Evaluation of the strain variability of Salmonella enterica acid and heat resistance

Effects of intrinsic characteristics of Salmonella enterica strains isolated from foods and humans, and their interaction with food matrices during simulated gastric conditions

The stomach's acidic pH is a crucial barrier against foodborne pathogens such as Salmonella enterica. This study investigated the survival of S. enterica under simulated oral and gastric conditions (SGC; pH 2 for 120 min) as a function of intrinsic pathogen characteristics and food matrix. Fifty-seven S. enterica strains isolated from food and human infections (previously characterized by serotype, virulotype, multi-drug resistance, isolation source, and isolation season) were subjected to SGC using water as a vehicle. Population reduction among the 57 isolates ranged from 2.7 to 4.7 log CFU, revealing that human isolates were inactivated less than food isolates (p = 0.0008). Among food strains, strains isolated during the cold season (food sampled from December to February) displayed the highest reduction (p = 0.00002). Six representatives of the 57 S. enterica strains were selected according to their virulotype and antimicrobial profile. They were further used to evaluate their survival under SGC in four food matrices (water, mango, tomato, and chicken), measuring S. enterica at 30 min intervals. The strains in chicken showed the lowest reduction and inactivation rate (1.42 ± 0.35 log CFU; 0.03 ± 0.005 min-1), followed by tomato (3.75 ± 0.57 log CFU; 0.15 ± 0.02 min-1), water (4.23 ± 0.27 log CFU; 0.17 ± 0.02 min-1), and mango (4.49 ± 0.39 log CFU; 0.17 ± 0.03 min-1). These data suggest that not all S. enterica strains have the same ability to survive under SGC, influencing the probability of arriving into the small intestine.

Investigating desiccation resistance, post-rehydration growth, and heat tolerance in desiccation-injured cells of Salmonella enterica isolated from the soybean production chain

This study compared the resistance to different desiccation conditions of 190 Salmonella enterica strains previously isolated from the soybean meal production chain and belonging to 23 serovars. Additionally, the post-rehydration growth and heat tolerance of the strains previously exposed to desiccation were determined. Variability in desiccation resistance was observed both within and between serovars. Strains belonging to S. Havana and S. Schwarzengrund serovars were the most resistant, regardless of storage condition. The drying temperature (20 °C and 30 °C) did not influence the desiccation resistance of the Salmonella strains. On the other hand, increasing drying time from 1 to 7 days reduced Salmonella counts. The origin (isolation sources) also influenced the desiccation resistance of the Salmonella strains. The growth of the Salmonella strains after rehydration varied considerably depending on the drying conditions and incubation temperature during cultivation. An increase in the time and temperature of drying led to a reduction in population of most Salmonella strains after rehydration. Salmonella strains previously desiccated also showed differences in the heat tolerance in all temperature-time binomials tested. Some strains were highly resistant to heat tolerance conditions, presenting <1 log CFU/mL reduction from the initial population. The results obtained in this study suggest that the strategies to mitigate Salmonella in low-aw foods must consider the existence of high-stress resistant strains and their multiple-stress adaptability profiles, including effects of processing, food composition, and storage conditions.

Relationship between growth ability, virulence, and resistance to food-processing related stresses in non-typhoidal Salmonellae

The ability of Salmonella to resist and adapt to harsh conditions is one of the major features that have made this microorganism such a relevant health hazard. However, the impact of these resistance responses on other aspects of Salmonella physiology, such as virulence and growth ability, is still not fully understood. The objective of this study was to determine the maximum growth rates (in three different media), virulence (adhesion and invasion of Caco-2 cells), and other phenotypic characteristics (biofilm-forming ability and antimicrobial resistance) of 23 Salmonella strains belonging to different serovars, and to compare them with their previously determined stress resistance parameters. Significant differences (p < 0.05) in growth rates, virulence, and biofilm-forming ability were found among the 23 strains studied. Nevertheless, whereas less than 3-fold change between the lowest and the highest growth rate was observed, the percentage of cells capable of invading Caco-2 cells varied more than 100-fold, that to form biofilms more than 30-fold, and the antibiotic MICs varied up to 512-fold, among the different strains. Results indicate that those strains with the highest cell adhesion ability were not always the most invasive ones and suggest that, in general terms, a higher stress resistance did not imply a reduced growth ability (rate). Similarly, no association between stress resistance and biofilm formation ability (except for acid stress) or antibiotic resistance (with minor exceptions) was found. Our data also suggest that, in Salmonella, acid stress resistance would be associated with virulence, since a positive correlation of that trait with adhesion and a negative correlation with invasion was found. This study contributes to a better understanding of the physiology of Salmonella and the relationship between bacterial stress resistance, growth ability, and virulence. It also provides new data regarding intra-specific variability of a series of phenotypic characteristics of Salmonella that are relevant from the food safety perspective.

