Survival of Lactobacillus rhamnosus strains inoculated in cheese matrix during simulated human digestion

Survival of probiotic bacteria during transit through the gastrointestinal (GI) tract is influenced by a number of environmental variables including stomach acidity, bile salts, digestive enzymes and food matrix. This study assessed survival of seven selected Lactobacillus rhamnosus strains delivered within a model cheese system to the human upper GI tract using a dynamic gastric model (DGM). Good survival rates for all tested strains were recorded during both simulated gastric and duodenal digestion. Strains H12, H25 and N24 demonstrated higher survival capacities during gastric digestion than L. rhamnosus GG strain used as control, with H12 and N24 continuing to grow during duodenal digestion. Strains L. rhamnosus F17, N24 and R61 showed adhesion properties to both HT-29 and Caco-2 cells. The ability to attach to the cheese matrix during digestion was confirmed by scanning electron microscopy, also indicating production of extracellular polysaccharides as a response to acid stress.     

Strain dependent expression of stress response and virulence genes of Listeria monocytogenes in meat juices as determined by microarray

A subgenomic array, encompassing 54 probes targeting genes responsible for virulence, adhesion and stress response in Listeria monocytogenes, was used in order to study their expression in food systems. RNA extracted from L. monocytogenes inoculated in BHI and in situ (i.e. in minced meat and fermented sausage juices) and incubated at 4 °C, was hybridized on the array and the results obtained were compared in order to understand the effect that the food juice has on the expression. Three different strains of L. monocytogenes were tested, in order to determine the effect of the strain provenience. As determined by cluster analysis, each strain behaved in a different way when inoculated in food juices. The goal was to respond to acidic and osmotic stresses encountered in the food, particularly in the fermented sausage juice. No differences in the expression profile between the three strains were observed, when they were inoculated in BHI. On the other hand, in the meat and sausage juices, the iap, gadC and gadE genes, together with different internalin encoding genes, were significantly differentially expressed in the three strains.     

Prevalence, characterization and sources of Listeria monocytogenes in blue crab (Callinectus sapidus) meat and blue crab processing plants

Seven blue crab processing plants were sampled to determine the prevalence and sources of Listeria spp. and Listeria monocytogenes for two years (2006–2007). A total of 488 raw crabs, 624 cooked crab meat (crab meat) and 624 environmental samples were tested by standard methods. Presumptive Listeria spp. were isolated from 19.5% of raw crabs, 10.8% of crab meat, and 69.5% of environmental samples. L. monocytogenes was isolated from 4.5% of raw crabs, 0.2% of crab meat, and 2.1% of environmental samples. Ninety-seven percent of the isolates were resistant to at least one of the ten antibiotics tested. Eight different serotypes were found among 76 L. monocytogenes isolates tested with the most common being 4b, 1/2b and 1/2a. Automated EcoRI ribotyping differentiated 11 ribotypes among the 106 L. monocytogenes isolates. Based on ribotyping analysis, the distribution of the ribotypes in each processing plant had a unique contamination pattern. A total of 92 ApaI and 88 AscI pulsotypes among the 106 L. monocytogenes isolates were found and distinct pulsotypes were observed in raw crab, crab meat and environmental samples. Ribotypes and serotypes recovered from crab processing plants included subtypes that have been associated with listeriosis cases in other food outbreaks. Our findings suggest that molecular methods may provide critical information about sources of L. monocytogenes in crab processing plants and will augment efforts to improve food safety control strategies such as targeting specific sources of contamination and use of aggressive detergents prior to sanitizing.     

Application of bacteriocinogenic Enterococcus mundtii CRL35 and Enterococcus faecium ST88Ch in the control of Listeria monocytogenes in fresh Minas cheese

Several strains of Enterococcus spp. are capable of producing bacteriocins with antimicrobial activity against important bacterial pathogens in dairy products. In this study, the bacteriocins produced by two Enterococcus strains (Enterococcus mundtii CRL35 and Enterococcus faecium ST88Ch), isolated from cheeses, were characterized and tested for their capability to control growth of Listeria monocytogenes 426 in experimentally contaminated fresh Minas cheese during refrigerated storage. Both strains were active against a variety of pathogenic and non-pathogenic microorganisms and bacteriocin absorption to various L. monocytogenes, Enterococcus faecalis ATCC 19443 and Lactobacillus sakei ATCC 15521 varied according to the strain and the testing conditions (pH, temperature, presence of salts and surfactants). Growth of L. monocytogenes 426 was inhibited in cheeses containing E. mundtii CRL35 up to 12 days at 8 °C, evidencing a bacteriostatic effect. E. faecium ST88Ch was less effective, as the bacteriostatic affect occurred only after 6 days at 8 °C. In cheeses containing nisin (12.5 mg/kg), less than one log reduction was observed. This research underlines the potential application of E. mundtii CRL35 in the control of L. monocytogenes in Minas cheese.     

