Heat resistance of different serovars ofListeria monocytogenes

Listeria monocytogenes - How This Pathogen Survives in Food-Production Environments?

The foodborne pathogen Listeria monocytogenes is the causative agent of human listeriosis, a severe disease, especially dangerous for the elderly, pregnant women, and newborns. Although this infection is comparatively rare, it is often associated with a significant mortality rate of 20-30% worldwide. Therefore, this microorganism has an important impact on food safety. L. monocytogenes can adapt, survive and even grow over a wide range of food production environmental stress conditions such as temperatures, low and high pH, high salt concentration, ultraviolet lights, presence of biocides and heavy metals. Furthermore, this bacterium is also able to form biofilm structures on a variety of surfaces in food production environments which makes it difficult to remove and allows it to persist for a long time. This increases the risk of contamination of food production facilities and finally foods. The present review focuses on the key issues related to the molecular mechanisms of the pathogen survival and adaptation to adverse environmental conditions. Knowledge and understanding of the L. monocytogenes adaptation approaches to environmental stress factors will have a significant influence on the development of new, efficient, and cost-effective methods of the pathogen control in the food industry, which is critical to ensure food production safety.

Behavior of Native Food Isolates of Listeria monocytogenes and Listeria innocua under the Influence of Selected Cultural Attributes and Heat and Cold Treatments

The present investigation assesses the influence of cultural attributes and heat and cold treatments on the behaviour of native toxigenic L. monocytogenes CFR 1302 and non-toxigenic L. innocua CFR 1304 in selected medium. The growth responses of L. monocytogenes and L. innocua under the influencing factors of storage temperature (10-40°C), pH level (5.5-7.5), and storage period (6-48 h) in brain heart infusion and nutrient broths revealed closeness between observed and predicted populations. Response surface plots were generated for the growth behavior of the two test cultures as a function of pH level. The toxigenic L. monocytogenes CFR 1302 could reach a higher viable population. The effect of heat treatment on Listeria spp. in selected five heating menstra showed the lowest D-value of 3.7 min at 60°C for L. innocua CFR 1304 in Milli-Q water to the highest of 8.4 min at 56°C for L. monocytogenes CFR 1302 in skim milk. The average z-value across the heating menstra for L. monocytogenes was 27.3°C as against that of 22°C for L. innocua. In the case of cold treatment, storage of 4 and 8°C resulted in appreciable increase in counts of L. monocytogenes CFR 1302 from the initial inoculum introduced in selected media. At -20°C, there was a slight decrease in the viable population. The research data helps to predict the viable populations of L. monocytogenes as a part of risk assessment in the food chain. This is of significance in providing safe and healthy food to human population.

Listeria Species Occurrence and Associated Factors and Antibiogram of Listeria monocytogenes in Beef at Abattoirs, Butchers, and Restaurants in Ambo and Holeta in Ethiopia

PURPOSE

Listeriosis is one of the globally distributed foodborne diseases with the highest fatality rate. The objectives of this study were to isolate and identify Listeria species, assess factors for contamination of beef, and antibiogram of Listeria monocytogenes in Ambo and Holeta towns, Central Ethiopia.

MATERIALS AND METHODS

A total of 450 meat samples were collected from abattoirs (n=150), butchers (n=150), and restaurants (n=150) for isolation and identification of Listeria species. Logistic regression analysis was used to assess the association between the occurrence of Listeria species in meat and potential risk factors. The antimicrobial susceptibility test was done using the Kirby Bauer test.

RESULTS

The overall occurrence of Listeria species in Ambo and Holeta towns was 28.4% (128/450; 95% confidence interval [CI]: 24.3-32.9%). The isolation rate of Listeria monocytogenes was 4.4%, Listeria ivanovii 2.2%, Listeria seeligeri 1.8%, Listeria welshimeri 3.8%, Listeria innocua 6.2%, and Listeria grayi 10.2%. The probability of contamination of meat in butchers and restaurants was higher in Holeta than Ambo [OR=3.4; 95%; p=0.001], in dry than wet season [OR=5.2; p=0.009], and where the hygiene of cutting boards was poor (OR=7.7; p=0.008). Of the 20 Listeria monocytogenes isolates, 80%, 70%, 60%, and 55% were resistant to oxacillin, amikacin, and nalidixic acid, chloramphenicol, and tetracycline, respectively. The Listeria monocytogenes isolates were 95%, 90%, and 85% susceptible to amoxicillin, vancomycin, and clindamycin, respectively. About 95% of Listeria monocytogenes isolates were multidrug-resistant. One isolate (5%) had developed resistance to 10 classes of antimicrobial drugs.

CONCLUSION

Listeria species are widespread and study towns, season, and hygiene of cutting boards are independent predictors of isolation of Listeria species. Multidrug resistance among Listeria monocytogenes was very high. Therefore, adequate cooking of meat, regular training of beef handlers, prudent use of drugs, and further molecular studies on Listeria species are important.

