Comparative evaluation of culture- and BAX polymerase chain reaction-based detection methods for Listeria spp. and Listeria monocytogenes in environmental and raw fish samples

Design Elements of Listeria Environmental Monitoring Programs in Food Processing Facilities: A Scoping Review of Research and Guidance Materials

Occurrence of Listeria monocytogenes (Lm), the causative agent of listeriosis, in food processing facilities presents considerable challenges to food producers and food safety authorities. Design of an effective, risk-based environmental monitoring (EM) program is essential for finding and eliminating Lm from the processing environment to prevent product contamination. A scoping review was conducted to collate and synthesize available research and guidance materials on Listeria EM in food processing facilities. An exhaustive search was performed to identify all available research, industry and regulatory documents, and search results were screened for relevance based on eligibility criteria. After screening, 198 references were subjected to an in-depth review and categorized according to objectives for conducting Listeria sampling in food processing facilities and food sector. Mapping of the literature revealed research and guidance gaps by food sector, as fresh produce was the focus in only 10 references, compared to 72 on meat, 52 on fish and seafood, and 50 on dairy. Review of reported practices and guidance highlighted key design elements of EM, including the number, location, timing and frequency of sampling, as well as methods of detection and confirmation, and record-keeping. While utilization of molecular subtyping methods is a trend that will continue to advance understanding of Listeria contamination risks, improved study design and reporting standards by researchers will be essential to assist the food industry optimize their EM design and decision-making. The comprehensive collection of documents identified and synthesized in this review aids continued efforts to minimize the risk of Lm contaminated foods.

Lateral-flow enzyme immunoconcentration for rapid detection of Listeria monocytogenes

Lateral-flow enzyme immunochromatography coupled with an immunomagnetic step was developed for rapid detection of Listeria monocytogenes in food matrices. The target bacteria was first separated and concentrated by magnetic nanoparticles containing the enzyme and directly applied to the assay system to induce an antigen-antibody reaction without any additional steps. The color signals produced by an enzyme-substrate reaction at a specific site on the immunostrip were found to be directly proportional to the concentration of L. monocytogenes in the sample. The detection concept was demonstrated by performing an enzyme immunoassay on a microtiter well prior to applying it to the lateral-flow assay. Results of the chromatographic analysis yield a limit of detection of 95 and 97 ± 19.5 CFU/mL in buffer solution and 2 % milk sample, respectively. In addition to the high sensitivity, it was also possible to shorten the separation and detection time to within 2 h. The system also showed no cross-reactivity with other bacteria (e.g., Escherichia coli O157:H7, Salmonella typhimurium, and Salmonella enteritidis). The analytical procedure developed will enable us to not only utilize the assay in the field where fast screening for pathogenic agents is required but also as a preventive measure to contain disease outbreak.

Comparison of automated BAX PCR and standard culture methods for detection of Listeria monocytogenes in blue Crabmeat (Callinectus sapidus) and blue crab processing plants

This study compared the automated BAX PCR with the standard culture method (SCM) to detect Listeria monocytogenes in blue crab processing plants. Raw crabs, crabmeat, and environmental sponge samples were collected monthly from seven processing plants during the plant operating season, May through November 2006. For detection of L. monocytogenes in raw crabs and crabmeat, enrichment was performed in Listeria enrichment broth, whereas for environmental samples, demi-Fraser broth was used, and then plating on both Oxford agar and L. monocytogenes plating medium was done. Enriched samples were also analyzed by BAX PCR. A total of 960 samples were examined; 59 were positive by BAX PCR and 43 by SCM. Overall, there was no significant difference (P ≤ 0.05) between the methods for detecting the presence of L. monocytogenes in samples collected from crab processing plants. Twenty-two and 18 raw crab samples were positive for L. monocytogenes by SCM and BAX PCR, respectively. Twenty and 32 environmental samples were positive for L. monocytogenes by SCM and BAX PCR, respectively, whereas only one and nine finished products were positive. The sensitivities of BAX PCR for detecting L. monocytogenes in raw crabs, crabmeat, and environmental samples were 59.1, 100, and 60%, respectively. The results of this study indicate that BAX PCR is as sensitive as SCM for detecting L. monocytogenes in crabmeat, but more sensitive than SCM for detecting this bacterium in raw crabs and environmental samples.

