Prevalence and antimicrobial resistance patterns of Listeria species isolated from poultry products marketed in Iran

In the present study, a total of 402 poultry product samples composed of raw, ready-to-cook (RTC) and ready-to-eat (RTE) products were examined for the presence of Listeria spp. The total contamination rate with Listeria spp. in poultry products was 33.3% with a higher rate of contamination in warm seasons than in cold seasons. The most species recovered was Listeria innocua (46.3%); the remaining isolates were Listeria monocytogenes (38.8%), Listeria ivanovii (9.7%) and Listeria seeligeri (5.22%). L. monocytogenes was detected in 14.1%, 12.2% and 11.4% of raw, RTC and RTE poultry products, respectively. Serotype 4b (44.9%) was the predominant serotype of L. monocytogenes isolates followed by 1/2a (40.8%), 1/2b (10.2%) and 1/2c (4.08%). Considering seasonal variability, 1/2a was the most prevalent serotype in warm seasons, while 4b was predominant in cold seasons. The Listeria spp. particularly L. monocytogenes isolates were highly resistant to ampicillin, penicillin, fluroquinolones and tetracycline. The results indicate that high prevalence of Listeria spp. especially L. monocytogenes in poultry products, and resistance of the isolates to the antimicrobials commonly used to treat human listeriosis could be a potential health hazard for consumers. In addition, prevalence of L. monocytogenes serotype 4b that involved in the majority of foodborne outbreaks of human listeriosis is a public health concern.     

Prevalence of Listeria spp. in ready to eat foods (RTE) from Algiers (Algeria)

The aim of this study was to establish the occurrence of Listeria spp., especially Listeria monocytogenes in ready to eat RTE food marketed in Algiers (Algeria).A total of 227 samples were collected from different producers and retailers.All samples were analyzed using a conventional cultivation method AFNOR V08-055.Out of 227 samples tested, 21 (9.3%) tested positive for Listeria spp. among them, 6 (2.6%) tested positive for L. monocytogenes. L. innocua was the most common Listeria species found being detected in 11 samples (4.8%), although both Listeria ivanovii and Listeria welshimeri were detected in 3 (1.3%) and 1 (0.4%) food samples respectively.The study of the antimicrobial sensitivity of Listeria monocytogenes strains showed no resistance.The study has enabled us to detect these contaminants in a wide range of RTE foods, to suggest that contamination likely occurs after heat treatment, and to assess the danger represented by this category of food for populations at risk.     

Growth inhibition effects of ferulic acid and glycine/sodium acetate on Listeria monocytogenes in coleslaw and egg salad

Listeria monocytogenes is a bacterium responsible for food poisoning through ready-to-eat (RTE) food products. In particular, salads are RTE products that lead to many cases of listeriosis. Such concerns have made it necessary to find a method of inhibiting Listeria growth. In this study, coleslaw and egg salads were inoculated with L. monocytogenes, followed by addition of either ferulic acid or ferulic acid + glycine/sodium acetate, and were incubated at 10 °C for a maximum of 5 days. In coleslaw, the addition of 1500 ppm ferulic acid resulted in a 1.5 log CFU/g reduction in L. monocytogenes after 5 days. In egg salad, for 5 days following the addition of 3000 ppm ferulic acid + 1% glycine/sodium acetate compound, no additional L. monocytogenes growth was observed. This study demonstrates that under particular conditions, ferulic acid has anti-bacterial properties against L. monocytogenes. Our results suggest that ferulic acid could be highly useful for inhibiting the growth of L. monocytogenes in salad products.     

