temperature (22 ± 1°C) for the duration of the study (52 weeks). Viable counts of strains from EHEC serotypes O45:NM, O111:H8, and O26:H11 were only detected for 44 weeks, and strains from serotypes O157:H7, O121:H19, and O103:H2 were only detected for 40 weeks until they passed below the limit of detection (2.0 log CFU/g). The D-values were found to be significantly different between Salmonella and EHEC (adjusted P ≤ 0.05) with Salmonella D-values ranging from 22.9 ± 2.2 weeks to 25.2 ± 1.0 weeks and EHEC D-values ranging from 11.4 ± 0.6 weeks to 13.1 ± 1.8 weeks. There were no significant differences among the four Salmonella strains or among the six EHEC strains (adjusted P > 0.05). These observations highlight the wide range of survival capabilities of enteric pathogens in a low-moisture environment and confirm these pathogens are a food safety concern when considering the long shelf life of wheat grain and its products.

HIGHLIGHTS

Induction of a Viable but Nonculturable State, Thermal and Sanitizer Tolerance, and Gene Expression Correlation with Desiccation-Adapted Biofilm and Planktonic Salmonella in Powdered Infant Formula

ABSTRACT

This study was conducted to investigate the effects of the physiological state, desiccation adaptation, and storage of powdered infant formula on Salmonella cell survival and expression of desiccation stress-related genes. Powdered infant formula was inoculated with Salmonella Typhimurium in the biofilm state on beads and in the planktonic state on nitrocellulose filters and stored at 25°C for up to 270 days. Both 5-cyano-2,3-ditoyl tetrazolium chloride flow cytometry and xylose lysine deoxycholate agar plate counts revealed that biofilm-forming Salmonella cells tended to enter the viable but nonculturable (VBNC) state (P < 0.05). The population reduction of all desiccation-adapted Salmonella Typhimurium decreased significantly in both physiological states after exposure to mild heat (60°C) compared with nonadapted control cells (P < 0.05). Salmonella cells were cross-protected from heat in both physiological states, but cross-protection against hydrogen peroxide was induced for only planktonic Salmonella cells. The reverse transcription quantitative PCR results revealed that the rpoS gene in biofilm Salmonella cells on all desiccation adaption days and in planktonic Salmonella cells on day 7 of dry storage was significantly upregulated (P < 0.05). The rpoE, grpE, and invA genes in Salmonella cells in both physiological states were significantly down-regulated (P < 0.05). Physiological state and storage time might affect expression of these genes. Prior exposure to adverse conditions, including low water activity, and the physiological state impacted Salmonella survival, and its ability to enter the VBNC state and gene expression.

HIGHLIGHTS

Models for factors influencing pathogen survival in low water activity foods from literature data are highly significant but show large unexplained variance

Factors that control pathogen survival in low water activity foods are not well understood and vary greatly from food to food. A literature search was performed to locate data on the survival of foodborne pathogens in low-water activity (<0.70) foods held at temperatures <37 °C. Data were extracted from 67 publications and simple linear regression models were fit to each data set to estimate log linear rates of change. Multiple linear stepwise regression models for factors influencing survival rate were developed. Subset regression modeling gave relatively low adjusted R² values of 0.33, 0.37, and 0.48 for Salmonella, E. coli and L. monocytogenes respectively, but all subset models were highly significant (p < 1.0e-9). Subset regression models showed that Salmonella survival was significantly (p < 0.05) influenced by temperature, serovar and strain type, water activity, inoculum preparation method, and inoculation method. E. coli survival was significantly influenced by temperature, water activity, and inoculum preparation. L. monocytogenes survival was significantly influenced by temperature, serovar and strain type, and inoculum preparation method. While many factors were highly significant (p < 0.001), the high degrees of variability show that there is still much to learn about the factors which govern pathogen survival in low water activity foods.