Detection, isolation, and incidence of Listeria spp. in small-scale artisan cheese processing facilities: a methods comparison

Environmental sampling, focused on environmental sites with a high probability of contamination, was conducted in eight artisan cheese processing facilities. Samples (n = 236) from 86 food contact surfaces and 150 non-food contact surfaces were examined for the presence of Listeria spp. by comparing the efficacy of three different primary enrichment media used in five detection and isolation methods. University of Vermont broth was the most sensitive primary enrichment medium for the detection of Listeria spp., including Listeria monocytogenes. These results, however, did not differ significantly from those obtained with Listeria repair broth or Oxoid 24 Listeria enrichment broth. When full methods were considered, the use of dual enrichment methods identified the most Listeria spp.-positive samples, whereas the BAX System PCR Assay for the Genus Listeria 24E provided the most rapid results (approximately 30 h). Cultural results from the direct plating of secondary enrichment broths were generally in agreement with PCR results when compared within methods. Despite minor differences in efficacy, all five methods were in agreement with one another. Overall, 24 (10.7%) of the 236 environmental samples were positive for Listeria spp., all of which were collected from non-food contact surfaces. Nine of these sites were also positive in previous sampling events, suggesting that these sites serve as Listeria niches and that certain ribotypes are particularly persistent, inhabiting environments and specific sites for over 2 years. Overall, our results suggest that the extent of Listeria spp. contamination, notably L. monocytogenes, in small-scale artisan cheese processing environments is low.

Evaluation of environmental sampling methods and rapid detection assays for recovery and identification of Listeria spp. from meat processing facilities

Studies that isolated Listeria spp. from the environment of two meat processing facilities were conducted. Samples were collected in the processing environment of the facilities with three different sampling methods (cotton swab, sterile sponge, and composite-ply tissues) to evaluate their ability to recover Listeria spp. A total of 240 samples for each sampling method were collected and tested. The cotton swab method of sampling was significantly (P < 0.01) less efficient in recovery of Listeria spp. than the sterile-sponge and composite-ply tissue methods, which were similar (P > 0.05) in their ability to recover Listeria spp. The specificity and sensitivity of four detection methods (conventional culture, Petrifilm Environmental Listeria Plates [ELP], lateral-flow immunoprecipitation [LFI], and automated PCR) were evaluated for identification of Listeria spp. Facilities were visited until a minimum of 100 positive and 100 negative samples per detection method were collected. The LFI and PCR methods were highly sensitive (95.5 and 99.1%, respectively) and specific (100%) relative to the culture method. The ELP method was significantly less efficient (P < 0.01) than LFI and PCR in detection of Listeria spp., with lower sensitivity (50.6%) and specificity (91.5%). Kappa values indicated excellent agreement of the LFI and PCR assays and moderate agreement of the ELP method to the culture method. Overall, ELP was easy to use but less efficient in detection of Listeria spp. from environmental samples, while the LFI and PCR methods were found to be excellent alternatives to culture, considering performance and time and labor inputs.

Increased detection of Listeria species and Listeria monocytogenes in raw beef, using the Assurance GDS molecular detection system with culture isolation

Testing for Listeria is challenging because of its slow growth rate. Recently, we described a rapid Listeria culture isolation method. This method can be improved by utilizing a rapid molecular detection test such as the Assurance GDS tests for Listeria and Listeria monocytogenes. These two methods (culture isolation and Assurance GDS) use different enrichment strategies that may affect the number of Listeria and L. monocytogenes cells detected. Therefore, after first determining that the Assurance GDS accurately identified common Listeria strains isolated from raw beef, the two methods were compared by using paired ground beef samples (n = 256) that had been gathered from commercial sources. The agreement of the two methods was > 76% for the culture and GDS Listeria method and > 77% for the culture and GDS L. monocytogenes method. The molecular tests then were evaluated as endpoint tests in selected culture isolation enrichments. In this comparison, culture isolation and the molecular Listeria test agreed 100 and 84.4% of the time for Listeria-positive and -negative enrichments, respectively. An analysis of the discrepant samples in both experiments revealed that approximately 50% of the samples identified as positive by the molecular method but not by the culture method could be confirmed by subsequent testing, indicating that the immunomagnetic concentration step of the GDS test likely provides a more sensitive level of detection than does culture alone. The culture results were available 2 days earlier when the molecular tests were used instead of plating media. However, because the Assurance GDS Listeria test cannot distinguish L. monocytogenes from other Listeria species such as Listeria innocua, samples containing both species could not be distinguished.