SigB plays a major role in Listeria monocytogenes tolerance to bile stress

The ability of Listeria monocytogenes to tolerate high levels of bile stress is critical to its successful infection and colonization in the human gastrointestinal tract. L. monocytogenes encodes bile salt hydrolase by a bsh gene which plays a significant role in hydrolyzing high concentrations of bile salt when L. monocytogenes grows under hypoxemic condition. As the bsh promoter contains consensus SigB and PrfA binding sites, we investigated the role of SigB (σ^B) and PrfA in L. monocytogenes tolerance against bile stress by comparing the survival of isogenic deletion mutants of L. monocytogenes EGD_ΔsigB, EGD_ΔprfA and EGD_ΔprfAΔsigB with their parent strain EGD at high levels of bile salt. Our results show that the sigB deletion significantly reduced the MICs of bile salt for EGD_ΔsigB and EGD_ΔprfAΔsigB (2.6% and 2.2% vs 3.5% in wild type strain EGD), while the growth rates of these two sigB deletion mutants (EGD_ΔsigB and EGD_ΔprfAΔsigB) were affected the most in the presence of 3% bile salt. Pre-exposure to alkali (pH 9.0) and osmotic (0.3 M NaCl) stresses for a short period of time (30 min) resulted in improved growth of L. monocytogenes as well as its prfA-sigB isogenic mutants even under sublethal concentrations of bile salt, while pre-exposure to acid pH (pH 4.5) failed to provide cross-protection against subsequent bile stress. Furthermore, the sigB gene had more remarkable influence than that of prfA on bsh expression, as much lower levels of bsh transciption were observed in EGD_ΔsigB and EGD_ΔprfAΔsigB. Meanwhile, bsh expression in the deletion mutants did not respond to elevated levels of bile salt. These data indicate that σ^B might play a crucial role in Listeria survival under bile salt environment in the gastrointestinal tract before its successful colonization, invasion and intracellular propagation.     

Bacterial stress response in Listeria monocytogenes: jumping the hurdles imposed by minimal processing

Minimal processing relies on the use of multiple sub-lethal stresses (or processes) to achieve a similar level of microbial control as that traditionally achieved using a single lethal stress. The benefit to the consumer is products which are less obviously processed than a frozen or canned, acidified or heavily salted food item. However, our increasing understanding of how bacteria can adapt to sub-lethal stresses in a manner which can render them less susceptible to additional insults, should be borne in mind when designing safety or extended shelf-life into a minimally processed product. Listeria monocytogenes is a target organism for many minimally processed food manufacturers because of its ability to tolerate adverse conditions such as low Aw and low temperature. In this communication we use L. monocytogenes as a model system to describe some of the consequences of stress adaptation in terms of improved survival in minimally processed foods and, importantly, the consequences in terms of the virulence of the target organism.     

Application of Central Composite Design to evaluate the antilisterial activity of hydro-alcohol berry extract of Myrtus communis L.

The antimicrobial activity of the hydro-alcohol extract of Myrtus communis L. (ME) berries was investigated against six Listeria monocytogenes strains (2 type strains and 4 isolates). Sub-lethal ME concentrations reduced L. monocytogenes counts by at least 2 log cycles. A Central Composite Design was used to investigate the combined effects of sub-lethal concentrations of ME (0.039–0.195 mL/100 mL), NaCl (0–2.0 g/100 mL) and pH (5.0–7.0) on strains growth. ME affected growth parameters, generally extending lag phase length and reducing maximum growth, sometimes with interactive effects with pH. The highest ME concentrations (0.117–0.195 mL/100 mL) combined with the lowest pH values (5.0–6.0) strongly reduced or even inhibited strains growth.     

