Viable but non-culturable Listeria monocytogenes on parsley leaves and absence of recovery to a culturable state

AIMS: To investigate the presence of viable but non-culturable Listeria monocytogenes during survival on parsley leaves under low relative humidity (RH) and to evaluate the ability of L. monocytogenes to recover from VBNC to culturable state under satured humidity. METHODS AND RESULTS: Under low RH (47-69%) on parsley leaves, the initial number of L. monocytogenes populations counted on non selective media (10(9) L. monocytogenes per leaf on TSA) was reduced by 6 log10 scales in 15 days, whereas number of viable L. monocytogenes counted under the microscope was reduced by 3-4 log10 scales, indicating the presence of VBNC cells. This was demonstrated on three L. monocytogenes strains (EGDe, Bug 1995 and LmP60). Changing from low to 100% RH permitted an increase of the culturable counts of L. monocytogenes and this growth was observed only when residual culturable cells were present. Moreover, VBNC L. monocytogenes inoculated on parsley leaves did not become culturable after incubation under 100% RH. CONCLUSIONS: Dry conditions induced VBNC L. monocytogenes on parsley leaves but these VBNC were likely unable to recover culturability after transfer to satured humidity. SIGNIFICANCE AND IMPACT OF STUDY: Enumeration on culture media presumably under-estimates the number of viable L. monocytogenes on fresh produce after exposure to low RH.     

Expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments

AIMS: To study the expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies (MAbs) C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments. METHODS AND RESULTS: The reaction patterns of antibodies to L. monocytogenes held in stressful environments for a short duration (3 h) or grown for extended periods (16-72 h) were investigated. During both short or prolonged exposure to stress environments of high temperature (45 degrees C) and NaCl (>1.5%, w/v), reactions of whole cells of L. monocytogenes to antibodies were severely affected as determined by ELISA and by the reduced expression of the antibody-reactive 66 kDa antigen in the Western blot assay. Conversely, cold (4-15 degrees C) or acid (pH 2-3) stress environments had very little effect on antigen expression or antibody reaction. Additionally, heat-killed cells showed reduced reactions to these antibodies when compared with unheated cells. Artificially created stress environments in hotdog slurry also affected the antigen expression in L. monocytogenes. Immunoelectron microscopy revealed that the antibody-reactive antigens were uniformly present on the surface of the cells. Morphological characteristics following growth in stressed environments revealed that heat stress at 45 degrees C caused L. monocytogenes cells to be elongated and to form clumps; whereas, osmotic stress (5.5% NaCl, w/v) caused filamentous appearance with multiple septa along the length of the cell. CONCLUSIONS: These results indicated that MAb C11E9 or EM-7G1 could detect L. monocytogenes from cold or acid-stress environments; however, they may show weaker reactions with heat or osmotically stressed cells or cells grown at 4 degrees C. SIGNIFICANCE AND IMPACT OF THE STUDY: Bacteria in food are routinely subjected to various stresses, induced by cold, heat, salt or acid during processing and storage. Whether stresses would modify the expression of cellular antigens of L. monocytogenes is of a great concern for immunodetections in food products.     

Evaluation of ALOA plating medium for its suitability to recover high pressure-injured Listeria monocytogenes from ground chicken meat

AIMS: To evaluate a chromogenic plating medium for the isolation of sublethally injured cells of Listeria monocytogenes from processed foods. METHODS AND RESULTS: The inactivation of L. monocytogenes at pressures up to 400 MPa and 12 degrees C in ground chicken meat was employed to examine the recovery of high-pressure injured cells. Before and after different repair incubation periods at 30 degrees C in a nonselective broth, samples were plated onto a selective and differential agar [Agar Listeria according to Ottaviani and Agosti (ALOA)] and in the same medium supplemented with 4% sodium chloride (ALOA-S), and incubated at 37 degrees C. Sublethally injured cells were able to grow when directly plated onto the ALOA medium, without a previous repair incubation period. However, only uninjured cells grew on the ALOA-S medium. CONCLUSIONS: Sublethally injured cells of L. monocytogenes can be quantified by subtracting counts on ALOA-S medium from counts on ALOA medium. SIGNIFICANCE AND IMPACT OF THE STUDY: Possible applications include direct enumeration on ALOA of stressed cells of L. monocytogenes in foods with more than 100 colony forming units per gram.     