Prevalence of Listeria spp. and antibiotic susceptibility of Listeria monocytogenes isolated from raw chicken and ready-to-eat chicken products in Jordan

Listeria monocytogenes is the causal agent of listeriosis, a disease that can be serious and is often fatal in susceptible individuals. The objective of the study was to determine the prevalence of Listeria spp. in raw chicken and ready-to-eat (RTE) chicken products in Amman, Jordan and the antimicrobial resistance of L. monocytogenes isolates. A total of 280 raw chicken and RTE chicken products (chicken-shawirma, chicken-burger, chicken-sausage and mortadella) were collected from Amman abattoir and local retail markets in Amman city. Listeria spp. were isolated by the conventional International Organization for Standardization (ISO) method and L. monocytogenes identified by biochemical and Polymerase Chain Reaction (PCR). Results of conventional method showed that out of total 280 samples, 141 (50%) were found to be contaminated with Listeria spp. [L. monocytogenes (18.2%), Listeria ivanovi (26.1%), Listeria grayi (3.5%), Listeria seeligeri (1.8), Listeria welshimeri (0.7%)]. The PCR confirmed all L. monocytogenes isolates (51 isolates: 15 from raw dressed broiler chicken, 23 from chicken-burger, 9 from chicken-sausage, and 4 from chicken-shawirma). Five of the tested L. monocytogenes isolates were resistance to two antibiotics (tilimicosin and tetracycline) among the ten tested antibiotics as determined by microbroth dilution method. The results presented in this study indicate the potential risk of contamination of RTE chicken products with L. monocytogenes.     

Listeria monocytogenes in ready-to-eat vacuum and modified atmosphere packaged meat and fish products of Estonian origin at retail level

The prevalence, counts and genetic diversity of Listeria monocytogenes in ready-to-eat (RTE) vacuum and modified atmosphere packaged meat and fish products was studied in Estonia. Within two consecutive years 370 RTE food samples were collected at retail level from which 11% were found to be positive for L. monocytogenes. Contamination was higher among RTE fish products (17%) than in RTE meat products (6%). Generally, the counts of L. monocytogenes in positive products remained under ten colony forming units (CFU) per gram of product. Only 1.6% of the RTE meat and fish products contained L. monocytogenes in range of 10–100 CFU/g and 0.3% more than 100 CFU/g at the end of shelf-life. The food category containing highest L. monocytogenes prevalence was RTE lightly salted fish products with the prevalence of 32%. Only one (0.3%) RTE food sample exceeded the 100 CFU/g food safety criterion set out in the EU Regulation 2073/2005. Pulsed-field gel electrophoresis (PFGE) characterization of the isolates showed an overall similarity higher than 70%, and nine clusters based on 100% similarity were revealed. PFGE genotyping revealed that the few predominant pulsotypes were associated with particular food plants.     

Antimicrobial resistance profiles of Listeria monocytogenes isolated from ready-to-eat products in Poland in 2007–2011

The aim of the study was to characterize strains of Listeria monocytogenes isolated from ready to eat (RTE) products collected as part of official food control and monitoring in Poland. A total of 105 L. monocytogenes isolates from RTE products: 54- cakes and 51 – delicatessen products were examined. The presence L. monocytogenes in cakes and delicatessen products was 0.4% and 0.7% respectively suggesting the level of contamination of RTE products with L. monocytogenes is very low.     

Occurrence of Listeria spp. in dairy plants in Southern Italy and molecular subtyping of isolates using AFLP

We report the detection of Listeria spp. and Listeria monocytogenes in 34 dairy plants. In total, 547 of food, product contact surface and floor drain samples were collected along the product lines. Nineteen cheese factories (55.8%) were contaminated by Listeria spp. Of these 20.6% were L. monocytogenes positive. Listeria spp. was found in 6.8% of food samples, 11.3% of product contact surfaces and 40.6% of floor drains. L. monocytogenes was found in 2.4% of food samples, 4.9% of product contact surfaces and 18.8% of floor drains. Twentyfive L. monocytogenes isolates were serotyped using commercial specific antisera and genotyped using Amplified Fragment Length Polymorphism (AFLP) analysis. AFLP genotyping discriminated the four species of Listeria isolated and different genotypes for each species, moreover it could identify persistent genotypes in some dairy facilities. Listeria spp. and L. monocytogenes are widely spread in the dairy sector and probably contaminate foods during the production process. Facility-based monitoring can identify possible routes of transmission and thus allow establishment of more effective strategies to prevent food contamination.     

