Effect of various environmental parameters on the recovery of sublethally salt-damaged and acid-damaged Listeria monocytogenes

Efficacy of Manufacturer Recommendations for the Control of Salmonella Typhimurium and Listeria monocytogenes in Food Ink Capsules Utilized in 3D Food Printing Systems

The adoption of 3D food printing systems has allowed for the personalization of food properties such as color, shape, and texture. This study aimed to determine if manufacturer cleaning recommendations for stainless steel food ink capsules utilized in 3D food printers adequately control foodborne pathogens of concern, as the recommendations have not been tested. A cocktail of ∼9 log₁₀ CFU/mL each of Salmonella Typhimurium and Listeria monocytogenes was inoculated onto the interior surface of the capsules. Capsules were either unsoiled or soiled with one of the following: butter, protein powder solution, powdered sugar solution, or a mixture containing all three food components. The prepared capsules underwent one of three hygienic protocols: manual washing (MW), a dishwasher speed cycle (DSC), or a dishwasher heavy cycle (DHC). The interaction effect between DSC and the soil mixture was significant (P = 0.01), with the combination achieving an estimated mean log reduction of 5.28 (95% CI: 4.61, 6.05) for L. monocytogenes and 6.69 (95% CI: 6.03, 7.41) for S. Typhimurium. The DSC was the least effective method of cleaning when compared with MW and the DHC. No significant differences were found by placing capsules on the right or left side of the dishwasher (P > 0.1). The interaction effect between wash type and capsule position was significant (P = 0.0007), with the soil mixture and DSC combination achieving an estimated mean log reduction of 3.48 (95% CI: 2.72, 4.45) for the front-most position versus 7.92 (95% CI: 6.72, 9.31) for the back-most position. Soil matrix, cleaning protocol, and capsule position all significantly impact capsule cleanability and therefore food safety risk. The DHC is recommended, and the corners should be avoided during dishwasher loading. The current study provides practical information for consumers, restaurants, industry, and regulatory industries regarding the best practices for cleaning 3D food printers.

Distribution, diversity and persistence of Listeria monocytogenes in swine slaughterhouses and their association with food and human listeriosis strains

Listeria monocytogenes is the etiological agent of listeriosis, a major foodborne disease and an important public health concern. Contamination of meat with L. monocytogenes occurs frequently at the slaughterhouse. Our aims were; 1) to investigate the distribution of L. monocytogenes in the processing areas of four swine slaughterhouses; 2) to describe the diversity of L. monocytogenes strains by pulsed-field gel electrophoresis; 3) to identify persistent L. monocytogenes strains and describe their distribution; 4) to investigate the associations between persistence of strains and their following characteristics: detection in food isolates, detection in human clinical isolates, and the presence of benzalkonium chloride (BAC) resistance genes. Various operation areas within the four swine slaughterhouses were sampled on four occasions. A total of 2496 samples were analyzed, and L. monocytogenes was successfully isolated from 243 samples. The proportion of positive samples ranged from 32 to 58% in each slaughterhouse and from 24 to 68% in each operation area. Fifty-eight different pulsotypes were identified and eight pulsotypes, present in samples collected during 4 visits, were considered persistent. The persistent pulsotypes were significantly more likely to be detected in food (P < 0.01, exact χ²) and human clinical cases (P < 0.01, exact χ²), respectively. Among pulsotypes harboring the BAC bcrABC resistance cassette or the emrE multidrug transporter gene, 42.8% were persistent compared to 4.5% for pulsotypes without these resistance genes (P < 0.01, exact χ²). Our study highlights the importance of persistent L. monocytogenes strains in the environmental contamination of slaughterhouses, which may lead to repeated contamination of meat products. It also shows that the presence of disinfectants resistance genes is an important contributing factor.