Induction of viable but nonculturable state in Vibrio parahaemolyticus and its susceptibility to environmental stresses

AIMS: This work analysed factors that influence the induction of viable but nonculturable (VBNC) state in the common enteric pathogen, Vibrio parahaemolyticus. The susceptibility of the VBNC cells to environmental stresses was investigated. METHODS AND RESULTS: Bacterium was cultured in tryptic soy broth-3% NaCl medium, shifted to a nutrient-free Morita mineral salt-0.5% NaCl medium (pH 7.8) and further incubated at 4 degrees C in a static state to induce the VBNC state in 28-35 days. The culturability and viability of the cells were monitored by the plate count method and the Bac Light viable count method, respectively. Cells grown at the optimum growth temperature and in the exponential phase better induced the VBNC state than those grown at low temperature and in the stationary phase. Low salinity of the medium crucially and markedly shortened the induction period. The VBNC cells were highly resistant to thermal (42, 47 degrees C), low salinity (0% NaCl), or acid (pH 4.0) inactivation. CONCLUSIONS: Optimal conditions for inducing VBNC V. parahaemolyticus were reported. The increase in resistance of VBNC V. parahaemolyticus to thermal, low salinity and acidic inactivation verified that this state is entered as part of a survival strategy in an adverse environment. SIGNIFICANCE AND IMPACT OF THE STUDY: The methods for inducing VBNC V. parahaemolyticus in a markedly short time will facilitate further physiological and pathological study. The enhanced stress resistance of the VBNC cells should attract attention to the increased risk presented by this pathogen in food.     

Survival of Listeria monocytogenes and Enterococcus faecium in sludge evaluated by real-time PCR and culture methods

AIMS: This study evaluates the behaviour in spiked sludge of a pathogenic bacteria, Listeria monocytogenes, by cultural and molecular techniques, and compares its survival with the one of a faecal indicator, Enterococcus faecium. METHODS AND RESULTS: Listeria monocytogenes strain Scott A and E. faecium(T) were followed for 17 days after inoculation in sludge. Kinetics of survival depended on the bacteria and on the technique used [most probable number method, direct plate count or real-time quantitative PCR (qPCR)]. The concentration of L. monocytogenes decreased rapidly regardless of the technique, but the decrease was much more dramatic with culture techniques than with qPCR. On the contrary, the concentrations of culturable E. faecium(T) were stable. CONCLUSIONS: The results suggest that the cells of L. monocytogenes strain Scott A might have entered a viable, but nonculturable (VBNC) status, whereas cells of the indicator bacteria, E. faecium(T), maintained themselves better and stayed culturable. SIGNIFICANCE AND IMPACT OF THE STUDY: The difference of survival kinetics in the sludge of a faecal indicator (E. faecium) and a pathogenic bacterium (L. monocytogenes) may be linked to the fact that they either enter or do not enter into a VBNC status.     

Induction of Viable but Nonculturable Escherichia coli O157:H7 by High Pressure CO2 and Its Characteristics

The viable but nonculturable (VBNC) state is a survival strategy adopted by many pathogens when exposed to harsh environmental stresses. In this study, we investigated for the first time that whether high pressure CO₂ (HPCD), one of the nonthermal pasteurization techniques, can induce Escherichia coli O157:H7 into the VBNC state. By measuring plate counts, viable cell counts and total cell counts, E. coli O157:H7 in 0.85% NaCl solution (pH 7.0) was able to enter the VBNC state by HPCD treatment at 5 MPa and four temperatures (25°C, 31°C, 34°C and 37°C). Meanwhile, with the improvement of treatment temperature, the time required for E. coli O157:H7 to enter VBNC state would shorten. Enzymatic activities in these VBNC cells were lower than those in the exponential-phase cells by using API ZYM kit, which were also reduced with increasing the treatment temperature, but the mechanical resistance of the VBNC cells to sonication was enhanced. These results further confirmed VBNC state was a self-protection mechanism for some bacteria, which minimized cellular energetic requirements and increased the cell resistance. When incubated in tryptic soy broth at 37°C, the VBNC cells induced by HPCD treatment at 25°C, 31°C and 34°C achieved resuscitation, but their resuscitation capabilities decreased with increasing the treatment temperature. Furthermore, electron microscopy revealed changes in the morphology and interior structure of the VBNC cells and the resuscitated cells. These results demonstrated that HPCD could induce E. coli O157:H7 into the VBNC state. Therefore, it is necessary to detect if there exist VBNC microorganisms in HPCD-treated products by molecular-based methods for food safety.     