Invasiveness of Listeria monocytogenes strains of Caco-2 cells in response to a period of extreme salt stress reflecting salt-curing and rehydration of cod (Gadus morhua L.)

Products like salt-cured fish contain approximately 15–21% NaCl and are rehydrated to 2–3% NaCl before preparation and consumption. These products are regarded as safe, but it has been shown that Listeria spp. is able to survive at extreme levels of salt and start to grow after rehydration. Thus, the ability of salt stressed Listeria monocytogenes to cause listeriosis, measured as its ability to invade Caco-2 cells was studied in this paper. Seven strains of L. monocytogenes and one Listeria innocua were cultivated in BHI to early and late stationary phase at 4 °C. At both phases, the strains were exposed to either no salt or to salt stress comparable to that applied in the production of rehydrated salt-cured cod, i.e. 21% NaCl followed by dilution to 2% NaCl. In addition, the eight strains were cultivated in BHI with 2% NaCl, which is similar to the salt content as in rehydrated salt-cured cod and other ready-to-eat (RTE) products as well. The ability of non salt stressed L. monocytogenes strains to enter Caco-2 cells was significant higher (p > 0.05) compared to the corresponding strains exposed to 21% NaCl for 96 h, followed by 2% NaCl for 48 h. On the other hand, L. monocytogenes cultivated in BHI with 2% NaCl showed a higher invasiveness of Caco-2 cells than both the other sample categories. As the ability to invade Caco-2 cells correlates with bacterial virulence, the results suggests that L. monocytogenes represent a lower food safety risk when exposed to salt-curing with extreme NaCl concentrations than exposure of a constant and moderate level of salt commonly used in RTE products.     

Hygienic conditions and microbiological status of chilled Ready-To-Eat products served in Southern Spanish hospitals

Chilled Ready-To-Eat (RTE) foods are of concern in hospital foodservices because they can support microbial growth when subjecting to time/temperature abuses during processing and distribution together with poor handling practices. This study was conducted in five different hospitals (A–E) of Southern Spain during 2008–2009 to perform an evaluation of their sanitary conditions and microbiological quality of two RTE meals: lettuce salads and cooked ham. A checklist based on hygiene principles embedded in Food European legislation was developed and applied in each hospital. In parallel, microbiological analysis of food contact surfaces, air quality and time/temperature measurements along the distribution chain were carried out. RTE samples (n = 150) were examined for mesophilic aerobic bacteria (MAB), total coliforms, coagulase-positive Staphylococci (CPS), Escherichia coli, Listeria spp. and Salmonella spp. Differences were found between hospitals regarding handling practices and cleanliness of working surfaces. Cooked ham samples presented lower counts of MAB and total coliforms (<10³ and <10 cfu/g respectively) than lettuce salads (10⁴ to 10⁵ and 10 to 10⁴ cfu/g respectively), although concentration of CPS was higher in cooked ham samples reaching maximum levels close to 10³ cfu/g. Neither Listeria spp. nor Salmonella spp. were detected in any food sample. Prevalence of E. coli was low (3%). Surface counts and air quality presented high variability among the different hospitals evaluated. It was concluded that good manufacturing practices and HACCP principles should be followed together with special training of food handlers. This study can help risk managers to better define the control measures to be adopted in healthcare settings in order to prevent foodborne infections.     

Investigation on prevalence of Listeria spp. and Listeria monocytogenes in animal-derived foods by multiplex PCR assay targeting novel genes

