Survey of Listeria monocytogenes in ready-to-eat foods

The purpose of this study was to develop data on the risk of listeriosis to support a science-based strategy for addressing Listeria monocytogenes in foods in the United States. Eight categories of ready-to-eat foods were collected over 14 to 23 months from retail markets at Maryland and northern California FoodNet sites. The product categories included luncheon meats, deli salads, fresh soft "Hispanic-style" cheeses, bagged salads, blue-veined and soft mold-ripened cheeses, smoked seafood, and seafood salads. The presence and levels of L. monocytogenes in the samples were determined by rapid DNA-based assays in combination with culture methods. Of 31,705 samples tested, 577 were positive. The overall prevalence was 1.82%. with prevalences ranging from 0.17 to 4.7% among the product categories. L. monocytogenes levels in the positive samples varied from <0.3 MPN (most probable number) per g to 1.5 x 10(5) CFU/g, with 402 samples having levels of <0.3 MPN/g, 21 samples having levels of >10(2) CFU/g, and the rest of the samples having intermediate levels. No obvious trends with respect to seasonality were observed. Significant differences (P < 0.05) between the sampling sites were found, with higher prevalences for threes categories in northern California and for two categories in Maryland. Significantly (P < 0.001) higher prevalences were found for in-store-packaged samples than for manufacturer-packaged samples of luncheon meats, deli salads, and seafood salads, while 16 of the 21 samples with higher counts were manufacturer packaged. The data collected in this study help to fill gaps in the knowledge about the occurrence of L. monocytogenes in foods, and this new information should be useful in the assessment of the risk posed by L. monocytogenes to consumers.     

Enumeration and growth of naturally occurring Listeria spp. in unpackaged ham

A total of 301 unpackaged retail ham samples were tested for the presence and number of Listeria spp. after 7 days at 5 degrees C to simulate domestic storage. Thirteen samples (4.3%) contained Listeria monocytogenes, with the highest count being 1.6 x 10(3)cfu g(-1). Thirteen samples contained other Listeria spp. Genotyping showed that only one L. monocytogenes isolate from the 14 tested was of a type previously identified in New Zealand human cases. Listeria-contaminated batches were incubated at 5 degrees C over approximately 3 weeks to assess the growth rate of natural contaminants. None contained L. monocytogenes, but growth occurred in one sample containing Listeria welshimeri and four containing Listeria innocua. Growth was usually slow at 0.002-0.004 log h(-1). In one sample, L. innocua grew at 0.02 log h(-1) although the maximum number reached was only 4.0-5.0 x 10(3)cfu g(-1). In five other samples little growth, if any, occurred. Growth of naturally occurring Listeria spp. at 5 degrees C was therefore generally slower than predicted by the Pathogen Modelling Programme (PMP) or did not occur.     

Listeria monocytogenes in foods in Norway

Three-hundred-and-eighty-two samples of different retail food items in Norway (imported soft cheese, raw chicken, minced meat, fermented sausages, vacuum-packed processed meat products, smoked salmon, peeled shrimps, raw minced fish) and 78 carcass samples (sheep, pig, cattle), were screened for Listeria monocytogenes. Of the 460 samples investigated, 78 were found to contain L. monocytogenes. Five of these contained greater than 10(3) cfu/g, four greater than 10(2) cfu/g, while the remainder were shown to contain L. monocytogenes only after enrichment. L. monocytogenes was isolated most frequently from raw chicken, sporadically from soft cheese, shrimps, processed meat products and smoked salmon, and not at all from carcasses and fermented sausages.     

Incidence of Listeria species in seafood and seafood salads

A total of 128 samples of seafood on the Icelandic market were tested for the presence of Listeria monocytogenes and other Listeria species. The samples included raw, smoked and dried fish, frozen shellfish and shrimps as well as several fish salads. These products are generally consumed without heating. Listeria spp. were present in 56% of the samples of raw fish, 29% of the smoked fish, 9% of the shrimps and 32% of the salads. No Listeria spp. were present in the shellfish or dried fish. In 46% of the positive samples L. monocytogenes could be demonstrated, either alone or together with L. innocua. The other positive samples contained L. innocua and, in one sample, L. welshimeri. All products sampled had been processed and packed in Iceland, mostly for use on the domestic market. It is suggested that consuming certain fish products and fish salads may form an additional risk factor for listeriosis in humans.     