Strain Variability in Growth and Thermal Inactivation Characteristics of Listeria monocytogenes Strains after Acid Adaptation

ABSTRACT

Given the importance of strain variability to predictive microbiology and risk assessment, this study aimed to quantify the magnitude of strain variability in growth and thermal inactivation kinetics behaviors after acid adaptation. Thirty-three Listeria monocytogenes strains were exposed to acid-adapted tryptic soy broth supplemented with yeast extract (TSBYE; pH 5.5) and non-acid-adapted TSBYE (pH 7.0) for 20 h. Next, the growth parameters of these adapted and nonadapted strains that grew in nonbuffered TSBYE at 25°C were estimated. The tested strains were inactivated at 60°C in nonbuffered broth to obtain the heat resistance parameters. The results revealed that strain variability was present in the growth and thermal inactivation characteristics. The maximum specific growth rate ranged from 0.21 to 0.44 h-1 and from 0.20 to 0.45 h-1 after acid and nonacid adaptation, respectively. The lag times were from 0.69 to 2.56 h and from 0.24 to 3.36 h for acid-adapted and non-acid-adapted cells, respectively. The apparent D-values at 60°C of the pathogen ranged between 0.56 and 3.93 min and between 0.52 and 3.63 min for the presence and absence of acid adaptation condition, respectively. Acid adaptation significantly (P < 0.05) increased the magnitude of strain variability in the thermal inactivation characteristics of L. monocytogenes, with the coefficient of variation increasing to 0.17, whereas acid adaptation did not significantly (P ≥ 0.05) influence the variabilities in the growth parameters of the tested strains. Furthermore, the subsequent growth behaviors of all strains did not exhibit significant (P > 0.05) changes after exposure to acidic broth. However, the thermal resistance of most (25 of 33) of the tested strains increased (P < 0.05) after growing in acid-adapted broth. The relevant data generated in the present study can be used to describe the strain variability in predictive microbiology and to deeply understand the behavioral responses of different strains to acid adaptation.

HIGHLIGHTS

Assessing the Impact of Heat Treatment of Food on Antimicrobial Resistance Genes and Their Potential Uptake by Other Bacteria-A Critical Review

The dissemination of antibiotic resistance genes (ARGs) is a global health concern. This study identifies and critically reviews the published evidence on whether cooking (heating) food to eliminate bacterial contamination induces sufficient damage to the functionality of ARGs. Overall, the review found that there is evidence in the literature that Antimicrobial Resistant (AMR) bacteria are no more heat resistant than non-AMR bacteria. Consequently, recommended heat treatments sufficient to kill non-AMR bacteria in food (70 °C for at least 2 min, or equivalent) should be equally effective in killing AMR bacteria. The literature shows there are several mechanisms through which functional genes from AMR bacteria could theoretically persist in heat-treated food and be transferred to other bacteria. The literature search found sparce published evidence on whether ARGs may actually persist in food after effective heat treatments, and whether functional genes can be transferred to other bacteria. However, three publications have demonstrated that functional ARGs in plasmids may be capable of persisting in foods after effective heat treatments. Given the global impact of AMR, there is clearly a need for further practical research on this topic to provide sufficient evidence to fully assess whether there is a risk to human health from the persistence of functional ARGs in heat-treated and cooked foods.

Evaluation of antimicrobial activities of plant aqueous extracts against Salmonella Typhimurium and their application to improve safety of pork meat

Nine odorless laboratory-collected hydro-distilled aqueous extracts (basil, calendula, centrifuged oregano, corn silk, laurel, oregano, rosemary, spearmint, thyme) and one industrial steam-distilled oregano hydrolate acquired as by-products of essential oils purification were screened for their in vitro antimicrobial activity against three Salmonella Typhimurium strains (4/74, FS8, FS115) at 4 and 37 °C. Susceptibility to the extracts was mainly plant- and temperature-dependent, though strain dependent effects were also observed. Industrial oregano hydrolate eliminated strains immediately after inoculation, exhibiting the highest antimicrobial potential. Hydro-distilled extracts eliminated/reduced Salmonella levels during incubation at 4 °C. At 37 °C, oregano, centrifuged oregano, thyme, calendula and basil were bactericidal while spearmint, rosemary and corn silk bacteriostatic. A strain-dependent effect was observed for laurel. The individual or combined effect of marinades and edible coatings prepared of industrial hydrolate and hydro-distilled oregano extracts with or without oregano essential oil (OEO) was tested in pork meat at 4 °C inoculated with FS8 strain. Lower in situ activity was observed compared to in vitro assays. Marinades and edible coatings prepared of industrial oregano hydrolate + OEO were the most efficient in inhibiting pathogen. Marination in oregano extract and subsequent coating with either 50% oregano extract + OEO or water + OEO enhanced the performance of oregano extract. In conclusion, by-products of oregano essential oil purification may be promising alternative antimicrobials to pork meat stored under refrigeration when applied in the context of multiple hurdle approach.