Chilled and frozen animal-derived food can be contaminated by Listeria spp., emerging foodborne pathogens in food industry. The objective of this study was to mine novel target genes by comparative genomics approach for multiplex PCR detection and differentiation of Listeria monocytogenes and other Listeria spp. in food. Multiplex PCR assay targeting the genetic markers LMOf2365_2721, AX25_00730, lin1814, int, lwe1673, and Oxidoreductase gene, resulted in the amplification of DNA fragments of 583 bp, 703 bp, 421 bp, 994 bp, 345 bp, and 201 bp from L. monocytogenes, Listeria ivanovii, Listeria innocua, Listeria seeligeri, Listeria welshimeri, and Listeria grayi, respectively. The detection limits of the multiplex assays were as low as 89 fg/μL genomic DNA and 910 CFU/mL of bacterial culture. The prevalence of Listeria spp. was determined using the developed multiplex PCR assay and standard microbiological method in a total of 200 food samples collected from different supermarkets and traditional agri-product markets in Nanjing, China. A total of 28 samples were found to be positive for the presence of Listeria, including 10.9% (6/55) of livestock meat samples, 22% (11/50) of poultry samples, 15% (6/40) of shellfish samples, 13.3% (4/30) of octopus samples and 4% (1/25) of freshwater fish samples. Of these, 13 isolates were classified as L. monocytogenes, 11 were classified as L. innocua, 2 were classified as L. ivanovii and 3 were classified as L. welshimeri. These results demonstrate that the multiplex PCR assay based on novel target genes is able to rapidly detect the Listeria spp. in 12 h with high accuracy and sensitivity, which may be used in the future for detection of Listeria spp. in animal-derived food products.     

Occurrence and traceability of Listeria monocytogenes strains isolated from sheep's milk cheese-making plants environment

The aim of the study was to conduct an extensive survey on Listeria monocytogenes and Listeria spp. environmental contamination in 13 cheese-making plants. A total of 409 environmental and food samples were collected during years 2011–2013. Listeria spp. contamination was observed in all the facilities, while L. monocytogenes was recovered from 12 facilities with a prevalence ranging between 3.0% and 22.6%. Floor drains were the most contaminated sampling sites (48.8% of positive samples), serving as harbourage site for subsequent contamination. Out of 616 isolates, 277 (45.0%) were Listeria innocua, 274 (44.5%) L. monocytogenes, 41 (6.6%) Listeria ivanovii, 14 (2.3%) Listeria welshimeri and 10 (1.6%) Listeria gravyi. Serotyping carried out by PCR and agglutination method for L. monocytogenes revealed that 169 strains (61.7%) were serotype 1/2a, 65 (23.7%) 4b, 20 (7.3%) 1/2b, 10 (3.6%) 3a, 7 (2.5%) 1/2c and 3 (1.1%) 3b. PFGE conducted on L. monocytogenes isolates using AscI and ApaI restriction enzymes, yielded 6 clusters. Two predominant PFGE clusters were observed including respectively 36 and 32 strains. Within cheese-making plants, L. monocytogenes showed wide variability with strains distributed up to 4 different clusters. Pulsotypes isolated from raw milk filter were never detected in the processing environment, indicating that the contamination originated from sources other than raw milk. The isolation of strains with similar profile from different sampling sites, within and among cheese-making plants, indicated the possible transfer of L. monocytogenes contamination along production lines and from one facility to another. Strains recovered from food were confirmed as originating from the processing environment.     

Microbiological quality of ready-to-eat minimally processed vegetables consumed in Brazil

Minimally processed leafy vegetables are ready-to-eat (RTE) products very attractive to consumers looking for healthy and convenient meals. However, the microbiological safety of these foods is of special concern due to the absence of lethal treatments during processing. In the present study, indicator microorganisms, Listeria spp. and Salmonella spp. were determined for 162 samples of minimally processed leafy vegetables commercialized in Brazil. Psychrotrophic aerobic bacterial populations >5 log CFU/g were found in 96.7% of the samples, while total and thermotolerant coliforms were detected respectively in 132 (81.5%) and 107 (66%) of vegetables analyzed. Escherichia coli was present in 86 (53.1%) samples analyzed and Listeria spp. and Salmonella spp. were detected respectively in 6 (3.7%) and 2 (1.2%) samples. These results indicate the need of implementing quality programs in the production chain of RTE vegetables to improve shelf life and microbiological safety.     