Cold-adaptation of Antarctic microorganisms – possible involvement of viable but nonculturable state

Occurrence of the viable but nonculturable (VBNC) form of microorganisms in nature is well known. Evidence available in the literature indicates that low temperature may induce microorganisms to form VBNC cells. The VBNC state of some bacteria, collected from Antarctic lakes, has been recently reported. Hence it is postulated that formation of VBNC cells may be one of the strategies adopted by Antarctic microorganisms to survive in the extreme cold climate. Accepted: 16 October 1999     

Cross‐protective effect of acid‐adapted Salmonella enterica on resistance to lethal acid and cold stress conditions

Aims: To evaluate the cross‐protected Salmonella enterica cells under acid and cold stress conditions. Methods and Results: The acid‐adapted S. enterica cells were exposed to pH 4·0 at 4 and 20°C. Recovery of sublethally injured cells was estimated by the difference between the counts obtained on trypticase soy agar (TSA) and xylose lysine desoxycholate (XLD) agar. The survival curves of nonadapted and acid‐adapted S. enterica cells at pH 4·0 were fitted with Weibull distribution model. The recovery behaviour of injured S. enterica cells was estimated by the modified Gompertz parameters. Acid‐adapted S. enterica were more resistant to subsequent acid shock than the nonadapted cells. The numbers of nonadapted S. enterica cells were decreased by 4·57 and 7·55 log CFU ml^?1 at 4 and 20°C after 12‐day acid challenge, respectively. The acid adaptation induced cross‐protection and viable nonculturable (VBNC) state against low acid and cold stresses. The 7‐h adaptation showed the least recovery of injured cells. Conclusion: The results suggest that acid‐adapted S. enterica cells induced acid tolerance response and VBNC state. Significance and Impact of the Study: These results provide useful information for understanding the induction of cross‐protected and VBNC pathogens under various stresses, which might be needed in designing new food preservation strategies.     

Resuscitation rate in different enterococcal species in the viable but non-culturable state

AIMS: The viable but non-culturable (VBNC) state is a survival strategy adopted by bacteria when exposed to environmental stress. When in this state bacteria are no longer culturable on conventional growth media, but cells display metabolic activity and maintain pathogenicity factors/genes and, in some cases, resuscitation from the VBNC state has been shown. This state has been described for both human pathogens and faecal pollution indicators. In this study, we present evidence for entry of different enterococcal species into the VBNC state in an oligotrophic microcosm. METHODS AND RESULTS: The duration of the viability of the cells in the VBNC state was measured either by detecting the presence of pbp5 mRNA or by quantifying their resuscitation capability. Enterococci showed different behaviours. Enterococcus faecalis and Enterococcus hirae entered into the VBNC state within 2 weeks and remained in that state for 3 months. In the experiments described the resuscitation rate was 1:10 000 cells as soon as the cells entered the VBNC state and decreased gradually to undetectable levels over the following 3 months. Enterococcus faecium, however, remained culturable up to 4 weeks. After this time period, when the population was totally unculturable, the cells were far less resuscitable than other enterococci and only over a narrow time interval (2 weeks). CONCLUSIONS: These results suggest that Ent. faecalis and Ent. hirae enter the VBNC state but that Ent. faecium, in an oligotrophic laboratory environment, tends to die instead of entering the VBNC state. SIGNIFICANCE AND IMPACT OF THE STUDY: These experiments may mimic what happens when enterococci are released by humans and animals in natural environments.     

Detection of viable and dead Listeria monocytogenes on gouda-like cheeses by real-time PCR

AIMS: Surface contamination by Listeria monocytogenes of gouda-like cheeses during processing represents a potential public health problem. The aim of this work was to develop novel real-time PCR diagnostics to detect the presence of viable, dead or viable but not culturable (VBNC) cells on gouda-like cheeses. METHODS AND RESULTS: We used ethidium monoazide bromide (EMA)-PCR for direct quantification of viable and dead cells, while semiquantitative detection of culturable cells below the PCR detection limit (c. 100 CFU g(-1)) was obtained by combining growth and real-time PCR. We were able to quantify the fraction of >0.5% viable cells in a background of dead cells by EMA-PCR, given that the viable cell concentration was above the PCR detection limit. The combined growth and real-time PCR complemented the EMA-PCR, and enabled semiquantitative detection of low levels of culturable cells (10 and 100 CFU g(-1)). SIGNIFICANCE AND IMPACT OF THE STUDY: The significance of this work is that we have developed a novel concept for detection of viable and potentially infectious L. monocytogenes.     