Enhanced detection of Listeria spp. in farmstead cheese processing environments through dual primary enrichment, PCR, and molecular subtyping

The incidence and ecology of Listeria spp. in farmstead cheese processing environments were assessed through environmental sampling conducted in nine different plants over a 10-week period. Environmental samples (n = 705) were examined for the presence of Listeria spp. by using three detection/isolation protocols. The use of dual enrichment methods, which allowed for the recovery of injured Listeria spp. (mUSDA), identified more Listeria species-positive samples with higher sensitivity than the standard USDA method. The addition of PCR to the mUSDA method identified the most Listeria monocytogenes-positive samples, achieving greater sensitivity of detection while substantially reducing time. Overall, 7.5% of samples were positive for Listeria spp., yielding 710 isolates, 253 of which were subtyped by automated ribotyping to examine strain diversity within and between plants over time. The isolation of specific ribotypes did not appear to be affected by the enrichment protocol used. Fifteen (2.1%) samples yielded L. monocytogenes isolates differentiated almost equally into ribotypes of lineages I and II. Of most concern was the persistent and widespread contamination of a plant with L. monocytogenes DUP-1042B, a ribotype previously associated with multiple outbreaks of listeriosis. Our results suggest that the extent of contamination by Listeria spp., notably L. monocytogenes, in farmstead cheese plants is comparatively low, especially for those with on-site farms. The results of this study also identified points of control for use in designing more effective Listeria spp. control and monitoring programs with a focus on ribotypes of epidemiological significance.

Evaluation of Effects of Primary and Secondary Enrichment for the Detection ofListeria monocytogenesby Real-Time PCR in Retail Ground Chicken Meat

A SYBR Green I based real-time PCR assay with inlA-specific oligonucleotide primers was developed for easy and rapid detection of Listeria monocytogenes in a model food that usually has a high incidence of contamination with this pathogen. Results with pure cultures and artificially contaminated chicken meat samples indicate that the PCR assay was highly specific and sensitive. The melting point analysis of the 160 bp amplified DNA fragment was different for L. monocytogenes isolates of the two major phylogenetic divisions of the species, 1 and 2. The assay was then used to survey retail ground chicken meat for contamination with L. monocytogenes. Thirty-seven samples were enriched according to the United States Department of Agriculture culture assays to detect L. monocytogenes on meat. The use and efficiency of PCR assay was examined following both primary and secondary enrichments, which were also plated on chromogenic agar for enumeration of L. monocytogenes and nonpathogenic Listeria spp. to investigate the discrepancies between culture and PCR. Overall, L. monocytogenes was detected in 75% of the samples. Primary enrichment yielded detection rates of 70% and 37% for culture and PCR, respectively. The corresponding rates for secondary enrichment were 54% and 70%, respectively. Test sensitivity is therefore influenced by the type of enrichment and is probably related not only to the limited growth of L. monocytogenes in the primary enrichment media (false-negative PCR results), but also to the high populations of nonpathogenic Listeria spp. in the secondary enrichment broths (false-negative culture results). The main challenge of rapid PCR-based detection of L. monocytogenes from food is the poor sensitivity of primary enrichment media. The improvement of enrichment conditions may help increase assay sensitivity.

Cross-contamination between processing equipment and deli meats by Listeria monocytogenes