Effect of Hyperosmotic Salt Concentration and Temperature on Viability of Escherichia coli during Cold Storage

Escherichia coli cells were suspended in phosphate-buffered saline solutions (pH 7.4) at physiological (0.9 %) and hyperosmotic (3.5, 5.0, and 10.0 %) concentrations of sodium chloride (NaCl) and stored at 5, 10, 15, 20, and 25 °C up to 48 d. During storage at 5 and 10 °C, viable cell counts decreased approximately from 9 log CFU/ml to 6-7 log CFU/ml, and NaCl showed slight protective effect on the decrease. When stored at 15, 20, and 25 °C, the counts decreased with increases in NaCl concentration and/or storage temperature. The cells in 10.0 % NaCl suspension became nondetectable after storage at 25 °C for 28 d. Under some storage conditions (NaCl ≤ 5 %, 20 and 25 °C), the counts approached constant values, indicating possible adaptation to NaCl. Injured cells were observed at 5.0 and 10.0 % NaCl. However, recovery was observed only at 5.0 % NaCl during storage at 20 °C. In addition, more cells were detected on nonselective medium when incubated at 37 °C than at 25 °C. Higher hyperosmotic NaCl solutions at higher storage temperatures reduced more viable cells of E. coli.

Effects of 405-nm LED Treatment on the Resistance of Listeria monocytogenes to Subsequent Environmental Stresses

Listeria monocytogenes can persist under a wide range of stress conditions, contributing to its ubiquitous distribution and unique pathogenic traits. Light from light-emitting diodes (LEDs) has recently been shown to inactivate various pathogens. Thus, the aim of the present study was to evaluate the effects of light treatment using a 405-nm LED on the subsequent resistance of L. monocytogenes to environmental stresses, including oxidative stress, ultraviolet (UV) irradiation, low temperature, osmotic pressure, simulated gastric fluid (SGF), and bile salts. Following 405-nm LED illumination at 4°C for 150 min, the survival of L. monocytogenes was examined after exposure to oxidative stress (0.04% H2O2), UV irradiation (253.7 nm), low temperature (4°C), osmotic pressure (10, 15, or 20% NaCl), SGF (pH 2.5), or bile salts (2%). The mechanisms responsible for changes in stress tolerance were identified by assessing the transcriptional responses and membrane integrity of L. monocytogenes. The 405-nm LED treatment reduced the resistance of L. monocytogenes to all the stresses tested. Reverse transcription quantitative real-time polymerase chain reaction analysis indicated that the transcription of multiple genes associated with stress resistance, including betL, gbuA, oppA, fri, bsh, and arcA, was reduced by 405-nm LED. Confocal laser scanning microscopy revealed that 405-nm LED treatment disrupted the integrity of the L. monocytogenes cell membrane compared with untreated bacteria. Therefore, 405-nm LED illumination appears to reduce the resistance of L. monocytogenes to various stress conditions. These findings suggest that 405-nm LED treatment could be used to effectively prevent and/or control with L. monocytogenes contamination along the entire food-processing chain, from production to consumption.

Sodium Chloride Does Not Ensure Microbiological Safety of Foods: Cases and Solutions

Addition of salt or salt-containing water to food is one of the oldest and most effective preservation methods in history; indeed, salt-cured foods are generally recognized as microbiologically safe due to their high salinity. However, a number of microbiological risks remain. The microbiological hazards and risks associated with salt-cured foods must be addressed more in-depth as they are likely to be underestimated by previous studies. This review examined a number of scientific reports and articles about the microbiological safety of salt-cured foods, which included salted, brined, pickled, and/or marinated vegetables, meat, and seafood. The following subjects are covered in order: (1) clinical cases and outbreaks attributed to salt-cured foods; (2) the prevalence of foodborne pathogens in such foods; (3) the molecular, physiological, and virulent responses of the pathogens to the presence of NaCl in both laboratory media and food matrices; (4) the survival and fate of microorganisms in salt-cured foods (in the presence/absence of additional processes); and (5) the interaction between NaCl and other stressors in food processes (e.g., acidification, antimicrobials, drying, and heating). The review provides a comprehensive overview of potentially hazardous pathogens associated with salt-cured foods and suggests further research into effective intervention techniques that will reduce their levels in the food chain.