Evaluation of enrichment broths for their ability to recover heat-injured Listeria monocytogenes

J.R. PATEL AND L.R. BEUCHAT. 1995. Listeria selective enrichment broth (LEB), University of Vermont (UVM) broth, modified UVM (MUVM) broth and Fraser broth (FB) were compared for their ability to recover cells of L. monocytogenes from heated tryptose phosphate broth. Three strains of L. monocytogenes were heated at 54C for 30 min, inoculated into enrichment broths supplemented with 400 µg catalase ml⁻¹, and incubated for 8 h at 30°C. After incubation for 4 h, the total viable cell populations either decreased or did not change, whereas the number of healthy (non-injured) cells of all strains increased significantly in all broths except FB inoculated with the LCDC strain. With an increase in incubation time to 8 h, the number of healthy cells of all strains increased in all broths. At 8 h, the difference between populations of total (injured plus healthy cells) and healthy cells detected in LEB inoculated with two strains was not significant. Overall, recovery of heat-treated cells was significantly higher in LEB, followed by MUVM broth, UVM broth and FB. The addition of catalase to enrichment broths significantly enhanced recovery of heat-injured cells. A slight reduction of catalase activity of heated cells of all test strains in all enrichment broths except FB was observed by extending the incubation period from 4 to 8 h. A test strain that produces relatively higher catalase activity compared to the other strains exhibited the greatest resistance to exogenous hydrogen peroxide. Enumeration of viable L. monocytogenes cells in heated foods should be done using LEB supplemented with 400 µg catalase ml⁻¹ to maximize the recovery of injured cells.     

Effect of a bacteriocin-activated polythene film on Listeria monocytogenes as evaluated by viable staining and epifluorescence microscopy

AIMS: To evaluate the effect of a bacteriocin-activated polythene film on resting and growing populations of Listeria monocytogenes. METHODS AND RESULTS: The active polythene films were industrially obtained by coating a solution of bacteriocin 32Y from Lactobacillus curvatus upon the surface of the film to be in contact with the packaged material. The behaviour of live Listeria populations was examined in liquid suspensions directly in contact with the bacteriocin-activated film, packed in antimicrobial film, and in a challenge test of storage of frankfurters superficially contaminated by L. monocytogenes and packed in antimicrobial film. In all the experiments, live and dead cells of L. monocytogenes were counted in epifluorescence microscopy after viable staining, which proved to be a suitable method to evaluate the action of bacteriocins on populations of L. monocytogenes. The results showed that the direct contact between active film surface and L. monocytogenes cells is effective for a fast and irreversible inactivation of the population by determining a direct cell disruption. This was confirmed by the results of the challenge test indicating that the antimicrobial package was effective in inhibiting the growth and survival of the pathogen on the surface of frankfurters during storage. CONCLUSIONS: The use of the antimicrobial film is encouraged especially for solid food products where the superficial contaminants come immediately in contact with the antimicrobial film. SIGNIFICANCE AND IMPACT OF THE STUDY: A fast inactivation of the bacterial population, coupled with appropriate conditions of storage, can improve the quality and safety and prolong the shelf-life of the food products packed in antimicrobial films.     

Inhibition of Listeria monocytogenes in chicken cold cuts by addition of sakacin P and sakacin P-producing Lactobacillus sakei