Microbiological safety of sandwiches from hospitals and other health care establishments in the United Kingdom with a focus on Listeria monocytogenes and other Listeria species

In the United Kingdom between 1999 and 2004, there were four outbreaks of Listeria monocytogenes infection associated with sandwiches purchased from or provided in hospitals. Elderly or immunocompromised individuals and pregnant women are particularly vulnerable to infection; therefore, the focus of this study was on sandwiches served in health care establishments. Of 3,249 sandwich samples collected between April 2005 and March 2006, 3.3% were of unsatisfactory microbiological quality because of high levels of Enterobacteriaceae (2.0%; > or = 10(4) CFU/g for sandwiches not containing salad), Escherichia coli (0.8%; > or = 10(2) CFU/g), Staphylococcus aureus (0.6%; > or = 10(2) CFU/g), and/or Listeria spp. (0.1%; two samples with L. welshimeri at 1.8 x 10(2) and 7.4 x 10(3) CFU/g and one sample with L. seeligeri at 1.8 x 10(3) CFU/g). Overall, 7.6% of sandwiches were contaminated with Listeria spp. L. monocytogenes was detected in 2.7% (88) of samples: 87 samples at < 10 CFU/g and 1 sample at 20 CFU/g. More frequent contamination with Listeria spp. and L. monocytogenes was found in sandwiches collected from hospital cafeterias, shops, or wards and in sandwiches stored and/or displayed at temperatures higher than 8 degrees C. The presence of Listeria spp. and L. monocytogenes also was associated with sandwiches that (i) were supplied from outside the establishment, (ii) were prepacked, (iii) had a main sandwich filling of poultry meat, or (iv) contained salad ingredients, soft cheese, and/or mayonnaise. Control of L. monocytogenes in sandwich manufacturing and within health care establishments is essential to minimize the potential for consumption of this bacterium at levels hazardous to health. The findings from this study support the proposal that manufacturers supplying sandwiches to health care establishments should follow the British Sandwich Association recommended guidelines of complete absence of L. monocytogenes in sandwiches at the point of production.     

Prevalence of Listeria monocytogenes and other Listeria species in butter from United Kingdom production, retail, and catering premises

Two recent listeriosis outbreaks involving butter prompted this first cross-sectional study on the prevalence, levels, and types of Listeria species in 3229 samples of butter from production, retail, and catering premises in the United Kingdom during May and June 2004. When the criteria of the Microbiological Guidelines were used, 99.4% of samples were found to be of satisfactory microbiological quality, 0.5% were of acceptable quality, and 0.1% were of unsatisfactory quality as a result of high levels (>100 CFU/g) of Listeria spp. The butter samples with Listeria spp. present at more than 100 CFU/g were negative for L. monocytogenes. L. monocytogenes was detected in 0.4% (n=13) of samples, all at levels of less than 10 CFU/g, and were therefore of acceptable quality. Butter was contaminated more frequently with Listeria spp., including L. monocytogenes, when packed in plastic tubs, when in pack sizes of 500 g or less, when stored or displayed above 8 degrees C, when a hazard analysis system was not in place, and when the manager had received no food hygiene training. This study demonstrates that although butter is regarded as a low-risk product, it may provide an environment for the persistence and growth of Listeria spp., including L. monocytogenes. The control of L. monocytogenes in food processing and supply systems is critical in order to minimize the potential for this bacterium to be present in foods at the point of consumption at levels hazardous to health.     