Growth of Stressed Strains of Four Non-O157 Shiga Toxin-Producing Escherichia coli Serogroups in Five Enrichment Broths

The purpose of this study was to evaluate (i) the behavior of several strains of non-O157 Shiga toxin-producing Escherichia coli (STEC) serogroups (O26, O103, O111, and O145) exposed to different stress conditions and (ii) the growth dynamics of stressed and nonstressed non-O157 STEC cells in five enrichment media. STEC strains were exposed to acid, cold, and freeze stresses. Lethal and sublethal injuries were determined by plating in parallel on selective and nonselective agar media. Freeze stress (8 days, 20°C) caused the most lethal (95.3% ± 2.5%) injury, as well as the most sublethal (89.1% ± 8.8%) injury in the surviving population. Growth of stressed and nonstressed pure cultures of non-O157 STEC on modified tryptic soy broth, buffered peptone water (BPW), BPW with sodium pyruvate, Brila, and STEC enrichment broth (SEB) was determined using total viable counts. To compare growth capacities, growth after 7 and 24 h of enrichment was measured; lag phases and maximum growth rates were also calculated. In general, growth on BPW resulted in a short lag phase followed by a high maximum growth rate during the enrichment of all tested strains when using all three stress types. Furthermore, BPW ensured the highest STEC count after 7 h of growth. Supplementing the medium with sodium pyruvate did not improve the growth dynamics. The two selective media, Brila and SEB, were less efficient than BPW, but Brila's enrichment performance was remarkably better than that of SEB. This study shows that irrespective of the effect of background flora, BPW is still recommended for resuscitation of non-O157 STEC.

Real-time PCR detection of Listeria monocytogenes in infant formula and lettuce following macrophage-based isolation and enrichment

AIMS

To develop a rapid detection procedure for Listeria monocytogenes in infant formula and lettuce using a macrophage-based enrichment protocol and real-time PCR.

METHODS AND RESULTS

A macrophage cell culture system was employed for the isolation and enrichment of L. monocytogenes from infant formula and lettuce for subsequent identification using real-time PCR. Macrophage monolayers were exposed to infant formula and lettuce contaminated with a serial dilution series of L. monocytogenes. As few as approx. 10 CFU ml(-1) or g(-1) of L. monocytogenes were detected in infant formula and lettuce after 16 h postinfection by real-time PCR. Internal positive PCR controls were utilized to eliminate the possibility of false-negative results. Co-inoculation with Listeria innocua did not reduce the L. monocytogenes detection sensitivity. Intracellular L. monocytogenes could also be isolated on Listeria selective media from infected macrophage lysates for subsequent confirmation.

CONCLUSIONS

The detection method is highly sensitive and specific for L. monocytogenes in infant formula and lettuce and establishes a rapid identification time of 20 and 48 h for presumptive and confirmatory identification, respectively.

SIGNIFICANCE AND IMPACT OF THE STUDY

The method is a promising alternative to many currently used q-PCR detection methods which employ traditional selective media for enrichment of contaminated food samples. Macrophage enrichment of L. monocytogenes eliminates PCR inhibitory food elements and contaminating food microflora which produce cleaner samples that increase the rapidity and sensitivity of detection.

Role of growth morphology in the formulation of NaCl-based selective media for injury detection of Escherichia coli, Salmonella Typhimurium and Listeria innocua

Sublethal injury (SI) poses major public health concerns since injured cells are responsible for serious limitations in food diagnostics and are susceptible to recovery, often developing adaptive stress responses. Detection of SI is based on the difference in plate counts between non-selective media, which represent the total cell population, and selective media, to which injured cells become sensitive. Selective media for detection of sublethal membrane damage are often based on NaCl supplement, although there is a lack of consensus in the literature about appropriate levels. Planktonic cells are generally used to investigate SI mechanisms, although they often exhibit different stress tolerance than cell colonies in/on solid food (model) systems. In this work, the effect of growth morphology, colony size and concentration of the gelling agent in the growth media, on the maximum non-inhibitory NaCl concentration in the plating medium was assessed for Escherichia coli, Salmonella Typhimurium and Listeria innocua. Stationary phase cultures of planktonic cells and large and small colonies grown in either 1.5% (w/v) xanthan gum-based system or 2.5% (w/v) xanthan gum-based system exhibited significantly different viable counts and osmotolerance. The effect of cell arrangement and xanthan gum percentage in the growth media depended on the microorganism under investigation. Additionally, differences in the maximum non-inhibitory concentration were evident, with 5.0% (w/v) NaCl for the Gram-negative bacteria and 6.5% (w/v), for L. innocua. Different extent of colony shrinkage and morphological damage was observed as NaCl concentration in the plating medium increased. This information will contribute to define NaCl-based selective media for accurate SI detection under realistic scenarios.