Thermal Resistance of Single Strains of Shiga Toxin-Producing Escherichia coli O121:H19 and O157:H7 Based on Culture Preparation Method and Osmolyte-Reduced Water Activity

ABSTRACT

Bacterial exposure to stress, such as reduced water activity (aw), can increase thermal resistance. Pathogen thermal resistance studies on low-aw foods use a variety of methods to inoculate food, as well as strategies to reduce aw, which can influence observations. This study investigated effects of culture preparation method and osmolyte-induced aw on thermal resistance of two Shiga toxin-producing Escherichia coli (STEC) strains (O121:H19 and O157:H7) challenged with isothermal conditions, determining D- and z-values for each isolate (56, 59, and 62°C). Tryptic soy broth (TSB) and agar (lawn cultures) were compared. D-values of broth cultures were significantly and consistently larger than those of lawn cultures, and O121 was significantly more resistant than O157, but only at 56°C (P < 0.05). To compare potential effects of aw on STEC thermal resistance, cells were suspended in osmolyte solutions with varying aw: high (TSB, aw 0.99), intermediate (61% glycerol or 26% NaCl, aw 0.75), and low (82% glycerol, aw 0.5). In most instances, STEC strains in high-aw broth exhibited greater heat resistance compared to reduced-aw solutions, with the exception of the glycerol intermediate-aw solution (aw 0.75). Magnitudes varied with strain and temperature. The z-values of lawn cultures were significantly lower than those of broth cultures (P < 0.05), but there were few differences between high-aw and reduced-aw samples. There were no significant differences of z-values based on strain type. These results highlight that thermal resistance can be affected by culture preparation and that osmolyte-induced changes to aw influence thermal inactivation of STEC by varying magnitudes. These results emphasize the challenges of extrapolating results from laboratory inactivation kinetic experiments to determine the inactivation of low-aw foods, especially those considered dry in nature.

HIGHLIGHTS

Effect of Physical Structures of Food Matrices on Heat Resistance of Enterococcus faecium NRRL-2356 in Wheat Kernels, Flour and Dough

Nonpathogenic surrogate microorganisms, with a similar or slightly higher thermal resistance of the target pathogens, are usually recommended for validating practical pasteurization processes. The aim of this study was to explore a surrogate microorganism in wheat products by comparing the thermal resistance of three common bacteria in wheat kernels and flour. The most heat-resistant Enterococcus faecium NRRL-2356 rather than Salmonella cocktail and Escherichia coli ATCC 25922 was determined when heating at different temperature-time combinations at a fixed heating rate of 5 °C/min in a heating block system. The most heat-resistant pathogen was selected to investigate the influences of physical structures of food matrices. The results indicated that the heat resistance of E. faecium was influenced by physical structures of food matrices and reduced at wheat kernel structural conditions. The inactivation of E. faecium was better fitted in the Weibull distribution model for wheat dough structural conditions while in first-order kinetics for wheat kernel and flour structural conditions due to the changes of physical structures during heating. A better pasteurization effect could be achieved in wheat kernel structure in this study, which may provide technical support for thermal inactivation of pathogens in wheat-based food processing.

Investigations of Possible Multistate Outbreaks of Salmonella, Shiga Toxin-Producing Escherichia coli, and Listeria monocytogenes Infections - United States, 2016

PROBLEM/CONDITION

Salmonella, Shiga toxin-producing Escherichia coli (STEC), and Listeria monocytogenes are the leading causes of multistate foodborne disease outbreaks in the United States. Responding to multistate outbreaks quickly and effectively and applying lessons learned about outbreak sources, modes of transmission, and risk factors for infection can prevent additional outbreak-associated illnesses and save lives. This report summarizes the investigations of multistate outbreaks and possible outbreaks of Salmonella, STEC, and L. monocytogenes infections coordinated by CDC during the 2016 reporting period.

PERIOD COVERED

2016. An investigation was considered to have occurred in 2016 if it began during 2016 and ended on or before March 31, 2017, or if it began before January 1, 2016, and ended during March 31, 2016-March 31, 2017.