Influence of drying conditions, food composition, and water activity on the thermal resistance of Salmonella enterica

Salmonella contamination of low-water activity (a_w) foods poses a serious concern worldwide. The present study was conducted to assess the effects of drying conditions, food composition, and water activity on the desiccation tolerance and thermal resistance of S. Enteritidis FUA1946, S. Senftenberg ATCC43845 and S. Typhimurium ATCC13311 in pet food, binder formulation, and skim milk powder. The samples were wet inoculated with the individual Salmonella strains and were equilibrated to a_w 0.33 and 0.75, followed by an isothermal treatment at 70 °C. The thermal inactivation data was fitted to the Weibull model. Irrespective of the a_w, food composition and physical structure of the selected foods, strain S. Enteritidis FUA1946 displayed the highest desiccation and thermal resistance, followed by S. Senftenberg ATCC43845 and S. Typhimurium ATCC13311. The food matrix and strain type significantly (p < 0.05) influenced the thermal resistance of microorganisms in foods along with a_w change during thermal treatments. To further study the effect of food composition, an additional set of experiments using dry inoculation of the resistant Salmonella strain in the low-a_w foods was designed. Significant (p < 0.05) matrix-dependent interaction on Salmonella reduction was observed. The water adsorption isotherms of selected low-a_w foods were measured at 20 and 70 °C to relate the thermal inactivation kinetics with the change in the a_w. The characterization of thermal resistance of the Salmonella serovars in low-a_w products with different compositions and a_w in this study may be used for the validation of thermal challenge studies.

Impact of the Resistance Responses to Stress Conditions Encountered in Food and Food Processing Environments on the Virulence and Growth Fitness of Non-Typhoidal Salmonellae

The success of Salmonella as a foodborne pathogen can probably be attributed to two major features: its remarkable genetic diversity and its extraordinary ability to adapt. Salmonella cells can survive in harsh environments, successfully compete for nutrients, and cause disease once inside the host. Furthermore, they are capable of rapidly reprogramming their metabolism, evolving in a short time from a stress-resistance mode to a growth or virulent mode, or even to express stress resistance and virulence factors at the same time if needed, thanks to a complex and fine-tuned regulatory network. It is nevertheless generally acknowledged that the development of stress resistance usually has a fitness cost for bacterial cells and that induction of stress resistance responses to certain agents can trigger changes in Salmonella virulence. In this review, we summarize and discuss current knowledge concerning the effects that the development of resistance responses to stress conditions encountered in food and food processing environments (including acid, osmotic and oxidative stress, starvation, modified atmospheres, detergents and disinfectants, chilling, heat, and non-thermal technologies) exerts on different aspects of the physiology of non-typhoidal Salmonellae, with special emphasis on virulence and growth fitness.

Prior exposure to different combinations of pH and undissociated acetic acid can affect the induced resistance of Salmonella spp. strains in mayonnaise stored under refrigeration and the regulation of acid-resistance related genes

The innate and inducible resistance of six Salmonella strains (4/74, FS8, FS115, P167807, ATCC 13076, WT) in mayonnaise at 5 °C following adaptation to different pH/undissociated acetic acid (UAA) combinations (15mM/pH5.0, 35mM/pH5.5, 45mM/pH6.0) was investigated. The inherent and acid-induced responses were strain-dependent. Two strains (ATCC 13076, WT), albeit not the most resistant innately, exhibited the most prominent adaptive potential. Limited/no adaptability was observed regarding the rest strains, though being more resistant inherently. The individual effect of pH and UAA adaptation in the phenotypic and transcriptomic profiles of ATCC 13076 and WT was further examined. The type (pH, UAA) and magnitude of stress intensity affected their responses. Variations in the type and magnitude of stress intensity also determined the relative gene expression of four genes (adiA, cadB, rpoS, ompR) implicated in Salmonella acid resistance mechanisms. adiA and cadB were overexpressed following adaptation to some treatments; rpoS and ompR were downregulated following adaptation to 15mM/pH5.0 and 35mM/pH5.5, respectively. Nonetheless, the transcriptomic profiles did not always correlate with the corresponding phenotypes. In conclusion, strain variations in Salmonella are extensive. The ability of the strains to adapt and induce resistant phenotypes and acid resistance-related genes is affected by the type and magnitude of the stress applied during adaptation.