Molecular Methods in Food Safety Microbiology: Interpretation and Implications of Nucleic Acid Detection

Because of increasing demand for rapid results, molecular techniques are now applied for the detection of microorganisms in foodstuffs. However, interpretation problems can arise for the results generated by molecular methods in relation to the associated public health risk. Discrepancies between results obtained by molecular and conventional culture methods stem from the difference in target, namely nucleic acids instead of actively growing microorganisms. Nucleic acids constitute 5% to 15% of the dry weight of all living cells and are relatively stable, even after cell death, so they may be present in a food matrix after the foodborne microorganisms have been inactivated. Therefore, interpretation of the public health significance of positive results generated by nucleic acid detection methods warrants some additional consideration. This review discusses the stability of nucleic acids in general and highlights the persistence of microbial nucleic acids after diverse food‐processing techniques based on data from the scientific literature. Considerable amounts of DNA and RNA (intact or fragmented) persist after inactivation of bacteria and viruses by most of the commonly applied treatments in the food industry. An overview of the existing adaptations for molecular assays to cope with these problems is provided, including large fragment amplification, flotation, (enzymatic) pretreatment, and various binding assays. Finally, the negligible risks of ingesting free microbial nucleic acids are discussed and this review ends with the future perspectives of molecular methods such as next‐generation sequencing in diagnostic and source attribution food microbiology.

Effect of cell immobilization on heat-induced sublethal injury of Escherichia coli, Salmonella Typhimurium and Listeria innocua

The occurrence of sublethally injured cells in foods poses major public health concerns and is an essential aspect when assessing the microbial response to food preservation strategies, yet there is limited research dealing with its specific implications for mild heating. All available studies so far have been performed in broths colonized by planktonic cells, although their susceptibility to lethal agents has often been reported to be markedly different to the stress tolerance of cell colonies developed in solid foods. In this work, the effect of planktonic and colony growth, as well as the influence of colony density on sublethal injury induced by mild heating of Escherichia coli, Salmonella Typhimurium and Listeria innocua were assessed in food model systems. Detection of injured survivors relied on their inability to form visible colonies on salt-based selective media, which do not affect the growth of healthy cells. Sublethal injury (SI) increased rapidly with shorter exposure times and afterwards, decreased progressively, suggesting a mechanism of cumulative damage triggering lethal instead of SI. Cell arrangement affected the degree of SI, higher values being generally found for gelified systems, although the effect of colony density depended on the target microorganism. This information is essential for optimizing the design of food safety assurance systems.

Survival of Campylobacter jejuni in chicken meat at frozen storage temperatures

The aim of this study was to determine the survival of Campylobacter jejuni in chicken meat samples at frozen temperatures and given length of incubation and to determine the impact of aerobic bacteria on the survival of C. jejuni. The chicken meat samples were inoculated with C. jejuni NCTC 11351 suspensions and stored in bags at temperatures of -20°C and -70°C. The mean value of C. jejuni from meat samples decreased from 7.52 log10 CFU/g after 30 minutes of incubation at ambient temperature, to 3.87 log10 CFU/g on the eighth week of incubation at -20°C, and to 3.78 log10 CFU/g at incubation at -70°C after the same incubation period. Both freezing temperatures, -20°C and -70°C, decreased the number of campylobacters. The presence of aerobic mesophilic bacteria did not influence the survival of C. jejuni in chicken meet samples. Keeping poultry meat at freezing temperatures is important for the reduction of C. jejuni, which has a strong influence on the prevention of occurrence of campylobacteriosis in humans.

Behaviour of Listeria monocytogenes isolates through gastro-intestinal tract passage simulation, before and after two sub-lethal stresses

The effects of previous exposure to sub-lethal acidic and osmotic stresses on the survival of Listeria monocytogenes during exposure to gastro-intestinal (GI) tract simulation, was investigated. Six L. monocytogenes strains isolated from cheeses were selected and exposed to high salt concentrations or acidic conditions and their viability compared in quick and slow digestions. The results demonstrated that (i) all isolates were more sensitive to the exposure to acidic than to osmotic sub-lethal conditions (ii) significant differences (p < 0.05) between the two types of digestion were observed; in slow digestion, the log reduction was higher for all the tested isolates (iii) all isolates were inhibited in the presence of bile salts for both types of digestion (iv) differences between quick and slow digestion were not observed (p > 0.05) after exposure to either osmotic or acidic stress (v) a higher cellular inactivation (p < 0.001) was observed during the passage through the GI tract simulation after exposure to osmotic than to acidic stresses and (vi) neither osmotic nor acidic sub-lethal stresses conferred resistance to simulated GI tract conditions.