DESCRIPTION OF SYSTEM

CDC maintains a database of investigations of possible multistate foodborne and animal-contact outbreaks caused by Salmonella, STEC, and L. monocytogenes. Data were collected by local, state, and federal investigators during the detection, investigation and response, and control phases of the outbreak investigations. Additional data sources used for this report included PulseNet, the national molecular subtyping network based on isolates uploaded by local, state, and federal laboratories, and the Foodborne Disease Outbreak Surveillance System (FDOSS), which collects information from state, local, and territorial health departments and federal agencies about single-state and multistate foodborne disease outbreaks in the United States. Multistate outbreaks reported to FDOSS were linked using a unique outbreak identifier to obtain food category information when a confirmed or suspected food source was identified. Food categories were determined and assigned in FDOSS according to a classification scheme developed by CDC, the Food and Drug Administration (FDA), and the U.S. Department of Agriculture Food Safety and Inspection Service (FSIS) in the Interagency Food Safety Analytics Collaboration. A possible multistate outbreak was determined by expert judgment to be an outbreak if supporting data (e.g., temporal, geographic, demographic, dietary, travel, or food history) suggested a common source. A solved outbreak was an outbreak for which a specific kind of food or animal was implicated (i.e., confirmed or suspected) as the source. Outbreak-level variables included number of illnesses, hospitalizations, cases of hemolytic uremic syndrome (HUS), and deaths; the number of states with illnesses; date of isolation for the earliest and last cases; demographic data describing patients associated with a possible outbreak (e.g., age, sex, and state of residence); the types of data collected (i.e., epidemiologic, traceback, or laboratory); the outbreak source, mode of transmission, and exposure location; the name or brand of the source; whether the source was suspected or confirmed; whether a food was imported into the United States; the types of regulatory agencies involved; whether regulatory action was taken (and what type of action); whether an outbreak was publicly announced by CDC via website posting; beginning and end date of the investigation; and general comments about the investigation. The number of illnesses, hospitalizations, cases of HUS, and deaths were characterized by transmission mode, pathogen, outcome (i.e., unsolved, solved with suspected source, or solved with confirmed source), source, and food or animal category.

RESULTS

During the 2016 reporting period, 230 possible multistate outbreaks were detected and 174 were investigated. A median of 24 possible outbreaks was under investigation per week, and investigations were open for a median of 37 days. Of these 174 possible outbreaks investigated, 56 were excluded from this analysis because they occurred in a single state, were linked to international travel, or were pseudo-outbreaks (e.g., a group of similar isolates resulting from laboratory media contamination rather than infection in patients). Of the remaining 118 possible multistate outbreaks, 50 were determined to be outbreaks and 39 were solved (18 with a confirmed food source, 10 with a suspected food source, 10 with a confirmed animal source, and one with a suspected animal source). Sprouts were the most commonly implicated food category in solved multistate foodborne outbreaks (five). Chicken was the source of the most foodborne outbreak-related illnesses (134). Three outbreaks involved novel food-pathogen pairs: flour and STEC, frozen vegetables and L. monocytogenes, and bagged salad and L. monocytogenes. Eleven outbreaks were attributed to contact with animals (10 attributed to contact with backyard poultry and one to small turtles). Thirteen of 18 multistate foodborne disease outbreaks with confirmed sources resulted in product action, including 10 outbreaks with recalls, two with market withdrawals, and one with an FSIS public health alert. Twenty outbreaks, including 11 foodborne and nine animal-contact outbreaks, were announced to the public by CDC via its website, Facebook, and Twitter. These announcements resulted in approximately 910,000 webpage views, 55,000 likes, 66,000 shares, and 5,800 retweets.

INTERPRETATION

During the 2016 reporting period, investigations of possible multistate outbreaks occurred frequently, were resource intensive, and required a median of 37 days of investigation. Fewer than half (42%) of the 118 possible outbreaks investigated were determined to have sufficient data to meet the definition of a multistate outbreak. Moreover, of the 50 outbreaks with sufficient data, approximately three fourths were solved.

PUBLIC HEALTH ACTION

Close collaboration among CDC, FDA, FSIS and state and local health and agriculture partners is central to successful outbreak investigations. Identification of novel outbreak sources and trends in sources provides insights into gaps in food safety and safe handling of animals, which helps focus prevention strategies. Summarizing investigations of possible multistate outbreaks can provide insights into the investigative process, improve future investigations, and help prevent illnesses. Although identifying and investigating possible multistate outbreaks require substantial resources and investment in public health infrastructure, they are important in determining outbreak sources and implementing prevention and control measures.