AIMS: To evaluate the potential of sakacin P and sakacin P-producing Lactobacillus sakei for the inhibition of growth of Listeria monocytogenes in chicken cold cuts, by answering the following questions. (i) Is sakacin P actually produced in food? (ii) Is sakacin P produced in situ responsible for the inhibiting effect? (iii) How stable is sakacin P in food? METHODS AND RESULTS: Listeria monocytogenes, a Lact. sakei strain and/or the bacteriocin sakacin P were added to chicken cold cuts, vacuum packed and incubated at 4 or 10 degrees C for 4 weeks. Each of two isogenic Lact. sakei strains, one producing sakacin P and the other not, had an inhibiting effect on the growth of L. monocytogenes. The effect of these two isogenic strains on the growth of L. monocytogenes was indistinguishable, even though sakacin P was produced in the product by one of the two Lact. sakei strains. The addition of purified sakacin P had an inhibiting effect on the growth of L. monocytogenes. A high dosage of sakacin P (3.5 microg x g(-1)) had a bacteriostatic effect throughout the storage period of 4 weeks, while a low dosage (12 ng x g(-1)) permitted initial growth, but at a slow rate. After 4 weeks of storage, the number of L. monocytogenes in the samples with a low dosage of sakacin P was 2 logs below that in the untreated control. When using a high dosage of sakacin P, the bacteriocin was detected in samples stored for up to 6 weeks. CONCLUSIONS: (i) Sakacin P is produced by a Lact. sakei strain when growing on vacuum-packed chicken cold cuts. (ii) Inhibiting effects of Lact. sakei, other than sakacin P, are active in inhibiting the growth of L. monocytogenes growing on chicken cold cuts. (iii) Sakacin P is stable on chicken cold cuts over a period of 4 weeks. SIGNIFICANCE AND IMPACT OF THE STUDY: Both sakacin P and Lact. sakei were found to have potential for use in the control of L. monocytogenes in chicken cold cuts.     

Development of a direct viable count procedure for the investigation of VBNC state in Listeria monocytogenes

A viable but non-culturable (VBNC) bacterial state was originally detected in studies in environmental microbiology. In particular, this state has been demonstrated for a number of human pathogens (Escherichia coli, Salmonella enteritidis, Vibrio cholerae, Legionella pneumophila and Campylobacter jejuni). The presence of VBNC cells poses a major public health problem since they cannot be detected by traditional culturing methods and the cells remain potentially pathogenic under favourable conditions. But, as far as we know, the VBNC state has not been yet described in Listeria monocytogenes. In most studies, this has been assessed by the Kogure procedure based on cellular elongation in the presence of DNA gyrase inhibitors. The antibiotic used was nalidixic acid in order to prevent DNA replication, only efficient in Gram-negative bacteria studies. In this study, we describe a new DVC procedure to detect and count viable of L. monocytogenes suspended in filtered, sterilized distilled water. We used different concentrations of ciprofloxacin, efficient both in Gram-negative and Gram-positive bacteria. Bacteria cells were removed and resuspended in BHI broth, with yeast extract and ciprofloxacin. The mixture was incubated at different incubation times at 37 degrees C. After different incubation times, cells were filtered through an isopore polycarbonate black membrane filter and covered with a DAPI solution or orange acridine. The filters were prepared and examined by epifluorescence microscopy. Elongated cells were counted as viable cells, whereas normal size was regarded as nonactive ones. This method allows determination of ciprofloxacin concentration and incubation time optimal to detect maximum viable cells percentage in L. monocytogenes.     

Recovery of culturability of an HOCl-stressed population of Escherichia coli after incubation in phosphate buffer: resuscitation or regrowth?

An Escherichia coli population harvested in exponential phase at about 10(8) cells/ml was treated in phosphate buffer with HOCl at concentrations ranging from 0.4 to 1 mg/liter (7.7 to 19 microM). The HOCl stress resulted in the appearance of three cell subpopulations: a majority of dead (nonrespiring) cells, a few culturable cells (10(2) to 10(4)), and about 10(7) viable but nonculturable cells. In the absence of any added exogenous nutrient, a culturable population could be recovered after 1 day of incubation in phosphate buffer, and such a population would reach a cell density close to 10% of the initial density of the stressed population, whatever the initial number of survivors. When a small number of untreated cells were mixed with the stressed population, growth of the untreated cells was observed, demonstrating that damaged cells provided nutrients. Similarly, a filtrate and a disrupted-cell filtrate of the stressed population supported growth of untreated cells with the same efficiency. The number of CFU (untreated or stressed) at plateau phase depended on the initial density of the stressed cells. Taken together, these results suggest that recovery in phosphate buffer of an HOCl-stressed population is in large part due to growth of a few culturable cells at the expense of damaged cells. However, comparison of the growth rates of the stressed culturable population and of untreated bacteria growing in filtrate showed significantly faster growth of the stressed cells, a fact not fully compatible with the hypothesis that recovery is only the simple growth of survivors. We suggest, therefore, that in addition to growth of the few culturable stressed cells, there is repair and growth of some mildly injured viable but nonculturable cells.     

Concentrations of copper thought to be toxic to Escherichia coli can induce the viable but nonculturable condition.