Quantification methods for Bacillus cereus vegetative cells and spores in the gastrointestinal environment

There is an interest to understand the fate and behaviour of the food-borne pathogen Bacillus cereus in the gut, a challenging environment with a high bacterial background. We evaluated the current detection methods to select an appropriate strategy for B. cereus monitoring during gastrointestinal experiments. Application of quantitative real-time PCR (qPCR) in a gastrointestinal matrix required careful selection of the qPCR reaction and elaborate optimization of the DNA extraction protocol. Primer competition and depletion problems associated with qPCR reactions targeting general 16S rRNA gene can be avoided by the selection of a target sequence that is unique for and widespread among the target bacteria, such as the toxin gene nheB in the case of pathogenic B. cereus. Enumeration of B. cereus during the ileum phase was impossible by plating due to overgrowth by intestinal bacteria, while a carefully optimized qPCR enabled specific detection and quantification of B. cereus. On the other hand, plating allowed the distinction of viable, injured and dead bacteria and the germination of spores, which was not possible with qPCR. In conclusion, both plating and qPCR were necessary to yield the maximal information regarding the viability and physiology of the B. cereus population in various gastrointestinal compartments.

The overgrowth of Listeria monocytogenes by other Listeria spp. in food samples undergoing enrichment cultivation has a nutritional basis

The isolation of Listeria monocytogenes from food is carried out using a double enrichment. In cases where multiple Listeria species are present within the original sample, L. monocytogenes can be overgrown during enrichment by other species of listeria present in the original sample. From a practical perspective, this can result in a false negative or complicate the ability of public health investigators to match food and clinical isolates. We have further investigated this phenomenon by analysing the growth kinetics of single species and pairs of different species over the ISO 11290-1 enrichment process. The overgrowth of a strain of L. monocytogenes by a strain of Listeria innocua resulted primarily from interactions which occurred in late exponential phase, where it was observed that growth of both strains stopped when the dominant strain reached stationary phase. In a second mixed culture, the dominant L. monocytogenes strain suppressed the exponential growth rate of the second Listeria welshimeri strain. Both findings suggest that the overgrowth could partially be explained in terms of a nutritional competition. Multi-factor analysis of Fraser broth constituents and growth temperatures using both stressed and non-stressed inoculants failed to identify any single factor in the ISO 11290-1 methodology which would contribute to the overgrowth phenomenon in our model system. Furthermore, species was not a significant factor in observed differences in growth parameters among a wider array of strains which had been stressed or not stressed prior to grown in Fraser broths, even though some strains had significantly faster growth rates than others. Limiting diffusion in Fraser broth through the addition of agar significantly reduced the extent of the overgrowth in experiments using mixtures of strains originally isolated from foods where overgrowth had been previously observed. Taken together, these findings support that the overgrowth phenomenon in most instances has a nutritional basis.

Establishment of procedures provoking sub-lethal injury of Listeria monocytogenes, Campylobacter jejuni and Escherichia coli O157 to serve method performance testing

In this study procedures provoking sub-lethal injury for three different pathogens are described which may be used in determination of accuracy and robustness of methods, comparison studies and or validation of rapid detection methods. Three common food-borne pathogens were used, Listeria monocytogenes, Campylobacter jejuni and Escherichia coli O157. The pathogens were exposed to heat stress, cold stress, freeze stress, acid stress, oxidative stress and "food" stress. Sub-lethal injury was determined by plating in parallel on selective and non-selective media. The statistical significant differences in enumeration were established. The choice of stress to create sub-lethal injury to cells depended on the fact that the procedure must be easy to handle, repeatable and relevant for stress conditions in foods, but also on the micro-organism itself. Oxidative stress (1000 microM H(2)O(2)) was chosen to impose sub-lethal injury on L. monocytogenes and a specific "food" stress for E. coli O157. For C. jejuni a specific "food" stress as well as the oxidative stress (750 microM H(2)O(2)) were capable of creating a standardized procedure of provoking injury.