Incidence and control of Listeria monocytogenes in foods in Denmark

The Danish regulatory policy on Listeria monocytogenes in foods is based on the principles of HACCP and was developed using a health risk assessment approach. The Danish policy focuses examinations and criteria for L. monocytogenes in ready-to-eat foods and is based on a combination of inspection and product-testing. Based on current epidemiological information from several countries, a concentration of L. monocytogenes not exceeding 100 cfu/g of food at the time of consumption, seems to be of low risk to the consumers. In Denmark, ready-to-eat foods have been placed into six categories where absence of L. monocytogenes in 25 g is required in foods heat treated in the final package and in heat-treated as well as preserved, non heat-treated foods which can support growth within the shelf life. This level is necessary in foods capable of supporting growth, in order not to exceed 100 L. monocytogenes per g at the point of consumption. In heat-treated and preserved foods, which are not supportive of growth within the shelf-life and for raw, ready to eat foods, a level below 10 L. monocytogenes per g is regarded acceptable. A level between 10 and 100 L. monocytogenes per g is not satisfactory and a level above 100/g is not acceptable. Data on the qualitative and quantitative occurrence of L. monocytogenes in foods in Denmark are presented and discussed. In 1997 and 1998, greater than 15,000 samples from different categories of food were examined (semi-quantitatively) for the presence of L. monocytogenes. A significant difference could be seen in the number of samples containing more than 100 L. monocytogenes per g, between different categories of foods (1997, P = 0.001; 1998, P = 0.016). In 1997, preserved meat products and preserved fish products and to a lesser extent vegetables and meat or vegetable mayonnaise were more likely to contain high numbers (i.e. above 100 cfu/g) of L. monocytogenes than other food categories. In 1998, preserved meat products, but also heat-treated meat products, vegetables and meat or vegetable mayonnaise had the highest frequency of samples with > 100 L. monocytogenes per g. In a survey performed in 1994 and 1995, 1.3% of ready-to-eat food samples (heat-treated meat products, preserved meat and fish products) were found to be contaminated with L. monocytogenes at a level above 100 cfu/g. The samples included in this survey were primarily products produced by authorized companies and were comprised mainly of vacuum packed products or products packed in modified atmosphere and with long shelf lives, typically above several weeks. The corresponding percentages of positive samples primarily processed in the retail outlets (heat-treated meat products, preserved meat and fish products) in 1997 and 1998 were 0.3% and 0.6%, respectively. The results suggest that ready-to-eat meat and fish products with extended shelf-lives produced by authorized companies are more likely to contain high numbers (> 100 cfu/g) of L. monocytogenes than products processed in the retail sector which often have a shorter shelf life.     

Occurrence and characterization of Listeria spp. in ready-to-eat retail foods from Vancouver, British Columbia

The occurrence of Listeria spp. and Listeria monocytogenes in retail RTE meat and fish products in Vancouver, British Columbia (B.C.) was investigated. To assess potential consumer health risk, recovered L. monocytogenes isolates were subjected to genotypic and phenotypic characterization. Conventional methods were used to recover Listeria spp. from deli meat (n = 40) and fish (n = 40) samples collected from 17 stores. Listeria spp. were recovered only from fish samples (20%); 5% harboured Listeria innocua, 5% had L. monocytogenes and 10% contained Listeria welshimeri. L. monocytogenes isolates serotyped as 1/2a and 1/2b, possessed dissimilar PFGE patterns, and had full-length InlA. Three 1/2a clonal isolates encoded the 50 kb genomic island, LGI1. Antimicrobial resistance (AMR) profiling showed all Listeria spp. possessed resistance to cefoxitin and nalidixic acid. L. monocytogenes were resistant to clindamycin, two were resistant to streptomycin, and one to amikacin. Reduced susceptibility to ciprofloxacin was seen in all L. monocytogenes, L. innocua and three L. welshimeri isolates. Reduced susceptibility to amikacin and chloramphenicol was also observed in one L. monocytogenes and three L. welshimeri isolates, respectively. Recovery of L. monocytogenes in fish samples possessing AMR, full-length InlA, LGI1, and serotypes frequently associated with listeriosis suggest B.C. consumers are exposed to high-risk strains.     

Prevalence and contamination levels of Listeria monocytogenes in smoked fish and paté sold in Spain.

From March to November 2000, 170 samples of smoked fish and 182 samples of paté for sale in retail outlets and supermarkets in the nine provinces of Castilla and León (Spain) were analyzed for the prevalence of Listeria monocytogenes and other Listeria spp. L. monocytogenes was isolated from 38 (22.3%) of the 170 samples of smoked fish analyzed. Twenty of these positive samples contained L. monocytogenes at >100 CFU/g. Other Listeria spp., such as Listeria innocua (26 isolates), Listeria grayi (9), Listeria welshimeri (3), Listeria seeligeri (3), and Listeria ivanovii (2), were also detected. L. monocytogenes was isolated from 5.4% of the 182 samples of paté. Only 1 of the 10 positive samples harbored >100 L. monocytogenes CFU/g. Two other species of Listeria were observed in paté: L. innocua (12 isolates) and L. grayi (2).     