Contamination of luncheon meats by Listeria monocytogenes has resulted in outbreaks of listeriosis and major product recalls. Listeriae can survive on processing equipment such as meat slicers which serve as a potential contamination source. This study was conducted to determine (i) the dynamics of cross-contamination of L. monocytogenes from a commercial slicer and associated equipment onto sliced meat products, (ii) the influence of sample size on the efficacy of the BAX-PCR and U.S. Department of Agriculture-Food Safety and Inspection Service enrichment culture assays to detect L. monocytogenes on deli meat, and (iii) the fate of L. monocytogenes on sliced deli meats of different types during refrigerated storage. Three types of deli meats, uncured oven-roasted turkey, salami, and bologna containing sodium diacetate and potassium lactate, were tested. A five-strain mixture of L. monocytogenes was inoculated at ca.10(3) CFU onto the blade of a commercial slicer. Five consecutive meat slices were packed per package, then vacuum sealed, stored at 4 degrees C, and sampled at 1 and 30 days postslicing. Two sample sizes, 25 g and contents of the entire package of meat, were assayed. Total numbers of L. monocytogenes-positive samples, including the two sample sizes and two sampling times, were 80, 9, and 3 for turkey, salami, and bologna, respectively. A higher percentage of turkey meat samples were L. monocytogenes positive when contents of the entire package were assayed than when the 25-g sample was assayed (12.5 and 7.5%, respectively). Lower inoculum populations of ca. 10(1) or 10(2) CFU of L. monocytogenes on the slicer blade were used for an additional evaluation of oven-roasted turkey using two additional sampling times of 60 and 90 days postslicing. L. monocytogenes-positive samples were not detected until 60 days postslicing, and more positive samples were detected at 90 days than at 60 days postslicing. When BAX-PCR and enrichment culture assays were compared, 12, 8, and 2 L. monocytogenes-positive samples were detected by both the enrichment culture and BAX-PCR, BAX-PCR only, and enrichment culture only assays, respectively. The number of L. monocytogenes-positive samples and L. monocytogenes counts increased during storage of turkey meat but decreased for salami and bologna. Significantly more turkey samples were L. monocytogenes positive when the contents of the entire package were sampled than when 25 g was sampled. Our results indicate that L. monocytogenes can be transferred from a contaminated slicer onto meats and can survive or grow better on uncured oven-roasted turkey than on salami or bologna with preservatives. Higher L. monocytogenes cell numbers inoculated on the slicer blade resulted in more L. monocytogenes-positive sliced meat samples. In addition, the BAX-PCR assay was better than the enrichment culture assay at detecting L. monocytogenes on turkey meat (P < 0.05).

Detection of Listeria monocytogenes and the toxin listeriolysin O in food

Listeria monocytogenes is an emerging bacterial foodborne pathogen responsible for listeriosis, an illness characterized by meningitis, encephalitis, and septicaemia. Less commonly, infection can result in cutaneous lesions and flu-like symptoms. In pregnant women, the pathogen can cause bacteraemia, and stillbirth or premature birth of the fetus. The mortality rate for those contracting listeriosis is approximately 20%. Currently, the United States has a zero tolerance policy regarding the presence of L. monocytogenes in food, while Canada allows only 100 cfu/g of food. As such, it is essential to be able to detect the pathogen in low numbers in food samples. One of the best ways to detect and confirm the pathogen is through the detection of one of the virulence factors, listeriolysin O (LLO) produced by the microorganism. The LLO-encoding gene (hlyA) is present only in virulent strains of the species and is required for virulence. LLO is a secreted protein toxin that can be detected easily with the use of blood agar or haemolysis assays and it is well characterized and understood. This paper focuses on some of the common methods used to detect the pathogen and the LLO toxin in food products and comments on some of the potential uses and drawbacks for the food industry.

Achieving continuous improvement in reductions in foodborne listeriosis--a risk-based approach