Acid adaptation and survival of Listeria monocytogenes in Italian-style soft cheeses

AIMS

The ability of Listeria monocytogenes to survive and grow at high salt concentrations and low pH makes it a potential hazard after the consumption of milk and dairy products, often implicated in severe outbreaks of listeriosis. This study was designed to evaluate the behaviour of L. monocytogenes in traditional acid and salted Italian-style soft cheeses and to investigate whether Listeria occurrence and growth in these environments may represent a potential increase of hazard.

METHODS AND RESULTS

A first approach was addressed to in vitro evaluate survival, acid tolerance response, ability to produce biofilm, and capability to invade intestinal-like cells of a L. monocytogenes strain grown under experimental conditions mimicking environmental features that this pathogen encounters in soft cheeses (such as acid pH and high NaCl content). A second set of experiments was performed to monitor, during the storage at 4 degrees C, the survival of acid-adapted and nonadapted Listeriae in artificially contaminated soft cheeses. Both acid tolerance response and invasion efficiency of acid-adapted bacteria resulted in an increase, even when bacteria were simultaneously pre-exposed to increasing salt stress. The contamination of cheeses with acid-adapted and nonadapted bacteria evidenced in all products a good survival. A significant increased survival, the recovery of bacterial cells highly resistant to lethal pH exposure, and the prevalence of filamentous structures were observed in crescenza cheese during the storage.

CONCLUSIONS

The Listeria survival and acid pH tolerance observed during refrigerated storage are probably related to the intrinsic acid and saline features of soft cheeses analysed.

SIGNIFICANCE AND IMPACT OF THE STUDY

Italian soft cheeses tested may represent a potential hazard for the recovery of acid-adapted L. monocytogenes cells with enhanced ability to adhere to inert surfaces and/or to penetrate host cells.

Performance evaluation of a low conductive growth medium (LCGM) for growth of healthy and stressed Listeria monocytogenes and other common bacterial species

The performance of a low conductive growth medium (LCGM) (conductivity of <1300 microS) was evaluated for its ability to support growth of food borne bacterial pathogens including Listeria monocytogenes and to determine the expression of the two key virulence proteins in L. monocytogenes for possible applications in an impedance-based microfluidic biochip detection platform. Growth of Listeria was monitored spectrophotometrically and the lag phase, generation time, growth rate and maximum population density were determined using the Gompertz equation. LCGM had a lag phase of 2.3 h and showed a higher cell density compared to Luria Bertini (LB) broth. Length of lag phase was highly dependent on initial inoculum concentrations. The changes in conductivity with respect to growth in the low conductive medium were monitored using a conductivity probe. L. monocytogenes growth could be detected within 2 h (0.1 mS) in LCGM and within 6 h in LB. The performance of the media was also evaluated for the recovery of Listeria cells exposed to various stresses as 42 degrees C for 1, 2 or 6 h, an osmotic stress in 10.5% NaCl, an acidic stress at pH 2, 3 or 5 and a combined stress of 10.5% NaCl, pH 5 and 1 h exposure at 42 degrees C. The recovery rate was comparable with that of Tryptic soy broth containing yeast extract (TSBYE). L. monocytogenes in LCGM supported the expression of two key virulence markers, actin polymerization protein (ActA) and internalin B (InlB), which could be detected using specific antibodies. In general LCGM also supported the growth of several other bacterial species suggesting its implication in microbial quality monitoring of products. In conclusion, LCGM is a sensitive low conductive medium that supports the growth as well as the expression of virulence markers for potential applications in sensitive detection of L. monocytogenes or other food borne pathogens in impedance-based sensor platform.