Prevalence and biofilm-forming ability of Listeria monocytogenes in New Zealand mussel (Perna canaliculus) processing plants

Greenshell™ mussels are New Zealand’s largest seafood export species. Some export markets require compliance with ‘zero’ tolerance legislation for Listeria monocytogenes in 25 g of product. Even though individually quick frozen (IQF) mussel products are labeled ‘to be cooked’, and are not classified as ready-to-eat, some markets still require them to comply with the strict policy. Three mussel processing plants were assessed for the pattern of L. monocytogenes contamination on raw material, environment, food contact surfaces, and in the final product. Cultures (n = 101) obtained from an industrial Listeria monitoring program from August 2007 to June 2009 were characterized by serotyping and pulsed field gel electrophoresis. Using the crystal violet method, isolates were assessed for their ability to form biofilms. This work confirmed the presence of L. monocytogenes in raw and processed product, and the importance of cross-contamination from external and internal environments. Processing plants had L. monocytogenes pulsotypes that were detected more than once over 6 months. No correlation was found between biofilm-forming ability and persistent isolates. Two pulsotypes (including a persistent one), were previously isolated in human cases of listeriosis in New Zealand, but none of the pulsotypes matched those involved in international outbreaks.     

Biofilm in milking equipment on a dairy farm as a potential source of bulk tank milk contamination with Listeria monocytogenes

The objective of this study was to assess the presence of a Listeria monocytogenes-containing biofilm in milking equipment as a potential source of bulk tank milk contamination on a dairy farm where milk contamination had been previously documented. Samples were collected from milking equipment and milking parlor premises on 4 occasions and analyzed for the presence of L. monocytogenes. Pulsed-field gel electrophoresis (PFGE) typing was conducted on L. monocytogenes isolates from the milking equipment, parlor and storage room floors, bulk tank milk, and in-line milk filters. Pieces from milk meters and rubber liners were obtained to visually assess the presence of a biofilm using scanning electron microscopy. A total of 6 (15%), 4 (25%), and 1 (6%) samples were culture-positive for L. monocytogenes in the first, second, and third sample collection, respectively. Two samples were L. monocytogenes hly PCR-positive but were culture-negative in the fourth sample collection. Combined AscI and ApaI restriction analysis yielded 6 PFGE types for 15 L. monocytogenes isolates obtained from milking equipment, parlor, bulk tank milk, and milk filters. A predominant and persistent PFGE type (PFGE type T) was observed among these L. monocytogenes isolates (9/15 isolates). Scanning electron microscopy of samples from the bottom cover of 2 milk meters showed the presence of individual and clusters of bacteria, mainly associated with surface scratches. The presence of a bacterial biofilm was observed on the bottom covers of the 2 milk meters. Prevention of the establishment of biofilms in milking equipment is a crucial step in fulfilling the requirement of safe, high-quality milk.     

Heat and acid tolerance of Listeria monocytogenes after exposure to single and multiple sublethal stresses

The majority of published studies on the adaptive heat or acid tolerance response of Listeria monocytogenes have been performed with a single strain exposed to a single adaptation treatment; however, in food ecosystems, microorganisms commonly exist as multi-species communities and encounter multiple stresses, which may result in "stress hardening". Therefore, the present study evaluated the adaptive responses to heat (52, 57 and 63 degrees C) or lactic acid (pH 3.5) of a 10-strain composite of L. monocytogenes meat and human isolates at stationary phase, following exposure to combinations of osmotic (10% NaCl), acidic (pH 5.0 with HCl) and thermal (T; 46 degrees C) stresses, sequentially or simultaneously within 1.5h, in tryptic soy broth with 0.6% yeast extract (TSBYE). All treatments induced adaptive responses on L. monocytogenes at 57 degrees C, while no such cross-protection was observed at 52 and 63 degrees C. Survivor curves at 57 degrees C appeared convex with profound shoulders determined by a Weibull model. The highest thermotolerance was observed after combined exposure to acid and heat shock (pH-T), followed by exposure to osmotic shock, and by the combination of osmotic with heat shock (NaCl-T). Regarding acid tolerance, prior exposure to low pH, pH-T, or a combination of NaCl, pH and T resulted in a marked increase of resistance to pH 3.5, showing concave inactivation curves with tails at higher levels of survivors (log(10)CFU ml(-1)) than the control cultures. The sequence of exposure to sublethal stresses did not affect the thermotolerance of L. monocytogenes, whereas simultaneous exposure to most multiple stresses (e.g., NaCl-pH-T, NaCl-T and NaCl-pH) resulted in higher survivors of L. monocytogenes at pH 3.5 than exposure to the same stresses sequentially. The results indicate that combinations and sequences of sublethal hurdles may affect L. monocytogenes acid and heat tolerance, especially in acidic environments with mild heating or in low moisture environments.     