Listeria monocytogenes is a foodborne pathogen that can cause listeriosis, a severe disease that can lead to septicemia, meningitis, and spontaneous abortion. Ongoing efforts are needed to further reduce the incidence of listeriosis, due to its high mortality rate. The focus of this report is the use of a risk-based approach to identify strategies that will have the greatest impact on reducing foodborne listeriosis. A continuum of risk for listeriosis is observed in the human population, ranging from exquisitely sensitive groups, who are highly immunocompromised and at very high risk of listeriosis, through the normal healthy population younger than 65 years of age, who appear to have a minimal risk for listeriosis. In addition, unique subpopulations may exist; for example, pregnant Latina women appear to have a higher risk of listeriosis than pregnant women of other ethnic groups, most likely due to consumption of contaminated soft cheeses such as queso fresco and queso blanco. The International Life Sciences Institute Risk Science Institute Expert Panel concluded that certain foods pose a high risk for causing listeriosis. High-risk foods have all of the following properties: (1) have the potential for contamination with L. monocytogenes; (2) support the growth of L. monocytogenes to high numbers; (3) are ready to eat; (4) require refrigeration; and (5) are stored for an extended period of time. Control strategies are needed in the food chain from preharvest through consumption to minimize the likelihood that food will become contaminated by L. monocytogenes and to prevent the growth of the organism to high numbers. The Expert Panel identified three main strategies for ensuring continuous improvement in reducing foodborne listeriosis: (1) preventing contamination of foods with L. monocytogenes; (2) preventing growth of L. monocytogenes to high numbers in foods; and (3) science-based education messages targeted to susceptible populations and their caregivers. Of these strategies, the Expert Panel concluded that preventing growth of L. monocytogenes to high numbers would have the greatest impact in reducing cases of listeriosis. Dose-response models predict that the risk of listeriosis increases as the number of organisms in a food increases and can be used as a scientific basis for a target level below which the organism should be reduced to minimize the likelihood of listeriosis in high-risk populations. This requires implementation of effective food safety control measures and ensuring that these control strategies are consistently met. Most effective strategies to control L. monocytogenes in high-risk foods include (1) good manufacturing practices, sanitation standard operating procedures, and hazard analysis critical control point programs to minimize environmental L. monocytogenes contamination and to prevent cross-contamination in processing plants and at retail; (2) an intensive environmental sampling program in plants processing high-risk foods and an effective corrective action plan to reduce the likelihood of contamination of high-risk foods; (3) time and temperature controls throughout the entire distribution and storage period, including establishing acceptable storage times of foods that support growth of L. monocytogenes to high numbers; (4) reformulating foods to prevent or retard the growth of L. monocytogenes; and (5) using postpackaging treatments to destroy L. monocytogenes on products. Science-based education and risk communication strategies aimed at susceptible populations and focused on high-risk foods should be delivered through health care providers or other credible sources of information. Exquisitely sensitive consumers may become ill when exposed to low numbers of L. monocytogenes or other opportunistic pathogens, so reducing the risk to this population could be achieved by maintaining them on restricted low-microbe diets during those periods when they are most severely immunocompromised. High-risk individuals (i.e., the elderly, pregnant women, and most immunocompromised individuals) should be provided with guidance on healthy eating, including specific information on high-risk foods that they should avoid, and strategies to reduce their risk, such as thorough cooking, avoidance of cross-contamination, and short-term refrigerated storage of cooked perishable foods. Those at low risk for listeriosis should receive information on safe food handling practices, preferably starting at a preschool age.

Use of monoclonal antibodies that recognize p60 for identification of Listeria monocytogenes

Listeria monocytogenes causes major food-borne outbreaks of disease worldwide. Specific identification of this microorganism is of utmost importance to public health and industry. Listeria species are known to secrete a 60-kDa protein collectively termed p60, which is encoded by the iap (invasion-associated protein) gene and secreted in large quantities into the growth media. p60 is a highly immunogenic murein hydrolase that is essential for cell division. Due to these properties, p60 is an ideal diagnostic target for the development of immunological detection systems for L. monocytogenes. We report here two independent lines of monoclonal antibody (MAb): p6007, which specifically recognizes L. monocytogenes p60, and p6017, which reacts with a wide range of Listeria p60 proteins. By combining these antibodies with a polyclonal antibody, we developed efficient sandwich enzyme-linked immunosorbent assay (ELISA) systems which can specifically identify L. monocytogenes or generally detect Listeria species. Since an excess amount of the peptide corresponding to PepA or PepD did not interfere with the ELISA, and direct ELISAs were unable to detect both peptides, we concluded that the epitope presumed to be recognized by p6007 or p6017 could be distinguished from PepA and PepD as described by Bubert et al. (Appl. Environ. Microbiol. 60:3120-3127, 1997). To our best knowledge, this is the first example of an immunological identification system that uses p60-recognizing MAbs.