Stress resistance of emerging poultry-associated Salmonella serovars

In recent years, the on-farm prevalence of some poultry-related Salmonella serovars such as S. Kentucky, S. Heidelberg, S. Livingstone and S. Mbandaka has increased significantly, even replacing S. Enteritidis and S. Typhimurium as the most frequently isolated serovars in some production settings and countries. For this reason, the aim of this work was to determine the resistance to several stressing agents and food preservation technologies, in laboratory media and in egg products, of 4 strains of these emerging Salmonella serovars associated to poultry and poultry products and to make comparisons with 4 S. Enteritidis strains. First, the resistance to acid pH, hydrogen peroxide, NaCl, heat, HHP, PEF and UV of the 8 Salmonella strains studied was determined and compared in laboratory media. From this part of the study, it was concluded that variability in resistance to stress among the 8 studied strains varied depending on the investigated agent/technology. However, differences in resistance (2D-values) were always lower than 3.3-fold. Results obtained also indicated that the strains of the emerging serovars studied would display lower acid and NaCl resistance, higher heat resistance and similar oxidative, HHP, PEF and UV resistance than S. Enteritidis. Then, the resistance of these 8 strains was evaluated and compared in egg, egg products and poultry manure. For some agents -including osmotic stresses, UV and PEF- there was a very good correspondence between the results obtained in laboratory media and in real food matrices and poultry manure (r > 0.85; p < 0.01). A significant relationship was also found for acid and HHP resistance (p < 0.05) and a trend for heat resistance (p < 0.10). Therefore, in general terms, conclusions drawn from the study carried out in laboratory media - regarding intraspecific variability and the relative resistance of the different strains - might be extrapolated, although with caution, to real food scenarios. Results obtained in this investigation would help to better understand the physiology and ecology of Salmonella and to design better egg preservation strategies.

Salmonella survival after exposure to heat in a model meat juice system

The effect of heat against eleven Salmonella strains in model meat juices was examined. Juices from beef, lamb and goat were made from either the fatty layer (FL), muscle (M) or a mixture of both (FLM). The pH of each FLM sample was altered to match the pH of PBS and vice versa to determine the pH effect on the survival of Salmonella against the effect of heat. Salmonella were exposed to either gradual heating to 70 °C in FLM, M and FL or heat shock at 70 °C for 5 min in FLM. Fat, fatty acid profile and iron content of the juices were determined. Gradual heat treatment significantly (p ≤ 0.05) reduced Salmonella as compared to the untreated controls (~1.92-7.61 log CFU ml^-1) while heat shock significantly (p ≤ 0.05) reduced Salmonella as compared to the untreated controls (~5.80-7.36 log CFU ml^-1). Survival of Salmonella was higher in lamb juices than other juices. The fat content in lamb FL (3.25%) was significantly higher (p ≤ 0.05) than beef (1.30%) and goat FL (1.42%). Iron content in lamb FLM (~127 mg kg^-1) was significantly (p ≤ 0.05) lower than beef (~233 mg kg^-1) and goat FLM (~210 mg kg^-1). The omega 6 and linoleic acid content in goat FLM (~36.0% and ~34.4%) was significantly higher (p ≤ 0.05) than beef (~29.1% and ~27.1%). Fat, fatty acids and iron may differentially protect Salmonella against the effect of heat in these juices.

A Review of Salmonella and Campylobacter in Broiler Meat: Emerging Challenges and Food Safety Measures

Poultry is one of the largest sources of animal-based protein in the United States. Poultry processing has grown from a small local network of plants to nearly 500 plants nationwide. Two of the most persistent bacteria in poultry processing are Salmonella and Campylobacter. It was not until the introduction of Hazard Analysis and Critical Control Point systems in 1996 that major efforts to reduce bacterial contamination were developed. Traditionally, chlorine has been the industry standard for decontaminating chicken meat. However, antimicrobials such as peracetic acid, cetylpyridinium chloride, and acidified sodium chlorite have replaced chlorine as primary antimicrobials. Despite current interventions, the emergence of stress-tolerant and biofilm-forming Salmonella and Campylobacter is of primary concern. In an effort to offset growing tolerance from microbes, novel techniques such as cold plasma treatment, electrostatic spraying, and bacteriophage-based applications have been investigated as alternatives to conventional treatments, while new chemical antimicrobials such as Amplon and sodium ferrate are investigated as well. This review provides an overview of poultry processing in the United States, major microbes in poultry processing, current interventions, emerging issues, and emerging technologies in antimicrobial treatments.