Resistance of Listeria monocytogenes to Stress Conditions Encountered in Food and Food Processing Environments

Listeria monocytogenes is a human food-borne facultative intracellular pathogen that is resistant to a wide range of stress conditions. As a consequence, L. monocytogenes is extremely difficult to control along the entire food chain from production to storage and consumption. Frequent and recent outbreaks of L. monocytogenes infections illustrate that current measures of decontamination and preservation are suboptimal to control L. monocytogenes in food. In order to develop efficient measures to prevent contamination during processing and control growth during storage of food it is crucial to understand the mechanisms utilized by L. monocytogenes to tolerate the stress conditions in food matrices and food processing environments. Food-related stress conditions encountered by L. monocytogenes along the food chain are acidity, oxidative and osmotic stress, low or high temperatures, presence of bacteriocins and other preserving additives, and stresses as a consequence of applying alternative decontamination and preservation technologies such high hydrostatic pressure, pulsed and continuous UV light, pulsed electric fields (PEF). This review is aimed at providing a summary of the current knowledge on the response of L. monocytogenes toward these stresses and the mechanisms of stress resistance employed by this important food-borne bacterium. Circumstances when L. monocytogenes cells become more sensitive or more resistant are mentioned and existence of a cross-resistance when multiple stresses are present is pointed out.

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.

Low, medium, and high heat tolerant strains of Listeria monocytogenes and increased heat stress resistance after exposure to sublethal heat

A group of 37 strains representing all 13 serotypes of Listeria monocytogenes with an initial cell density of 10(7) CFU/ml were analyzed for their heat tolerance at 60°C for 10 min. These L. monocytogenes strains were categorized into three heat tolerance groups: low (<2 log CFU/ml survival), medium (2 to 4 log CFU/ml survival), and high (4 to 6 log CFU/ml survival). Serotype 1/2a strains had relatively low heat tolerance; seven of the eight tested strains were classified as low heat tolerant. Of the two serotype 1/2b strains tested, one was very heat sensitive (not detectable) and the other was very heat resistant (5.4 log CFU/ml survival). Among the 16 serotype 4b strains, survival ranged from not detectable to 4 log CFU/ml. When one L. monocytogenes strain from each heat tolerance group was subjected to sublethal heat stress at 48°C for 30 or 60 min, the survival of heat-stressed cells at 60°C for 10 min increased by 5 log CFU/ml (D60°C-values nearly doubled) compared with the nonstressed control cells. Sublethal heat stress at 48°C for 60 or 90 min increased the lag phase of L. monocytogenes in tryptic soy broth supplemented with 0.6% yeast extract at room temperature by 3 to 5 h compared with nonstressed control cells. The heat stress adaptation in L. monocytogenes was reversed after 2 h at room temperature but was maintained for up to 24 h at 4°C. Our results indicate a high diversity in heat tolerance among strains of L. monocytogenes, and once acquired this heat stress adaptation persists after cooling, which should be taken into account while conducting risk analyses for this pathogen.

Growth and stress resistance variation in culture broth among Listeria monocytogenes strains of various serotypes and origins

Twenty-five Listeria monocytogenes strains of various serotypes and sources, including clinical and food isolates associated with the same outbreaks, were characterized and compared based on growth rates and heat and acid death rates. Growth was monitored in tryptic soy broth supplemented with 0.6% yeast extract (TSBYE) at 4 and 30 degrees C for 32 days and 20 h, respectively. Heat and acid stress responses in TSBYE heated to 55 degrees C or acidified to pH 3.0 with lactic acid were evaluated for 240 or 120 min, respectively. Extensive variation in growth and stress resistance was observed among the tested strains. Growth rate differences were less evident at 30 than at 4 degrees C, where growth rates (log CFU per milliliter per day) ranged from 0.28 to 0.43. Thermal and acid death rates (log CFU per milliliter per minute) ranged from -0.023 to -0.052 and from -0.012 to -0.134, respectively. Serotype appeared to play a significant role (P < 0.05) only with respect to the heat resistance of the organism. Serotype 4b isolates as a group had lower heat resistance than did isolates representing all other serotypes combined. Although no clear origin-related (food versus clinical) trends were observed under the tested conditions, outbreak-related isolates of serotype 4b had lower acid death rates (higher acid resistance) (P < 0.05) than did the rest of the strains belonging to this serotype. Strain Scott A exhibited slow growth at 4 degrees C and low acid resistance, behavior that was distinct among both clinical and serotype 4b isolates. The results of this study highlight the risks associated with extrapolation to other strains of findings obtained with only one strain of L. monocytogenes. This information should be useful when test strains are to be selected for the evaluation of antimicrobial alternatives in ready-to-eat meat and other food products and when risk assessments are to be conducted.