A survey estimating the benefits of incorporating Listeria specific growth inhibitors in bulk luncheon meats to be sliced in retail delis

The rate of foodborne illness caused by Listeria monocytogenes continues to exceed the Healthy People 2020 goal of 0.2 cases per 100,000 persons. Listeria infections are primarily sporadic, most cases caused by eating contaminated, ready-to-eat (RTE) foods including luncheon meats sliced in retail delis which have been implicated as being responsible for as many as 83% of these illnesses. Listeria specific antimicrobials incorporated in RTE luncheon meats to be sliced in retail delis would lower the risk to consumers by as much as 96%, especially for high-risk consumers. Walmart and Sam's Club stores (Bentonville, AR), large retailers of RTE meats sliced in their delis, have required all their suppliers of bulk RTE meats which could support the growth of Listeria to include a verified inhibitor that will not allow an increase in L. monocytogenes of more than 1 log during the intended shelf-life and storage of the product. We surveyed these suppliers and determined that six of 15 suppliers had not added inhibitors to their bulk luncheon meat for Walmart prior to the 2010 requirement. One supplier reported using inhibitors in 60% of their products prior to Walmart's mandate and now uses Listeria specific inhibitors in 100% of the bulk deli meats it produces, regardless of customer. Three of the five manufacturers who needed to reformulate their products reported additional benefits: the Listeria specific antimicrobials extended their products' shelf life, improved food safety and provided better protection for their customers.     

Listeria detection in microscale solid state inoculation with minimal selective agents

The food industry needs a simple, reliable, and cost-effective primary screening protocol for routine inspection of bacterial contamination. Microscale inoculation technique is proposed as an alternative method for Listeria detection. This proposed novel technique shows a good correlation with standard spread plate technique for the enumeration of pure Listeria cultures (R² = 0.96, P < 0.0001). The commonly used selective agents in Listeria media (PALCAM, MOX, and OCLA) were assessed for their inhibitory effect on Listeria monocytogenes, Listeria innocua, and Listeria ivanovii using the microscale inoculation technique. The concentration of selective agents was lowered to inhibit competitive bacteria with minimum impact on the growth of Listeria. At the standard concentration, all three media showed less inhibition on L. monocytogenes and L. innocua than on L. ivanovii. OCLA had less inhibitory effect on the three species than did MOX and PALCAM. The comparison between ISO 11290-1 and microscale with 25% of selective agent concentration on detection of Listeria in 36 naturally contaminated food samples revealed that the microscale technique agreed well with the ISO method (Cohen KAPPA = 0.83). Reduction of selective agent concentration to 25% of the conventional formulas resulted in substantial improvement in detectability of Listeria spp. without significantly reducing specificity. The detection sensitivity of the Listeria colonies in food samples was significantly improved by microscale inoculation with 25% of the inhibitors on the three selective media at 24 and 48 h incubation (ANOVA, P = 0.039). At 48 h incubation the improvement was from 95%, 90%, and 85% (ISO method with regular strength inhibitors) to 100%, 100%, and 90% (microscale) on OCLA, MOX, and PALCAM respectively. There was no discrepancy between the microscale and the ISO method in detection of L. monocytogenes in the food samples. The optimization of Listeria detection improves sensitivity of detection as well as reducing media volume which may reduce costs and waste generated.     

Simultaneous detection of Listeria species isolated from meat processed foods using multiplex PCR