We have determined that concentrations of copper considered to be toxic can induce a fraction of a population of Escherichia coli to enter the viable but nonculturable (VBNC) condition. Copper-induced VBNC cells could be resuscitated for up to 2 weeks after entering the VBNC state.     

Profiling of recovery efficiencies for three standard protocols (FDA-BAM, ISO-11290, and modified USDA) on temperature-injured Listeria monocytogenes

There have been a number of studies conducted in order to compare the efficiencies of recovery rates, utilizing different protocols, for the isolation of L. monocytogenes. However, the severity of multiple cell injury has not been included in these studies. In the current study, L. monocytogenes ATCC 19112 was injured by exposure to extreme temperatures (60°C and -20°C) for a one-step injury, and for a two-step injury the cells were transferred directly from a heat treatment to frozen state to induce a severe cell injury (up to 100% injury). The injured cells were then subjected to the US Food and Drug Administration (FDA), the ISO-11290, and the modified United States Department of Agriculture (mUSDA) protocols, and plated on TSAyeast (0.6% yeast), PALCAM agar, and CHROMAgar Listeria for 24 h or 48 h. The evaluation of the total recovery of injured cells was also calculated based on the costs involved in the preparation of media for each protocol. Results indicate that the mUSDA method is best able to aid the recovery of heat-injured, freeze-injured, and heat-freeze-injured cells and was shown to be the most cost effective for heat-freeze-injured cells.     

Increased thermal and osmotic stress resistance in Listeria monocytogenes 568 grown in the presence of trehalose due to inactivation of the phosphotrehalase-encoding gene treA

The food-borne pathogen Listeria monocytogenes is a problem for food processors and consumers alike, as the organism is resistant to harsh environmental conditions and inimical barriers implemented to prevent the survival and/or growth of harmful bacteria. One mechanism by which listeriae mediate survival is through the accumulation of compatible solutes, such as proline, betaine and carnitine. In other bacteria, including Escherichia coli, the synthesis and accumulation of another compatible solute, trehalose, are known to aid in the survival of stressed cells. The objective of this research was to investigate trehalose metabolism in L. monocytogenes, where the sugar is thought to be transferred across the cytoplasmic membrane via a specific phosphoenolpyruvate phosphotransferase system and phosphorylation to trehalose-6-phosphate (T6P). The latter is subsequently broken down into glucose and glucose-6-phosphate by α,α-(1,1) phosphotrehalase, the putative product of the treA gene. Here we report on an isogenic treA mutant of L. monocytogenes 568 (568:ΔTreA) which, relative to the wild-type strain, displays increased tolerances to multiple stressors, including heat, high osmolarity, and desiccation. This is the first study to examine the putative trehalose operon in L. monocytogenes, and we demonstrate that lmo1254 (treA) in L. monocytogenes 568 indeed encodes a phosphotrehalase required for the hydrolysis of T6P. Disruption of the treA gene results in the accumulation of T6P which is subsequently dephosphorylated to trehalose in the cytosol, thereby contributing to the stress hardiness observed in the treA mutant. This study highlights the importance of compatible solutes for microbial survival in adverse environments.     

Comparative evaluation of adhesion, surface properties, and surface protein composition of Listeria monocytogenes strains after cultivation at constant pH of 5 and 7

AIMS: To analyse the cellular mechanisms that influence Listeria monocytogenes adhesion onto inert surfaces under acidic growth conditions. METHODS AND RESULTS: The adhesion capability of all the strains was significantly reduced after cultivation at constant pH 5 than at constant pH 7 and the cell surface was significantly less hydrophobic at pH 5 than at 7. At pH 5, the analyses of surface protein composition revealed that the flagellin was downregulated for all strains, which was confirmed by the absence of flagella and the P60 protein was upregulated for L. monocytogenes EGD-e, X-Li-mo 500 and 111. The use of L. monocytogenes EGD mutants revealed that flagellin could be involved in the adhesion process, but not P60 protein. It was also observed that the hydrophobic character was not linked to the presence or the absence of flagellin or P60 protein at the cell surface of L. monocytogenes. CONCLUSIONS: The decrease of L. monocytogenes adhesion at pH 5 could be attributed to the downregulation of the flagellin synthesis under the acidic conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: Conservation of food product at pH 5 will delay bacterial adhesion and biofilm formation during food processing on inert surfaces when the product is contaminated with L. monocytogenes.