Prior Exposure to Dry-Cured Meat Promotes Resistance to Simulated Gastric Fluid in Salmonella Typhimurium

This study assessed if exposure of foodborne Salmonella enterica in Brazilian dry-cured loin (BDL) affects pathogen inactivation in simulated gastric fluid (SGF). The acid tolerance responses of three Salmonella enterica serovars, Typhimurium, Derby and Panama, were assessed by an acid challenge trial at pH 3.0 for 4 h following pre-adaptation to three conditions: neutral pH, acidic pH (4.5) or BDL matrix. The influence of Salmonella exposure temperature and time in the BDL on pathogen gastric fluid resistance was evaluated by the response surface methodology. The Salmonella serovars acquired acid tolerance when exposed to the BDL matrix and their response to acid stress was strain-dependent, with S. Typhimurium being the most tolerant strain. S. Typhimuirum exposed to temperatures >25 °C in the BDL matrix displayed increased resistance to SGF. By using the response surface methodology, it was determined that S. Typhimurium becomes less resistant against SGF if maintained in the BDL matrix at temperatures <7 °C, reinforcing the recommendation to store dry-cured meat under refrigeration in order to minimize consumer risks. The results presented herein point to a novel aspect of hurdle technology that should be taken into account to further understand the risks associated with hurdle-stable meat product, such as dry-cured meats, concerning foodborne pathogen contamination.

Salmonella enterica subsp. enterica Serovar Heidelberg Food Isolates Associated with a Salmonellosis Outbreak Have Enhanced Stress Tolerance Capabilities

Salmonella enterica serovar Heidelberg is currently the 12th most common serovar of Salmonella enterica causing salmonellosis in the United States and results in twice the average incidence of blood infections caused by nontyphoidal salmonellae. Multiple outbreaks of salmonellosis caused by Salmonella Heidelberg resulted from the same poultry processor, which infected 634 people during 2013 and 2014. The hospitalization and invasive illness rates were 38% and 15%, respectively. We hypothesized that the outbreak strains of Salmonella Heidelberg had enhanced stress tolerance and virulence capabilities. We sourced nine food isolates collected during the outbreak investigation and three reference isolates to assess their tolerance to heat and sanitizers, ability to attach to abiotic surfaces, and invasiveness in vitro We performed RNA sequencing on three isolates (two outbreak-associated isolates and a reference Salmonella Heidelberg strain) with various levels of heat tolerance to gain insight into the mechanism behind the isolates' enhanced heat tolerance. We also performed genomic analyses to determine the genetic relationships among the outbreak isolates. Ultimately, we determined that (i) six Salmonella Heidelberg isolates associated with the foodborne outbreak had enhanced heat tolerance, (ii) one outbreak isolate with enhanced heat tolerance also had an enhanced biofilm-forming ability under stressful conditions, (iii) exposure to heat stress increased the expression of Salmonella Heidelberg multidrug efflux and virulence genes, and (iv) outbreak-associated isolates were likely transcriptionally primed to better survive processing stresses and, potentially, to cause illness.IMPORTANCE This study provides a deep analysis of the intrinsic stress tolerance and virulence capabilities of Salmonella Heidelberg that may have contributed to the length and severity of a recent salmonellosis outbreak. Additionally, this study provides a comprehensive analysis of the transcriptomic response of S. enterica strains to heat stress conditions and compares baseline stationary-phase gene expression among outbreak- and non-outbreak-associated Salmonella Heidelberg isolates. These data can be used in assay development to screen isolates for stress tolerance and subsequent survival. This study adds to our understanding of the strains associated with the outbreak and informs ongoing regulatory discussions on Salmonella in poultry.

Response to Acid Adaptation in Salmonella enterica Serovar Enteritidis

Acid adaptation in Salmonella Enteritidis was characterized by phenotypic and gene-expression analyses. S. Enteritidis cells at log-phase and stationary-phase were kept at pH 4.5 to 6.0 for 1 to 4 hours. All treatments induced various levels of acid tolerance response that were dependent on pH, exposure time and growth phase. This acid adaptation resulted in tolerance to 50 °C and 8% NaCl regardless of the growth phase. However, the tolerance of log-phase and stationary-phase cells to low temperatures (4 and -20 °C) was increased and decreased, respectively. RT-qPCR analysis revealed that genes involved in tolerance to acid (SEN1564A and cfa), heat (rpoH, uspB, and htrA), salt (proP, proV, and osmW), and cold (cspA, cspC, and csdA) stress were generally upregulated after acid adaptation. These results provide an initial insight into mechanisms of acid adaptation and induced cross protection in S. Enteritidis. PRACTICAL APPLICATION: Stress tolerance acquisition resulting from acid adaptation in foodborne pathogens poses a great threat to food safety. The current work showed that acid adaptation induced direct tolerance and cross-tolerance to high temperature, low temperature, and salt in Salmonella Enteritidis, possibly due to the upregulation of stress tolerance-related genes. These results provide key insights into acid adaptation mechanisms and efficient control of S. Enteritidis.