Listeriosis is a foodborne disease caused by the pathogenic Listeria monocytogenes and is considered as a serious health problem due to the severity of symptoms and its high mortality rate. Listeria genus is divided into six species and especially L. monocytogenes is an important foodborne pathogen in humans and livestock. Recently, other Listeria species are reported as pathogenic strains in decayed foods and environments as well. High mortality rate of listeriosis demands for rapid methods to detect the potential presence of the food pathogens in the food industry. We have developed a multiplex PCR for rapid and simultaneous detection of six Listeria species including Listeria grayi, Listeria innocua, Listeria ivanovii, L. monocytogenes, Listeria seeligeri and Listeria welshimeri to identify specific Listeria species in processed foods. The optimized multiplex PCR in this study utilized one Listeria genus specific and each Listeria species-specific primer pairs. Each primer pair yields the products of 370-bp for Listeria genus-specific, 201-bp for L. grayi-specific, 749-bp for L. innocua-specific, 463-bp for L. ivanovii-specific, 509-bp for L. monocytogenes-specific, 673-bp for L. seeligeri-specific and 281-bp for L. welshimeri-specific. We have successfully applied multiplex PCR strategy to 93 Listeria isolates from processed meat products to determine specific Listeria species and out of which 81 strains of L. monocytogenes, 10 strains of L. innocua and 2 strains of L. welshimeri were identified. This established multiplex PCR provides rapid and reliable results and will be useful for the detection of Listeria species in contaminated food products and clinical samples.     

Presence of Listeria monocytogenes in Chilean food matrices

ObjectivesTo compare Listeria monocytogenes strains obtained from food matrices with those from environmental samples in the same food processing plant.Methods and resultsBetween 2008 and 2012 the presence of L. monocytogenes was evaluated in 2647 food samples. A total of 448 work surfaces and 92 equipment's were also evaluated from 6 plants which produce ready-to-eat (RTE) foods in Santiago, Chile. An additional selected sample of hand and nails samples was also obtained from 13 food handlers working in a sausage elaboration plant.As a whole L. monocytogenes was present in 265 (10%) food samples and 22 (4%) environmental samples. The foods with highest recovery were red meats 14/60 (23%), poultry 223/1196 (19%), the remaining samples accounted a total of 27/1391 (2%). The environmental samples positive for L. monocytogenes were obtained from two food plants both the cheese 8/8 (100%) and from a fresh peaches exporter 3/3 (100%). Finally L. monocytogenes was isolated from 5/13 (38%) food handlers studied.ConclusionsThe study confirms the presence of L. monocytogenes in different matrices, especially in meat and RTE products. Analyses conducted on work surfaces revealed that contamination comes mostly from both raw materials and surfaces in indirect contact with foods.Significance and impact of studyThe study reinforces the need for companies to apply regulations related to food quality and safety systems (HACCP, Hazard Analysis & Critical Control Points) to prevent L. monocytogenes contamination from food processing plants.     

Listeria monocytogenes in raw salad vegetables sold at retail level in Malaysia

A range of commercially available vegetables (n = 306) that are consumed in the minimally processed state in Malaysia was examined for the presence of Listeria spp. and Listeria monocytogenes to provide information on the occurrence of such organisms in these vegetables. Analysis was carried out using the most probable number–polymerase chain reaction (MPN–PCR) method. It was found that Listeria spp. and L. monocytogenes could be detected in 33.3% and 22.5% of the vegetables respectively. L. monocytogenes was more frequently detected in Vigna unguiculata (Japanese parsley) at 31.3% and Oenanther stolonifera (yardlong bean) at 27.2%.     

Incidence and genetic variability of Listeria species from three milk processing plants

The presence of Listeria in three milk processing environments as a potential source of milk contamination was assessed. Swab samples (n = 210) taken from milk processing plants were examined. Sample sites included the milk processing equipment, besides areas handling raw and pasteurized milk. The USDA Listeria-selective enrichment procedure was used to process the samples. Forty one (19.52%) Listeria isolates were recovered. The isolates were further subjected to biochemical and genotypic characterization. Out of 41 isolates, 16 (7.62%) were confirmed as Listeria monocytogenes, 2 (0.95%) as L. ivanovii, 19 (9.05%) as L. innocua. 1 (0.48%) as L. seeligeri and 3 (1.43%) as L. grayi. All the L. monocytogenes isolates were positive for the hlyA gene. PCR based serotyping revealed all L. monocytogenes to be of 1/2a, 1/2c, 3a and 3c serovar group. AscI and ApaI restriction analysis yielded four PFGE clusters for 16 L. monocytogenes isolates obtained from raw milk collector, milk silos, buttermilk mixer, cheese and other milk product processor. No predominant PFGE cluster was observed among these L. monocytogenes isolates. The main sources of L. monocytogenes were found to be raw milk collector and milk silos. In the present study L. monocytogenes was isolated from milk and milk products processing plants which could cross-contaminate the processed products and may possess a potential threat to public health.     