Formic Acid as an Antimicrobial for Poultry Production: A Review

Organic acids continue to receive considerable attention as feed additives for animal production. Most of the emphasis to date has focused on food safety aspects, particularly on lowering the incidence of foodborne pathogens in poultry and other livestock. Several organic acids are currently either being examined or are already being implemented in commercial settings. Among the several organic acids that have been studied extensively, is formic acid. Formic acid has been added to poultry diets as a means to limit Salmonella spp. and other foodborne pathogens both in the feed and potentially in the gastrointestinal tract once consumed. As more becomes known about the efficacy and impact formic acid has on both the host and foodborne pathogens, it is clear that the presence of formic acid can trigger certain pathways in Salmonella spp. This response may become more complex when formic acid enters the gastrointestinal tract and interacts not only with Salmonella spp. that has colonized the gastrointestinal tract but the indigenous microbial community as well. This review will cover current findings and prospects for further research on the poultry microbiome and feeds treated with formic acid.

Influence of water activity on the heat resistance of Salmonella enterica in selected low-moisture foods

Low-moisture foods (LMF with water activity, a_w < 0.85) including pet foods and black pepper powder have consistently been associated with foodborne disease caused by Salmonella enterica. Increased heat resistance and prolonged survival at low-moisture conditions, however, remain major challenges to achieve effective inactivation of Salmonella in low-moisture foods. At low water activity (a_w) conditions, heat resistance of Salmonella is greatly enhanced when compared to high a_w conditions. This study aimed to quantify the effect of a_w on the heat resistance of Salmonella enterica in pet food pellets and black pepper powder. Pet food pellets were inoculated with two strains of heat resistant S. enterica and black pepper powder was inoculated with a 5-strain cocktail of Salmonella. Both inoculated food samples were equilibrated at 0.33, 0.54, and 0.75 a_w in controlled humidity chambers. Inoculated pet food pellets and black pepper powder in closed aluminum cells were heat treated at specific temperatures for selected times. The results showed that the Weibull model fitted well the inactivation data. At a specific temperature, the rate of inactivation increased with the increase in the a_w from 0.33 to 0.75, and the 3-log reduction times decreased for Salmonella in both food samples with the increase in a_w. Water adsorption isotherms of pet food pellets and black pepper powder at initial and treatment temperatures were developed to understand the change in a_w during heat treatments. The change in a_w during heat treatment was dependent on the type of food matrix, which possibly influenced the thermal inactivation of Salmonella in pet food pellets and black pepper powder. The quantitative analysis of heat reduction of Salmonella with respect to a_w aids in selection of the appropriate initial a_w to develop effective heat treatment protocols for adequate reduction of Salmonella in pet foods and black pepper powder.

Effect of Vacuum Steam Treatment of Hard Red Spring Wheat on Flour Quality and Reduction of Escherichia coli O121 and Salmonella Enteritidis PT 30

ABSTRACT

Recent outbreaks traced to contaminated flour have created a need in the milling industry for a process that reduces pathogens in wheat while maintaining its functional properties. Vacuum steam treatment is a promising technology for treatment of low-moisture foods. Traditional thermal treatment methods can compromise wheat functionality due to high temperatures; thus, maintaining the functional quality of the wheat protein was critical for this research. The objective of this study was to evaluate the effect of vacuum steam treatment of hard red spring (HRS) wheat kernels on final flour quality and the overall efficacy of vacuum stream treatment for reducing pathogens on HRS wheat kernels. HRS wheat samples were treated with steam under vacuum at 65, 70, 75, and 85°C for 4 and 8 min. Significant changes in dough and baked product functionality were observed for treatments at ≥70°C. Treatment time had no significant effect on the qualities evaluated. After determining that vacuum steam treatment at 65°C best preserved product quality, HRS wheat was inoculated with Escherichia coli O121 and Salmonella Enteritidis PT 30 and processed at 65°C for 0, 2, 4, 6, or 8 min. The treatments achieved a maximum average reduction of 3.57 ± 0.33 log CFU/g for E. coli O121 and 3.21 ± 0.27 log CFU/g for Salmonella. Vacuum steam treatment could be an effective pathogen inactivation method for the flour milling industry.

HIGHLIGHTS