Relative response of populations of Escherichia coli and Salmonella enterica to exposure to thermal, alkaline and acidic treatments

We evaluated the relative response of generic Escherichia coli (GEC), Shiga toxin-producing E. coli (STEC) and Salmonella enterica to heat, alkaline or acid treatments. GEC included strains from carcasses (n = 24) and trim (n = 25) at a small beef plant where no decontamination interventions are used and at a large plant where multiple decontamination interventions are used (carcass n = 25 and trim n = 25). STEC strains belonging to nine serogroups, included isolates from cattle (n = 53), beef (n = 16) and humans (n = 44). S. enterica strains belonging to 29 serotypes, included isolates from humans (n = 30), poultry (n = 26), pork (n = 10) and beef (n = 33). Strains were grown in Brain Heart Infusion (BHI) broth and subjected to the following treatments: 60 °C for 2 min, 5% lactic acid (pH 2.9) for 1 h at 4 °C, or NaOH (pH 11.0) for 2 h at 4 °C. Median log reductions of the GEC populations after heat, alkaline and acid treatment ranged from 2.3 to 3.8, 0.7 to 2.2 and 0.7 to 1.2 log CFU/mL, respectively. No statistically significant difference in reductions was observed between carcass GEC or trim GEC from the large or small plant, except for a greater reduction in trim GEC from the small plant. Median reductions of the STEC populations ranged from 3.3 to 3.5, 0.0 to 0.6, and 0.3 to 0.5 log CFU/mL after heat, alkaline and acid treatment, respectively. The median reductions were not dependent upon isolation source, except between STEC cattle and human isolates after alkaline treatment, where the reduction of the former was higher by 0.6 log unit. For the Salmonella populations, median log reductions ranged from 3.5 to 4.0, 1.7 to 2.4 and 3.7 to 4.1 log CFU/mL after heat, alkaline and acid treatment, respectively. The reductions were not isolation source related. The median log reductions were in the order GEC < STEC < Salmonella after heat treatment and STEC < GEC < Salmonella after alkaline or acid treatment. Overall, the relative response of GEC, STEC and Salmonella in the model system suggests that exposure to heat or pH-based decontamination interventions in meat plants is not associated with increased resistance among E. coli strains in these environments, and total E. coli counts on beef can be indicative of treatment efficacy for the control of Salmonella by heat, lactic acid and alkaline treatment and for the control of STEC subjected to heat.

Variability in the Adaptive Response of Antibiotic-Resistant Salmonella Typhimurium to Environmental Stresses

This study was designed to evaluate the resistance phenotype and genotype of wild type (WT)-, cefotaxime (CET)-, and ciprofloxacin (CIP)-induced Salmonella Typhimurium ATCC 19585, CIP-resistant Salmonella Typhimurium ATCC 19585, Salmonella Typhimurium CCARM 8009, and Salmonella Typhimurium KCCM 40253 before and after exposure to pH 4.5, 4% NaCl, and heat at 42°C. The susceptibilities of WT Salmonella Typhimurium ATCC 19585 and WT Salmonella Typhimurium KCCM 40253 to all antibiotics tested in this study were decreased after CET and CIP induction with the exception with kanamycin, meropenem, and polymyxin B. The highest β-lactamase activities were 2.8 and 3.3 nmol/(min·mL), respectively, at the WT- and CET-induced Salmonella Typhimurium CCARM 8009. FT-IR spectra were found to be dominant at the region from 1,700 to 1,500 cm^-1 corresponding to proteins such as amides I, II, and III. The relative expression levels of efflux pump-related genes (acrA, acrB, and TolC), porin-related gene (ompC), virulence-related gene (stn), adhesion-related gene (fimA), and stress-induced alternative sigma factor (rpoS) varied in the antibiotic resistance and stress exposure. This study provides useful information for understanding the antibiotic resistance profile, physicochemical property, and gene expression pattern in Salmonella Typhimurium in association with the induction of antibiotic resistance and exposure to environmental stresses.