A predictive model for heat inactivation of Listeria monocytogenes biofilm on stainless steel

Heat treatment of potential biofilm-forming sites is sometimes used for control of Listeria monocytogenes in food processing plants. However, little information is available on the heat treatment required to kill L. monocytogenes present in biofilms. The purpose of this study was to develop a predictive model for the heat inactivation of L. monocytogenes in monoculture biofilms (strains Scott A and 3990) and in biofilms with competing bacteria (Pseudomonas sp. and Pantoea agglomerans) formed on stainless steel in the presence of food-derived soil. Biofilms were produced on stainless steel coupons with diluted tryptic soy broth incubated for 48 h at 25 degrees C. Duplicate biofilm samples were heat treated for 1, 3, 5, and 15 min at 70, 72, 75, 77, and 80 degrees C and tested for survivors using enrichment culture. The experiment was repeated six times. A predictive model was developed using logistic regression analysis of the fraction negative data. Plots showing the probability of L. monocytogenes inactivation in biofilms after heat treatment were generated from the predictive equation. The predictive model revealed that hot water sanitation of stainless steel can be effective for inactivating L. monocytogenes in a biofilm on stainless steel if time and temperature are controlled. For example, to obtain a 75% probability of total inactivation of L. monocytogenes 3990 biofilm, a heat treatment of 80 degrees C for 11.7 min is required. The model provides processors with a risk management tool that provides predicted probabilities of L. monocytogenes inactivation and allows a choice of three heat resistance assumptions. The predictive model was validated using a five-strain cocktail of L. monocytogenes in the presence of food soil.

Thermal inactivation, growth, and survival studies of Listeria monocytogenes strains belonging to three distinct genotypic lineages

Twenty-one Listeria monocytogenes strains belonging to three different genotypic lineages were evaluated for differences between lineages and between individual strains with respect to thermal inactivation, growth, and survival. Three sets of heat inactivation conditions (60 degrees C, pH 6.0, and 0.5 M lactate; 55 degrees C, pH 6.0, and 0.5 M lactate; and 50 degrees C, pH 4.0, and 0.5 M lactate) were used on strains grown in modified brain heart infusion (BHI) broth with and without glucose. Two sets of growth conditions (35 degrees C, pH 6.5, and 0.1 M lactate and 5 degrees C, pH 6.5, and 0.1 M lactate) were used with modified BHI broths to determine lag phases and exponential growth rates. Two sets of conditions (28 degrees C, pH 4.0, and 1 M lactate and 28 degrees C, pH 4.5, and 0.5 M lactate) were used with modified BHI broth to determine survival times (D-values). Thermal inactivation D-values were consistently lowest for lineage III, but differences were not significant for any set of conditions tested. Some significant differences were observed between lineages with respect to some of the growth and survival conditions tested. Extensive strain-to-strain variation was observed for all parameters tested. Average coefficients of variation for the thermal inactivation, growth, and survival studies were 0.31, 0.18, and 0.26, respectively. Strain-to-strain variations were approximately equal to the uncertainties associated with the analytical procedures. The results obtained indicate a diversity among strains encountered in food processing that must be accounted for in process calculations and risk assessments.

Can food-related environmental factors induce different behaviour in two key serovars, 4b and 1/2a, of Listeria monocytogenes?

Listeria monocytogenes isolates (81 in total; 42 isolated from cases of human listeriosis: 39 isolated from food), belonging to serovars 1/2a or 4b, were studied for any group differences between serovars to selected factors associated with foods (two bacteriocins and mild heat treatment), growth kinetics at 37 degrees C and pathogenicity for chick embryos. The isolates were tested for sensitivity to two bacteriocins at 4 degrees C and 37 degrees C, and were tested for the remaining parameters both before and after exposure to cold storage (4 degrees C) with starvation. In addition, the isolates were typed using pulsed field gel electrophoresis (PFGE), multilocus enzyme electrophoresis (MEE) and phage typing to find any correlation between the types and group differences in the chosen parameters. Considerable strain diversity within each L. monocytogenes serovar with respect to the chosen parameters was observed, especially after exposure to cold storage. Nevertheless, the serovar 1/2a isolates, as a group, tended to be more resistant to the two antilisterial bacteriocins at 4 degrees C than the group of serovar 4b isolates. In contrast, after cold storage at 4 degrees C, L. monocytogenes serovar 4b isolates, as a group, tended to be more resistant to heat treatment at 60 degrees C than the group of 1/2a isolates. In addition, the serovar 4b group tended to have shorter lag phases and higher pathogenicity, when transferred from cold storage to body temperature (37 degrees C), than the group of serovar 1/2a isolates. No correlation between PFGE-, MEE- and phage-types and the tested parameters was found. Although the above serovar-related differences were observed only when mean values of the groups were compared (not all isolates within each group followed the group pattern), the results indicate interesting directions for further research.