The incidence of Listeria species in retail foods in Japan.

Meat, fish and vegetable products obtained at retail shops in or around Tokyo were examined for Listeria contamination. Listeria spp. were isolated from 43 (56.6%) out of 76 samples of meat products. L. monocytogenes occurred in 26 (34%) of the samples, L. monocytogenes was isolated from 7 (6.1%) out of 114 samples of fish and fish products including 'ready-to-eat' foods. Listeria was not isolated from any of 21 samples of vegetable and vegetable product including 'ready-to-eat' foods investigated.     

Prevalence of Listeria species in food products in Isfahan, Iran

A total of 617 meat and meat products, diary, vegetables and ready to eat food samples were collected. Listeria spp. isolated by using USDA method of isolation and L. monocytogenes identified by biochemical and Polymerase Chain Reaction (PCR). The incidence of Listeria spp. was 4.6% in all food samples. L. monocytogenes was found in 1.2% of food samples. It was found that Listeria spp. was present in 6.7% of meat and meat product samples, 1.3% of diary samples, 1.2% of vegetable samples and 12% ready to eat samples. The results presented in this study indicate, the potential risk of eating ready to eat food or raw and undercooked foods.     

Prevalence and concentration of Listeria monocytogenes in sliced ready-to-eat meat products in the Hellenic retail market

The aim of this work was to estimate the prevalence and concentration of Listeria monocytogenes in packaged precut (slices or cubes) ready-to-eat (RTE) meat products available in the Hellenic retail market. Samples of these RTE meat products (n = 209) were taken from local supermarkets during a 3-month period and analyzed for the presence of L. monocytogenes with an automated enzymatic qualitative immunoassay followed by biochemical confirmation of positive results. The concentration of the pathogen in the positive samples was also determined. Seventeen samples (8.1%) were positive for L. monocytogenes. Eight (47.1%) of these 17 samples were from the same manufacturer; 36.4% of the products tested from this manufacturer were positive for L. monocytogenes. When bacon samples were not considered, the estimated prevalence of L. monocytogenes in sliced RTE meat products was much lower (3.1%). The L. monocytogenes populations in all positive samples were low, < or = 10 CFU/g. In 64.7% of the L. monocytogenes-positive samples, other Listeria species, including L. innocua and L. welshimeri, were also present at <10 to 690 CFU/g. These results indicate that L. monocytogenes is present in low numbers but is in a considerable proportion of the packaged precut RTE meat products that are sold in the Hellenic retail market. Cooked ham and bacon cut in cubes were the sample types most often contaminated with L. monocytogenes. The higher level of handling (e.g., cutting) associated with these products may further increase the risk of contamination with L. monocytogenes.     

Detection, quantification and vitality of Listeria monocytogenes in food as determined by quantitative PCR

In this paper we describe the development of a quantitative PCR (qPCR) technique to detect, quantify and determine the vitality of Listeria monocytogenes in foods. The method was based on the amplification of the intergenic region spacer (IGS) between the 16S and 23S rRNA genes. A panel of more than 100 strains of Listeria spp. and non-Listeria was used in order to verify the specificity of the primers and Taqman probe and amplification signals were obtained only when L. monocytogenes DNA and RNA were loaded in the qPCR mix. Standard curves were constructed in several food matrices (milk, meat, soft cheese, fermented sausage, cured ham and ready-to-eat salad). The quantification limit was of 10(3)-10(4) cfu/g or ml, while for the determination of vitality it was 10(4)-10(5) cfu/g or ml. After an overnight enrichment in BHI at 37 degrees C also 10 cfu/g or ml could be detected in all the matrices used in this study. When we applied the protocol to food samples collected from the market or from small food processing plants, on a total number of 66 samples, 4 fresh cheeses from raw milk gave positive results prior to the overnight incubation, while 9 samples, of which only one represented by fresh meat and the others by cheeses from raw milk, were positive after the enrichment. Out of the 4 positive samples, only one could be quantified and it was determined to contain 4x10(3) cfu/g.     