Methods for the isolation and identification of Listeria spp. and Listeria monocytogenes: a review

Listeria monocytogenes is an important food-borne pathogen and is widely tested for in food, environmental and clinical samples. Identification traditionally involved culture methods based on selective enrichment and plating followed by the characterization of Listeria spp. based on colony morphology, sugar fermentation and haemolytic properties. These methods are the gold standard; but they are lengthy and may not be suitable for testing of foods with short shelf lives. As a result more rapid tests were developed based on antibodies (ELISA) or molecular techniques (PCR or DNA hybridization). While these tests possess equal sensitivity, they are rapid and allow testing to be completed within 48 h. More recently, molecular methods were developed that target RNA rather than DNA, such as RT-PCR, real time PCR or nucleic acid based sequence amplification (NASBA). These tests not only provide a measure of cell viability but they can also be used for quantitative analysis. In addition, a variety of tests are available for sub-species characterization, which are particularly useful in epidemiological investigations. Early typing methods differentiated isolates based on phenotypic markers, such as multilocus enzyme electrophoresis, phage typing and serotyping. These phenotypic typing methods are being replaced by molecular tests, which reflect genetic relationships between isolates and are more accurate. These new methods are currently mainly used in research but their considerable potential for routine testing in the future cannot be overlooked.

Eye infections due to Listeria monocytogenes in three cows and one horse

A retrospective study was conducted to determine case histories, microbiological characteristics, and molecular subtypes associated with Listeria monocytogenes infections of the eye in large animals. For selected cases, environmental L. monocytogenes contamination patterns on case farms were also evaluated to probe for potential sources and spread of listerial eye infections. Records of 170 L. monocytogenes isolates from animal infections were reviewed to determine the fraction of isolates associated with eye infections (conjunctivitis, keratitis, and uveitis) of animals and to gather information on the clinical history of these cases. Overall, 4 of 170 Listeria monocytogenes isolates were associated with eye infections; 3 of these had occurred in cows and 1 in a horse. Molecular subtyping (by EcoRI ribotying) showed that 4 different L. monocytogenes subtypes were isolated from these 4 cases; the same ribotypes had previously been found among invasive animal listeriosis infections. Although a variety of L. monocytogenes subtypes were isolated from environmental sources, on 1 farm, the same ribotype associated with the eye infection was also isolated from a fecal sample of a healthy animal and from a soil sample. The data reported in this study further suggest that L. monocytogenes can be a cause of eye infections in several animal species. Listerial eye infections do not seem to require specific pathogen-related virulence characteristics but rather seem to be a function of environmental or host factors, such as direct exposure of the eyes of susceptible animals to high numbers of the pathogen. Although listerial eye infections are rarely diagnosed because of its ubiquitous nature, L. monocytogenes may have to be considered more commonly as a causative agent of eye infections in ruminants and horses.

Molecular Subtyping and Tracking of Listeria monocytogenes in Latin-Style Fresh-Cheese Processing Plants

Latin-style fresh cheeses, which have been linked to at least 2 human listeriosis outbreaks in the United States, are considered to be high-risk foods for Listeria monocytogenes contamination. We evaluated L. monocytogenes contamination patterns in 3 Latin-style fresh-cheese processing plants to gain a better understanding of L. monocytogenes contamination sources in the manufacture of these cheeses. Over a 6-mo period, 246 environmental samples were collected and analyzed for L. monocytogenes using both the Food and Drug Administration (FDA) method and the Biosynth L. monocytogenes detection system (LMDS). Finished cheese samples from the same plants (n = 111) were also analyzed by the FDA method, which was modified to include L. monocytogenes plating medium (LMPM) and the L. monocytogenes confirmatory plating medium (LMCM) used in the LMDS method. Listeria monocytogenes was detected in 6.3% of cheese and 11.0% of environmental samples. Crates, drains, and floor samples showed the highest contamination rates, with 55.6, 30.0, and 20.6% L. monocytogenes positive samples, respectively. Finished products and food contact surfaces were positive in only one plant. The FDA method showed a higher sensitivity than the LMDS method for detection of L. monocytogenes from environmental samples. The addition of LMPM and LMCM media did not further enhance the performance of the FDA method for L. monocytogenes detection from finished products. Molecular subtyping (PCR-based allelic analysis of the virulence genes actA and hly and automated ribotyping) was used to track contamination patterns. Ribotype DUP-1044A, which had previously been linked to a 1998 multistate human listeriosis outbreak in the United States, was the most commonly identified subtype (20/36 isolates) and was isolated from 2 plants. This ribotype was persistent and widespread in one factory, where it was also responsible for the contamination of finished products. We hypothesize that this ribotype may represent a clonal group with a specific ability to persist in food processing environments. While previous listeriosis outbreaks were linked to Latin-style fresh cheeses made from unpasteurized milk, the presence of this organism in pasteurized cheese products illustrates that persistent environmental contamination also represents an important source of finished product contamination.