Evaluation of Peroxyacetic Acid for Reducing Low Levels of Salmonella on Laboratory-Inoculated and Naturally Contaminated In-Shell Hazelnuts

In-shell hazelnuts may be exposed to many sources of Salmonella during harvest and postharvest processing. Chemical sanitizers, such as peroxyacetic acid (PAA), are used by some postharvest processors in an attempt to reduce microbial loads; however, data are limited on its efficacy to reduce Salmonella on in-shell hazelnuts under conditions relevant to the industry. This study analyzed the efficacy of PAA to reduce small numbers of Salmonella on laboratory-inoculated (3.9 most probable number [MPN] per hazelnut) and naturally contaminated in-shell hazelnuts (8.2 MPN/33 g). Batches of hazelnuts (333 to 1,500 g) were agitated in water or PAA (80 or 200 ppm) for 1 min. Inoculated hazelnuts were analyzed for the presence of Salmonella on an individual nut basis. Naturally contaminated hazelnuts were analyzed for the presence of Salmonella in larger sample sizes (33 g). Water and PAA (80 ppm) were effective at reducing Salmonella contamination on laboratory-inoculated hazelnuts (untreated, 85.5% positive; water, 38.7%; and PAA, 29.5%). Neither water nor PAA treatments (80 and 200 ppm) were effective in reducing Salmonella on naturally contaminated hazelnuts. The use of PAA in wash lines is unlikely to reduce Salmonella contamination on in-shell hazelnuts; however, PAA may reduce cross-contamination during postharvest washing activities.

The locus of heat resistance (LHR) mediates heat resistance in Salmonella enterica, Escherichia coli and Enterobacter cloacae

Enterobacteriaceae comprise food spoilage organisms as well as food-borne pathogens including Escherichia coli. Heat resistance in E. coli was attributed to a genomic island called the locus of heat resistance (LHR). This genomic island is also present in several other genera of Enterobacteriaceae, but its function in the enteric pathogens Salmonella enterica and Enterobacter cloacae is unknown. This study aimed to determine the frequency of the LHR in food isolates of E. coli, and its influence on heat resistance in S. enterica and Enterobacter spp. Cell counts of LHR-positive strains of E. coli, S. enterica and E. cloacae were reduced by less than 1, 1, and 4 log (cfu/mL), respectively, after exposure to 60 °C for 5 min, while cell counts of LHR-negative strains of the same species were reduced by more than 7 log (cfu/mL). Introducing an exogenous copy of the LHR into heat-sensitive enteropathogenic E. coli and S. enterica increased heat resistance to a level that was comparable to LHR-positive wild type strains. Cell counts of LHR-positive S. enterica were reduced by less than 1 log(cfu/mL) after heating to 60 °C for 5 min. Survival of LHR-positive strains was improved by increasing the NaCl concentration from 0 to 4%. Cell counts of LHR-positive strains of E. coli and S. enterica were reduced by less than 2 log (cfu/g) in ground beef patties cooked to an internal core temperature of 71 °C. This study indicates that LHR-positive Enterobacteriaceae pose a risk to food safety.

Heat Survival and Phenotype Microarray Profiling of Salmonella Typhimurium Mutants

Contamination of food products by pathogenic microorganisms continues to be a major public health and food industry concern. Non-typhoidal Salmonella species have led to numerous outbreaks associated with various foods. A wide variety of methods have been applied and introduced for treatment of fresh foods to eliminate pathogenic as well as spoilage microorganisms. Salmonella can become exposed to elevated temperatures while associated with hosts such as poultry. In addition, heat treatment is also applied at various stages of processing to retain the shelf life of food products. Despite this, these microorganisms may overcome exposure to such treatments through the efficient expression of stress response mechanisms and result in illness following consumption. Thermal stress induces a range of destructive exposures to bacterial cells such as protein damage and DNA damage caused by reactive oxygen species. In this study, we chose three genes (∆recD, ∆STM14_5307, and ∆aroD) associated with conditionally essential genes required for different aspects of optimal growth at 42 °C and evaluated the responses of wild type and mutant Salmonella Typhimurium strains to uncover potential mechanisms that may enable survival and resistance under thermal stress. The RecBCD complex that initiates repair of double-stranded DNA breaks through homologous recombination. STM14_5307 is a transcriptional regulator involved in stationary phase growth and inositol metabolism. The gene aroD is involved in metabolism and stationary phase growth. These strains were characterized via high throughput phenotypic profiling in response to two different growth temperatures (37 °C (human host temperature) and 42 °C (poultry host temperature)). The ∆aroD strain exhibited the highest sensitivity to the various temperatures followed by the ∆recD and ∆STM14_5307 strains, respectively. Achieving more understanding of the molecular mechanisms of heat survival may lead to the development of more effective strategies to limit Salmonella in food products through thermal treatment by developing interventions that specifically target the pathways these genes are involved in.