The occurrence in the U.K. of Listeria species in raw chickens and soft cheeses

Retail samples of 100 raw chickens and 222 U.K. and imported soft cheeses were examined for the presence of Listeria species. 60% of raw chickens (fresh and frozen) were contaminated with L. monocytogenes and 28% with other Listeria spp. including L. welshimeri, L. seeligeri and L. innocua. Six serotypes of L. monocytogenes were represented (1/2, 3a, 3b, 3c, 4b, 4d) of which more than one were isolated from some samples. 10% of the soft cheeses examined were found to contain L. monocytogenes at levels from less than 10(2) cfu/g to 10(5) cfu/g. The incidences in cheeses from various countries were Italy (16%), France (14%), Cyprus (10%) and the U.K. (4%). Only 2 serotypes (1/2 and 4b) were isolated, some samples containing both. L. innocua was the only other Listeria sp. found. There was no correlation between either the contamination with E. coli or the processing of the original milk used to make the cheeses (raw or pasteurized) and the presence of L. monocytogenes or other Listeria spp. The contribution of contaminated food to the epidemiology of listeriosis in the U.K. is discussed.     

The occurrence of Listeria species in milk and dairy products: a national survey in England and Wales

A total of 4172 samples of milk, cheese and other dairy products were examined over a 1-year period for the presence of Listeria species. Strains of Listeria were found most frequently in soft, ripened cows milk cheese; 63 out of 769 (8.2%) samples contained Listeria monocytogenes, 25 samples contained species other than L. monocytogenes, and 18 samples contained both L. monocytogenes and other Listeria spp. Eleven samples of pasteurized cows milk (1.1%) from four dairies contained L. monocytogenes, and other Listeria spp. were isolated from a further five samples. Goats and ewes milk and their products, yogurt, cream and ice cream also occasionally contained Listeria spp. Levels of Listeria were usually low, but 20 samples of cheese contained more than 1000 cfu/g. Most strains of L. monocytogenes belonged to serotype 1/2 (58%) or serotype 4b (33%).     

The development of a combined surface adhesion and polymerase chain reaction technique in the rapid detection of Listeria monocytogenes in meat and poultry.

A procedure combining enrichment surface adhesion and polymerase chain detection (SA-PCR) was developed and applied in the detection of Listeria monocytogenes in meat products. Minced beef samples were inoculated with L. monocytogenes (log(10)3 cfu g(-1)) and incubated for 10 h at 30 degrees C in buffered peptone water. L. monocytogenes was recovered from the culture by attachment to a polycarbonate membrane immersed for 15 min in the enriched meat culture. The membrane and attached bacteria were removed from the culture and the membrane dissolved in phenol:chloroform. The DNA was extracted from the bacteria and a PCR assay was carried out using primers directed against the listerolysin O gene of L. monocytogenes. The combined (SA-PCR) technique had a detection limit of log10 4.0 cfu ml(-1) in enriched meat cultures.The rapid technique was applied to a small number of retail samples (n = 100) and was found to compare favourably to the standard cultural method. A total of 12 samples were confirmed positive for L. monocytogenes using the standard cultural method and the SA-PCR assay. No false positives or negatives were recorded by either method.     

The incidence of Salmonella and Listeria in raw milk from farm bulk tanks in England and Wales

Raw milks from farm bulk tanks in England and Wales were sampled over a 15 month period in 1992–93 and analysed for Salmonella spp, Listeria spp and Listeria monocytogenes. Of 1673 samples tested for Salmonella spp, six (0.36%) yielded a positive result. A total of 2009 samples were analysed for Listeria spp. Of these, 310 (15.43%) were found positive for Listeria spp, 102 (5.08%) of the positive samples yielded L monocytogenes, which represented 33% of the Listeria isolations. There was a significant rise in the isolation rate for listerias between October and March broadly in line with the period during which the cows were housed indoors. The highest isolation rate was in January, 25.89% in 1992 and 28.4% in 1993, and the lowest isolation rate (3.1%) in August. Although the incidence of L monocytogenes broadly paralleled the trend for total Listeria, the highest percentage of L monocytogenes to total Listeria was found in the months of April to September when the total Listeria isolations were at a minimum. Thirty two (1.59%) samples were found to be positive for Listeria spp prior to enrichment procedure, with 17 (0.85%) positive for L monocytogenes. The highest counts reported for L monocytogenes were 62 and 30 colony forming units (cfu)/ml with over 60% at a level of less than 10 cfu/ml. Of the direct isolations, some 78% were obtained between the months of March and July.     