Molecular subtyping and virulence gene analysis of Listeria monocytogenes isolates from food

A total of 67 Listeria monocytogenes isolates from 698 raw meat samples were characterized for molecular serogroup identification and antimicrobial susceptibility. Approximately one third (32.8%) of the isolates belonged to molecular serogroup 1/2a, 3a, followed by 1/2c, 3c (26.9%), 1/2b, 3b, 7 (22.4%), 4b, 4d, 4e (16.4%) and 4a, 4c (1.5%). Most of the L. monocytogenes isolates were susceptible to 14 antimicrobials tested but several were resistant to tetracycline, ciprofloxacin and nitrofurantoin. An additional 30 L. monocytogenes isolates from chicken and produce in our collection were also included to determine the presence of significant virulence markers. All 97 isolates carried inlC and inlJ except for a lineage III isolate 110-1. Most Listeriolysin S (LLS)-carrying isolates (11/12) belonged to lineage I, whereas the remaining one isolate belonged to lineage III. Five 4b, 4d, 4e isolates including two from turkey and three from produce belonged to Epidemic Clone I (ECI). Four molecular serogroup associated mutation types that lead to premature stop codons (PMSCs) in inlA were identified. PFGE and inlA sequence analysis results were concordant, and different virulence potential within 1/2a, 3a and 4b, 4d, 4e isolates were observed. The study revealed that a subset of isolates from meat and produce belonged to ECI, harbored inlC, inlJ and LLS, and produced full length InlA, suggesting that they be capable of causing human illness.     

Prevalence and characterization of antimicrobial resistance of foodborne Listeria monocytogenes isolates in Hebei province of Northern China, 2005-2007

A total of 2177 food samples collected from nine cities in northern China during 2005 to 2007 were screened for the presence of Listeria monocytogenes. All L. monocytogenes isolates were subjected to serotyping, antimicrobial susceptibility, pulsed-field gel electrophoresis (PFGE), as well as PCR screening to identify genes responsible for tetracycline resistance [tet(L), tet(M), tet(K), tet(S) and tet(B)], transposon Tn916, and class 1 integron. Contamination with L. monocytogenes was detected in 4.13% (90/2177) of the total samples representing various food products. The pathogen was mainly isolated from frozen food made of wheat flour or rice products (26/252, 10.32%) and raw meat products (46/733, 6.28%). Besides, 3.31% (10/302) of cooked meat, 1.17% (4/343) of seafood, 0.98% (2/204) of non-fermented bean products and 0.62% (2/323) of vegetables samples were contaminated by this bacterium. The L. monocytogenes isolates belonged to five serotypes (1/2a, 1/2b, 1/2c, 4b, and 3a), with serotype 1/2a being dominant (48.88%). Antimicrobial resistance was most frequently observed for ciprofloxacin (17.8%), tetracycline (15.6%) and streptomycin (12.2%). Overall, resistance was observed against 14 out of 18 antimicrobials tested while multiple resistances occurred among 18.9% (17/90) isolates. Interestingly, two isolates were resistant to more than five antimicrobials. Among 14 tetracycline-resistant isolates, 13 carried tet(M) gene including nine possessing Tn916, and one harbored tet(S) gene. PFGE analysis revealed genetic heterogeneity among individual serotypes as well as scattered occurrence of some genotypes without any clear-cut correlation to source or food type. The widespread distribution of epidemiologically important serotypes (1/2a, 1/2b and 4b) of L. monocytogenes, and their resistance to commonly used antibiotics indicate a potential public health risk. Our data also indicate that L. monocytogenes could act as a reservoir of mobile tet genes along the food chain.     