Quantifying Variability in Growth and Thermal Inactivation Kinetics of Lactobacillus plantarum

The presence and growth of spoilage organisms in food might affect the shelf life. In this study, the effects of experimental, reproduction, and strain variabilities were quantified with respect to growth and thermal inactivation using 20 Lactobacillus plantarum strains. Also, the effect of growth history on thermal resistance was quantified. The strain variability in μmax was similar (P > 0.05) to reproduction variability as a function of pH, aw, and temperature, while being around half of the reproduction variability (P < 0.05) as a function of undissociated lactic acid concentration [HLa]. The cardinal growth parameters were estimated for the L. plantarum strains, and the pHmin was between 3.2 and 3.5, the aw,min was between 0.936 and 0.953, the [HLamax], at pH 4.5, was between 29 and 38 mM, and the Tmin was between 3.4 and 8.3°C. The average D values ranged from 0.80 min to 19 min at 55°C, 0.22 to 3.9 min at 58°C, 3.1 to 45 s at 60°C, and 1.8 to 19 s at 63°C. In contrast to growth, the strain variability in thermal resistance was on average six times higher than the reproduction variability and more than ten times higher than the experimental variability. The strain variability was also 1.8 times higher (P < 0.05) than the effect of growth history. The combined effects of strain variability and growth history on D value explained all of the variability as found in the literature, although with bias. Based on an illustrative milk-processing chain, strain variability caused ∼2-log10 differences in growth between the most and least robust strains and >10-log10 differences after thermal treatment.

IMPORTANCE

Accurate control and realistic prediction of shelf life is complicated by the natural diversity among microbial strains, and limited information on microbiological variability is available for spoilage microorganisms. Therefore, the objectives of the present study were to quantify strain variability, reproduction (biological) variability, and experimental variability with respect to the growth and thermal inactivation kinetics of Lactobacillus plantarum and to quantify the variability in thermal resistance attributed to growth history. The quantitative knowledge obtained on experimental, reproduction, and strain variabilities can be used to improve experimental designs and to adequately select strains for challenge growth and inactivation tests. Moreover, the integration of strain variability in prediction of microbial growth and inactivation kinetics will result in more realistic predictions of L. plantarum dynamics along the food production chain.

Metabolic parameters linked by phenotype microarray to acid resistance profiles of poultry-associated Salmonella enterica

Phenotype microarrays were analyzed for 51 datasets derived from Salmonella enterica. The top 4 serotypes associated with poultry products and one associated with turkey, respectively Typhimurium, Enteritidis, Heidelberg, Infantis and Senftenberg, were represented. Datasets were partitioned initially into two clusters based on ranking by values at pH 4.5 (PM10 A03). Negative control wells were used to establish 90 respiratory units as the point differentiating acid resistance from sensitive strains. Thus, 24 isolates that appeared most acid-resistant were compared initially to 27 that appeared most acid-sensitive (24 × 27 format). Paired cluster analysis was also done and it included the 7 most acid-resistant and -sensitive datasets (7 × 7 format). Statistical analyses of ranked data were then calculated in order of standard deviation, probability value by the Student's t-test and a measure of the magnitude of difference called effect size. Data were reported as significant if, by order of filtering, the following parameters were calculated: i) a standard deviation of 24 respiratory units or greater from all datasets for each chemical, ii) a probability value of less than or equal to 0.03 between clusters and iii) an effect size of at least 0.50 or greater between clusters. Results suggest that between 7.89% and 23.16% of 950 chemicals differentiated acid-resistant isolates from sensitive ones, depending on the format applied. Differences were more evident at the extremes of phenotype using the subset of data in the paired 7 × 7 format. Results thus provide a strategy for selecting compounds for additional research, which may impede the emergence of acid-resistant Salmonella enterica in food.

Quantifying variability on thermal resistance of Listeria monocytogenes

Knowledge of the impact of strain variability and growth history on thermal resistance is needed to provide a realistic prediction and an adequate design of thermal treatments. In the present study, apart from quantifying strain variability on thermal resistance of Listeria monocytogenes, also biological variability and experimental variability were determined to prioritize their importance. Experimental variability was defined as the repeatability of parallel experimental replicates and biological variability was defined as the reproducibility of biologically independent reproductions. Furthermore, the effect of growth history was quantified. The thermal inactivation curves of 20 L. monocytogenes strains were fitted using the modified Weibull model, resulting in total 360 D-value estimates. The D-value ranged from 9 to 30 min at 55 °C; from 0.6 to 4 min at 60 °C; and from 0.08 to 0.6 min at 65 °C. The estimated z-values of all strains ranged from 4.4 to 5.7 °C. The strain variability was ten times higher than the experimental variability and four times higher than the biological variability. Furthermore, the effect of growth history on thermal resistance variability was not significantly different from that of strain variability and was mainly determined by the growth phase.