Bacterial populations associated with meat from the deboning room of a high throughput red meat abattoir

Developing countries are faced with high incidences of food poisoning outbreaks, with obvious economic consequences. In highly perishable foodstuffs such as fresh red meat the threat of food poisoning is particularly intense. In this study, red meat samples were collected from a deboning room of a high throughput abattoir. The samples were analysed for the presence of Bacillus cereus., Staphylococcus aureus., Pseudomonas spp., Listeria monocytogenes., Escherichia coli and Salmonella spp. The aerobic plate counts as well as Enterobacteriaceae were also enumerated. Almost without exception the counts exceeded the microbiological guidelines for raw meat as proposed by the South African Department of Health. The average B. cereus count over the sampling period was 8.32 × 10(3) cfu, g (-1), for S. aureus and Pseudomonas spp. 1.72 × 10(5) and 1.7 × 10(5) cfu g(-1) respectively and for E. coli 3.4 × 10(5) cfu g(-1). Sixty percent of the samples were positive for presumptive Salmonella spp. while 52% of the samples tested positive for the presence of L. monocytogenes. The aerobic plate and Enterobacteriaceae counts were 1.7 × 10(7) and 4.6 × 10(6) cfu g(-1), respectively. The data highlighted the need for a more systematic approach to ensuring safe food through implementing quality control methods to prevent the entry and proliferation of pathogens in meat and meat products, especially during processes with a high degree of handling, such as deboning.     

Incidence of Listeria monocytogenes in different types of meat products on the Belgian retail market

A survey was undertaken to determine the incidence and numbers of L. monocytogenes in a variety of meat products (cooked meat products, raw cured meat products (dried or not), mayonnaise based salads and prepared meals). As expected, raw cured meat products were significantly higher contaminated with L. monocytogenes than cooked meat products, 13.71% (113/824) and 4.90% (167/3405), respectively. Also a larger proportion of raw cured meat product samples contained a high initial level of the pathogen ( > 10 cfu/g). Higher incidence rates were obtained for whole cooked meat products (e.g. cooked ham, bacon) after slicing than before slicing, 6.65 and 1.56%, respectively, indicating cross-contamination. Due to multiple handling and processing steps, the incidence rate of the pathogen was higher for cooked minced meat products than for whole cooked meat products, 6.14 and 3.96%, respectively. No significant differences were obtained in the incidence of L. monocytogenes in whole cured meat products (e.g., raw ham) and minced cured meat products (e.g., dry fermented sausage), 14.92 and 11.69%, respectively. Lower incidence rates of L. monocytogenes were obtained for raw, cured meat products using beef or horse meat, 4.65 and 5.88%, respectively, A high incidence rate of L. monocytogenes was noted for the mayonnaise based salads (21.28% (186/874)) as well as for prepared meals (11.70% (92/786)), the latter especially due to contamination of vegetarian meals.     

Molecular ecology of Listeria monocytogenes and other Listeria species in small and very small ready-to-eat meat processing plants

A longitudinal study was conducted to track Listeria contamination patterns in ready-to-eat meats from six small or very small meat processing plants located in three states over 1 year. A total of 688 environmental sponge samples were collected from nonfood contact surfaces during bimonthly visits to each plant. Overall, L. monocytogenes was isolated from 42 (6.1%) environmental samples, and its prevalence ranged from 1.7 to 10.8% across different plants. Listeria spp., other than L. monocytogenes, were isolated from 9.5% of samples overall, with the prevalence ranging from 1.5 to 18.3% across different plants. The prevalence of L. monocytogenes correlated well with that of other Listeria spp. for some but not all plants. One L. monocytogenes isolate representing each positive sample was characterized by molecular serotyping, EcoRI ribotyping, and pulsed-field gel electrophoresis typing. Seven sample sites tested positive for L. monocytogenes on more than one occasion, and the same ribotype was detected more than once at five of these sites. Partial sigB sequencing was used to speciate other Listeria spp. isolates and assign an allelic type to each isolate. Other Listeria spp. were isolated more than once from 14 sample sites, and the same sigB allelic type was recovered at least twice from seven of these sites. One plant was colonized by an atypical hemolytic L. innocua strain. Our findings indicate that small and very small meat processing plants that produce ready-to-eat meat products are characterized by a varied prevalence of Listeria, inconsistent correlation between contamination by L. monocytogenes and other Listeria spp., and a unique Listeria molecular ecology.