Conductivity and pH dual detection of growth profile of healthy and stressed Listeria monocytogenes

In this study, growth of Listeria monocytogenes in a low conductivity growth medium (LCGM) was simultaneously monitored by conductivity and pH measurements. Detection times obtained from the conductivity and pH growth curves were inversely related to the initial concentration of L. monocytogenes in the medium. Linear responses were found by plotting detection times obtained from both conductivity and pH growth curves as a function of initial cell concentration in the range of 10(2) to 10(7) cfu/mL. The detection time was approximately 12 and 2 h for 10(2) and 10(7) cfu/mL of viable L. monocytogenes, respectively, using the conductivity growth curves, whereas it was approximately 1 h less using the pH growth curves. This dual detection system was used for evaluating the growth of acid-, temperature-, and salt-treated L. monocytogenes in the medium. Acid stress at pH 2 and 3 for 3 h caused approximately 12 and 4 h delay in the detection time on pH growth curves, while stress at pH 5 for 3 h did not cause a significant delay in detection time. Delay in detection times was also observed for L. monocytogenes cells exposed to 45 degrees C for more than 1 h (2 and 6 h). Exposure to 10% NaCl for 3 h did not cause visible delay in the detection time. These observations on detection times for stressed L. monocytogenes had a consistent trend with the cell number decrease determined by surface plating method.

Influence of the sigB gene on the cold stress survival and subsequent recovery of two Listeria monocytogenes serotypes

The influence of serotype and the role of the sigB gene of Listeria monocytogenes during the survival and recovery on different substrates were determined. Wild-type and sigB mutants of two serotypes of L. monocytogenes were inoculated into buffer and onto beef steaks, and incubated at 4 degrees C for 8 weeks during which samples were removed and Listeria numbers determined. Growth kinetics of stationary phase wild-type and sigB mutant cells were compared, without prechilling and after prechilling at 4 degrees C. The two serotypes had similar survival capabilities under the conditions examined, and the sigB gene was influential in survival of chill stress, but was dependent upon additional nutritional factors. Prechilling cells prior to growth extended the lag phase of both strains, and this lag phase extension was compounded by the absence of a functional sigB gene. In conclusion, the sigB gene is involved in the survival and recovery from chill stress by the two serotypes tested. Additional factors such as previous growth conditions, nutritional requirements and serotype susceptibility are also contributory. This study adds relevant information regarding the influence of the sigB gene, in conjunction with the historical growth conditions and serotype differences. Understanding the significance of these factors may be useful in creating improved recovery systems for the detection of L. monocytogenes from at-risk foods.

Different cellular fatty acid pattern behaviours of two strains of Listeria monocytogenes Scott A and CNL 895807 under different temperature and salinity conditions

Cells of two strains of Listeria monocytogenes CNL 895807 and Scott A were grown to late exponential phase at different growth temperatures (37, 20 and 4 degrees C) with or without NaCl (7%), and their fatty acid compositions were analysed. The results showed that low thermal adaptation response of L. monocytogenes CNL was different than that of the Scott A strain, and it was based on both an increase of anteiso-branched-chain fatty acids and a significant decrease of straight-chain fatty acids. However, the main modifications observed in the Scott A strain when grown at a low temperature were a decrease of the proportion of ai17:0 and an increase of ai15:0. In hyperosmotic medium and over the entire temperature range (4 degrees C, 20 degrees C and 37 degrees C) the two L. monocytogenes strains showed a cellular fatty acid profile dominated by ai15:0. In addition, a decrease of the two major straight-chain fatty acids (14:0 and 16:0) was observed in the CNL strain. These results demonstrated that the CNL strain showed different behaviours of low thermal and salt adaptation to maintain membrane fluidity, which are based both on an increase of anteiso-branched-chain fatty acids, and a significant decrease of straight-chain fatty acids.

Salt stress proteins induced in Listeria monocytogenes

The ability of Listeria monocytogenes to tolerate salt stress is of particular importance, as this pathogen is often exposed to such environments during both food processing and food preservation. In order to understand the survival mechanisms of L. monocytogenes, an initial approach using two-dimensional polyacrylamide gel electrophoresis was performed to analyze the pattern of protein synthesis in response to salt stress. Of 400 to 500 visible proteins, the synthesis of 40 proteins (P < 0.05) was repressed or induced at a higher rate during salt stress. Some of the proteins were identified on the basis of mass spectrometry or N-terminal sequence analysis and database searching. Twelve proteins showing high induction after salt stress were similar to general stress proteins (Ctc and DnaK), transporters (GbuA and mannose-specific phosphotransferase system enzyme IIAB), and general metabolism proteins (alanine dehydrogenase, CcpA, CysK, EF-Tu, Gap, GuaB, PdhA, and PdhD).