PFGE characterisation and adhesion ability of Listeria monocytogenes isolates obtained from bovine carcasses and beef processing facilities

Listeria monocytogenes is a pathogen capable of adhering to many surfaces and forming biofilms, which may explain its persistence in food processing environments. This study aimed to genetically characterise L. monocytogenes isolates obtained from bovine carcasses and beef processing facilities and to evaluate their adhesion abilities. DNA from 29 L. monocytogenes isolates was subjected to enzymatic restriction digestion (AscI and ApaI), and two clusters were identified for serotypes 4b and 1/2a, with similarities of 48% and 68%, respectively. The adhesion ability of the isolates was tested considering: inoculum concentration, culture media, carbohydrate source, NaCl concentration, incubation temperature, and pH. Each isolate was tested at 10⁸ CFU mL^− 1 and classified according to its adhesion ability as weak (8 isolates), moderate (17) or strong (4). The isolates showed higher adhesion capability in non-diluted culture media, media at pH 7.0, incubation at 25 °C and 37 °C, and media with NaCl at 5% and 7%. No relevant differences were observed for adhesion ability with respect to the carbohydrate source. The results indicated a wide diversity of PFGE profiles of persistent L. monocytogenes isolates, without relation to their adhesion characteristics. Also, it was observed that stressing conditions did not enhance the adhesion profile of the isolates.     

Effects of nisin, EDTA and salts of organic acids on Listeria monocytogenes, Salmonella and native microflora on fresh vacuum packaged shrimps stored at 4 °C

Nisin (500 IU ml⁻¹), EDTA (0.02 M), potassium sorbate (PS) (3%, w/v), sodium benzoate (SB) (3%, w/v) or sodium diacetate (SD) (3%, w/v); alone or in combination were used to dip uninoculated shrimps and shrimps inoculated with Listeria monocytogenes or Salmonella (∼4.0–5.0 log CFU g⁻¹). Shrimps were then drip-dried, vacuum packaged and stored at 4 °C for 7 days. Untreated shrimps were used as a control. Numbers of L. monocytogenes, Salmonella and native background microflora were determined on uninoculated and inoculated shrimps on days 0, 3 and 7. Nisin–EDTA–PS and nisin–EDTA–SD significantly reduced (p < 0.05) L. monocytogenes numbers by 1.07–1.27 and 1.32–1.36 log CFU g⁻¹, respectively, on day 0 and 3. However, all treatments failed to significantly reduce (p > 0.05) Salmonella counts on shrimps throughout storage. On day 7, numbers of aerobic bacteria, psychrotrophic bacteria and Pseudomonas on combined nisin–EDTA–salt of organic acids treated shrimps were significantly lower (p < 0.05) by 4.40–4.60, 3.50–4.01, and 3.84–3.99 log CFU g⁻¹ respectively, as compared to the control. Dipping in organic acids solutions followed by vacuum packaging and chilled storage can help reduce L. monocytogenes and native microflora, but not Salmonella, on fresh shrimps.     

Modeling the growth rate and lag time of different strains of Salmonella enterica and Listeria monocytogenes in ready-to-eat lettuce

The growth parameters (growth rate, μ and lag time, λ) of three different strains each of Salmonella enterica and Listeria monocytogenes in minimally processed lettuce (MPL) and their changes as a function of temperature were modeled. MPL were packed under modified atmosphere (5% O₂, 15% CO₂ and 80% N₂), stored at 7–30 °C and samples collected at different time intervals were enumerated for S. enterica and L. monocytogenes. Growth curves and equations describing the relationship between μ and λ as a function of temperature were constructed using the DMFit Excel add-in and through linear regression, respectively. The predicted growth parameters for the pathogens observed in this study were compared to ComBase, Pathogen modeling program (PMP) and data from the literature. High R² values (0.97 and 0.93) were observed for average growth curves of different strains of pathogens grown on MPL. Secondary models of μ and λ for both pathogens followed a linear trend with high R² values (>0.90). Root mean square error (RMSE) showed that the models obtained are accurate and suitable for modeling the growth of S. enterica and L. monocytogenes in MP lettuce. The current study provides growth models for these foodborne pathogens that can be used in microbial risk assessment.     