Genotypic characterization and antimicrobial resistance of Listeria monocytogenes from ready-to-eat foods

Listeria monocytogenes is an important foodborne pathogen. The aims of this study were to determine genetic relatedness of L. monocytogenes isolated from ready-to-eat (RTE) foods in Malaysia. L. monocytogenes isolates from RTE foods were characterized by multiplex-PCR serotyping, REP-PCR, BOX-PCR, RAPD, PFGE, virulotyping and antibiotyping. Of the 32 L. monocytogenes isolates analyzed, 21 (65.6%) were assigned to serogroup “1/2a, 3a”, seven (21.9%) serogroup “1/2c, 3c”, and four (12.5%) serogroup “4b, 4d, 4e”. All the L. monocytogenes harbored inlA, inlB, inlC and inlJ virulence genes. More than half (53%) L. monocytogenes isolates were resistant to penicillin G, followed by tetracycline (15.6%), amoxicillin-clavulanic acid (12.5%), vancomycin (9.4%) erythromycin (6.3%), clindamycin, streptomycin, kanamycin, and chloramphenicol (each 3.1%). REP-PCR, BOX-PCR, RAPD and PFGE generated 28 (D = 0.992), 31 (D = 0.998), 32 (D = 1), and 20 (D = 0.916) patterns, respectively. These results indicate that L. monocytogenes isolates from RTE food were heterogeneous. There was no correlation between antibiograms and serogroups or pulsotypes or PCR-typing and/or sources of isolates. Since different subtyping methods often give different discriminatory powers, the use of more than one subtyping approach is necessary in providing a more accurate picture of the genetic diversity of L. monocytogenes. In conclusion, L. monocytogenes isolates from RTE possess the internalin genes and are genetically diverse. Furthermore, the occurrence of resistant isolates belonging to epidemiologically important serogroups “1/2a, 3a” and “4b, 4d, 4e” in RTE foods is a matter of public health concern.     

Occurrence and diversity of Listeria spp. in seafood processing plant environments

Listeria contamination in processing plant environments is a major issue for the seafood industry worldwide; faster and more reliable results are therefore desired for early detection and monitoring of environmental Listeria spp. This study aimed to gain a better understanding of the prevalence and diversity of Listeria spp., and to evaluate a rapid detection method, the 3M Molecular Detection Assay (MDA) Listeria, for its ability to detect Listeria spp. in environmental samples from seafood processing plants. Duplicate environmental sponge samples (n = 444) were collected from 152 different sites within three seafood processing plants, and analyzed for Listeria spp. by the MDA method (after 26 and 48 h of enrichment) and the U.S. Food and Drug Administration Bacteriological Analytical Manual method. Overall, detection of Listeria spp. by the two methods did not differ significantly (p > 0.05); 11 (4.9%) and 13 (5.9%) samples were positive for Listeria spp. by the MDA and FDA-BAM method, respectively. The sensitivity of the MDS was 87.0% (95% CI: 77.4–96.6%), specificity was 97.6% (95% CI: 95.5–99.7%), accuracy was 95.3%, and the positive predictive value was 89.4% (95% CI: 80.5–98.2%). Classification of 19 Listeria isolates by partial SigB sequencing analysis identified three allelic types. Twelve of these isolates were ATs 58 and 60 which were classified as Listeria monocytogenes lineage I and serotypes 1/2b, 3b, 4b, 4d, 4e, by multiplex-PCR serotyping. Six Listeria isolates were classified as Listeria innocua (AT31). Our data show that the 3M Molecular Detection Assay Listeria provides rapid and reliable results for detection and monitoring of Listeria spp., which are important for seafood processing plants. Effective Listeria monitoring programs will allow for improved development of Listeria control measures in order to minimize cross-contamination in finished products.