Heat resistance of Listeria monocytogenes

The heat resistance data on Listeria monocytogenes in culture media and foods are summarized. Most heat resistance data for foods have been obtained in dairy, meat, poultry, and egg products. Limited data have been published on seafood, fruits, and vegetables. The methodologies employed have evolved over time; hence data from earlier experiments are not directly comparable to more recent studies. Many factors influence the heat resistance of L. monocytogenes. Variation exists among different strains in their ability to withstand heat treatment. In addition, heat resistance is influenced by age of the culture, growth conditions, recovery media, and characteristics of foods such as salt content, a(w), acidity, and the presence of other inhibitors. Listeriae are more heat resistant than most other nonspore-forming foodborne pathogens, and thus, processing recommendations based on data from experiments with Salmonella spp. or pathogenic Escherichia coli may not be sufficient to eliminate similar numbers of L. monocytogenes. The data provided in this review may prove useful for food processors in determining appropriate times and temperatures for producing foods free of vegetative pathogens.

Effects of above-optimum growth temperature and cell morphology on thermotolerance of Listeria monocytogenes cells suspended in bovine milk

The thermotolerances of two different cell forms of Listeria monocytogenes (serotype 4b) grown at 37 and 42.8 degrees C in commercially pasteurized and laboratory-tyndallized whole milk (WM) were investigated. Test strains, after growth at 37 or 42.8 degreesC, were suspended in WM at concentrations of approximately 1.5 x 10(8) to 3.0 x 10(8) cells/ml and were then heated at 56, 60, and 63 degrees C for various exposure times. Survival was determined by enumeration on tryptone-soya-yeast extract agar and Listeria selective agar, and D values (decimal reduction times) and Z values (numbers of degrees Celsius required to cause a 10-fold change in the D value) were calculated. Higher average recovery and higher D values (i.e., seen as a 2.5- to 3-fold increase in thermotolerance) were obtained when cells were grown at 42.8 degrees C prior to heat treatment. A relationship was observed between thermotolerance and cell morphology of L. monocytogenes. Atypical Listeria cell types (consisting predominantly of long cell chains measuring up to 60 micron in length) associated with rough (R) culture variants were shown to be 1.2-fold more thermotolerant than the typical dispersed cell form associated with normal smooth (S) cultures (P Collapse

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MESH Headings

• Animals
• Cattle
• Heat-Shock Proteins/biosynthesis
• Hot Temperature
• Listeria monocytogenes/cytology
• Listeria monocytogenes/growth & development
• Listeria monocytogenes/pathogenicity
• Milk/microbiology
• Sterilization
• Temperature
• Virulence

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Affiliation(s)

N J Rowan Department of Bioscience and Biotechnology, University of Strathclyde, Glasgow, Scotland.
J G Anderson

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Hazard identification in swine slaughter with respect to foodborne bacteria

Swine slaughter is an open process with many opportunities for the contamination of the pork carcass with potentially pathogenic bacteria; however, it does not contain any point where hazards are completely eliminated. Data on the prevalence of various pathogenic bacteria (Aeromonas hydrophila, Campylobacter coli/jejuni, Listeria monocytogenes, Salmonella spp., Staphylococcus aureus and Yersinia enterocolitica) in pigs, their growth and survival characteristics and ability to become established on the slaughter line are presented. The presentation covers the processing steps from lairage to chilling and is based on swine slaughter practices in Denmark, Norway and Sweden. The major contamination points during swine slaughter are pig-related, such as faecal and pharyngeal, and environmental. HACCP (Hazard Analysis Critical Control Point) and GMP (Good Manufacturing Practice) in swine slaughter must be focused on limiting this spread. The pathogenic bacteria show differences in their general mechanism of distribution. The major contamination source of Campylobacter spp., Salmonella spp. and Y. enterocolitica is the pig, and the contamination of carcasses with these bacteria may be limited, provided that only strict slaughtering procedures are used. Other organisms such as Aeromonas spp., L. moncytogenes/Listeria spp. and S. aureus can be endemic in the processing environment. Since endemic bacteria can be controlled by proper cleaning and disinfection, these organisms are useful as indicators for the success of GMP rules. The following affiliation to CPs or CCPs made for specific steps during slaughter and dressing may serve as a guidance: (i) lairage (CP), (ii) killing (CP), (iii) scalding (CP), (iv) dehairing (CP), (v) singeing/flaming (CP), (vi) polishing (CP), (vii) circumanal incision and removal of the intestines (CCP), (viii) excision of the tongue, pharynx, and in particular the tonsils (CCP), (ix) splitting (CP), (x) post mortem inspection procedures (CCP) and (xi) deboning of the head (CCP).