Longitudinal monitoring of Listeria monocytogenes contamination patterns in a farmstead dairy processing facility

Contamination of dairy products with Listeria monocytogenes is a concern because multiple human listeriosis outbreaks have been linked to contaminated cheese and dairy products. Dairy production on farmstead operations may be a particular concern because L. monocytogenes is also an animal pathogen that can be shed by ruminants with and without clinical symptoms; physical proximity between production animal and dairy processing facilities may thus provide a higher risk for introduction of L. monocytogenes into the dairy production process. To better understand the risks of L. monocytogenes contamination associated with farmstead dairy production, samples from a farmstead dairy processing operation and the milking barn of the directly adjacent dairy sheep operation were tested for L. monocytogenes over a 3-yr period. Prevalence of L. monocytogenes for samples collected on the farm (n = 85) and the dairy production facility (n = 674) was 9.4 and 2.7%, respectively. Molecular subtyping using automated EcoRI ribotyping of L. monocytogenes isolates revealed that distinct subtypes were associated with the dairy production facility and the farm's milking parlor. Although a total of 5 and 4 different ribotypes were identified among isolates obtained from the dairy production facility and the milking parlor, respectively, only 1 ribotype (DUP-1030A) was isolated from both. Different ribotypes were predominant among isolates from the dairy production facility (ribotype DUP-1052A, representing 15 of 18 isolates) and the farm's milking parlor (ribotype DUP-1039A, representing 4 of 8 isolates); each of these ribotypes appeared to persist over time in the respective area. Our data support that i) in farmstead dairy processing facilities, L. monocytogenes present on the farm can largely be prevented from being introduced into the processing facility; and ii) L. monocytogenes can persist on farm and in processing areas, providing a potential high-risk source for contamination. Preventing cross contamination between dairy production and processing facilities and control of persistent L. monocytogenes are thus critical to assuring the microbial safety of farmstead dairy products.     

Contributions to selected phenotypic characteristics of large species- and lineage-specific genomic regions in Listeria monocytogenes

We hypothesized that genomic regions specific to Listeria monocytogenes or selected L. monocytogenes strains may contribute to virulence and phenotypic differences among the strains. A whole genome alignment of two completed L. monocytogenes genomes and the one completed Listeria innocua genome initially identified 28 genomic regions of difference (RD) > 4 kb that were found in one or both L. monocytogenes genomes, but absent from the non-pathogenic L. innocua. In silico analyses using an additional 18 draft L. monocytogenes genomes showed that (i) 15 RDs were found in all or most L. monocytogenes genomes; (ii) three RDs were found in all or most lineage I genomes, but absent from lineage II genomes; and (iii) four RDs were found in all lineage II genomes, but no lineage I genomes. Null mutants in two L. monocytogenes-specific RDs (RD16 and RD30; found in most L. monocytogenes) and the lineage II-specific RD25 showed no evidence for impaired invasion or intracellular growth in selected tissue culture cells. Although, in pH 5.5 minimal media, the ΔRD30 null mutant showed reduced ability to compete with its parent strain, indicating that RD30 may have a role in L. monocytogenes growth under limited nutrient conditions at acidic pH.     

Sub-lethal stress effects on virulence gene expression in Enterococcus faecalis

Enterococci are ubiquitous lactic acid bacteria commonly associated with the human digestive tract as commensal organisms. Additionally, these organisms have a long history of use in foods improving flavor as well as providing protective mechanisms as either a probiotic or antimicrobial additive. However, Enterococcus faecalis accounts for up to 10% of all nosocomial infections of the bloodstream, wounds, urinary tract and heart. Knowledge about the regulation of virulence factors is limited and the involvement of environmental signals contributing to E. faecalis pathogenicity is poorly documented. In this study, two clinical E. faecalis isolates, TMW 2.63 and OG1RF, as well as one food isolate, TMW 2.629, were subjected to six sub-lethal food- and host-related stresses including 6.8% NaCl, 200 ppm nitrite, 51 °C, 80 MPa, pH 4.1 and 0.08% bile salts (cholic acid:chenodeoxycholic acid 1:1), respectively, reducing their growth rate to 10%. Relative gene expression of 15 stress and virulence-associated genes including dnaK, groEL, ctsR, clpPBCEX, gls24, efaAfs, ace, fsrB, gelE, sprE and cylB, was quantified by using real time PCR and Lightcycler^® technology (reference conditions: BHI broth, 37 °C, pH = 7.4). Apart from strain-dependent differences, sub-lethal environmental stress was capable of provoking significant alterations in the expression of virulence-associated genes in E. faecalis from clinical as well as food origins of isolation. These results help to avoid preconditioning enterococci in food production processes and to understand the complex mechanisms in E. faecalis' switch to pathogenicity.