New chromogenic plating media for detection and enumeration of pathogenic Listeria spp.—an overview

In recent years a number of selective chromogenic plating media for pathogenic Listeria spp. have been developed and marketed. Their advantages are direct detection and enumeration of pathogenic Listeria spp. utilizing cleavage of substrates by the virulence factor phosphatidylinositol-phospholipase C (PI-PLC) and, to a lesser extent, by phosphatidylcholin-phospholipase C (PC-PLC). There are two groups of such media: the first utilizes cleavage by PI-PLC of L-alpha-phosphatidyl-inositol, forming a white precipitation zone around the colony, combined with the chromogenic substrate 5-bromo-4-chloro-3-indoxyl-beta-D-glucopyranoside for detection of beta-d-glucosidase, which occurs in all Listeria spp. All Listeria spp. produce turquoise colonies on these media which include ALOA , CHROMagar Listeria, BBL CHROMagar Listeria, and OCLA. The second group of media utilizes 5-bromo-4-chloro-3-indoxyl-myoinositol-1-phosphate, forming blue-turquoise colonies of pathogenic Listeria spp. and white colonies of non-pathogenic Listeria spp. BCM trade mark Listeria monocytogenes plating medium, Rapid'L.mono and LIMONO-Ident-Agar belong to this group. Selective chromogenic L. monocytogenes plating media offer the attraction of rapid economic detection and enumeration of pathogenic Listeria spp. within 24 or 48 h of incubation at 36+/-1 degrees C. This overview summarises the characteristics of these chromogenic plating media, reviews important evaluations, and focuses on replacement of conventional by these chromogenic plating media, particularly for applications in the food industry.

The BAX PCR assay for screening Listeria monocytogenes targets a partial putative gene lmo2234

The BAX PCR for screening Listeria monocytogenes is a commercial PCR assay for specifically targeting L. monocytogenes, a foodborne pathogen that can contaminate a variety of foods and cause a potentially fatal disease, listeriosis, among high-risk populations. The high specificity (> 98%) of this PCR assay is achieved by targeting a species-specific genomic region (approximately 400 bp) presumably found only in L. monocytogenes. In this study, the identity of the BAX PCR-targeted genomic region was determined by using PCR cloning, DNA sequencing, and basic local alignment search tool (BLAST) analysis of the amplicon sequences of an L. monocytogenes serotype 1/2a strain. BLAST analysis identified the BAX PCR amplicon (GenBank accession no. AY364605) as a 423-bp genomic region between nucleotides 224,409 and 224,831 in the genome of L. monocytogenes (serotype 1/2a strain EGD-e), including a 145-bp noncoding region and a 278-bp partial coding sequence of a putative gene, lmo2234. The translated amino acid sequence (92 amino acids) of this partial coding region is highly conserved between L. monocytogenes and Listeria innocua (93% homology). Reverse-position-specific BLAST analysis identified a conserved domain in Lmo2234 that was similar (95.3% aligned, E value = 9E-18) to the consensus amino acid sequence of sugar phosphate isomerases/epimerases (National Center for Biotechnology Information conserved domain database accession no. COG 1082.1, IolE), indicating that Lmo2234 might be involved in bacterial carbohydrate transport and metabolism.

Listeria monocytogenes: diagnostic problems

The first isolation methods for the detection of Listeria spp. were generally based on the direct culture of samples on simple agar media, but isolation of the pathogenic Listeria monocytogenes was difficult. In time, new techniques were developed, based on a variety of selective and elective agents in isolation and enrichment media, which gained better and quicker results. Current reference methods allow the recovery of L. monocytogenes from a variety of foods with relative ease. However, more comparative studies are needed to select one horizontal method. It is suggested that the procedure of the International Organization for Standardization is a good base for such comparisons.

Detection of Listeria monocytogenes in salmon using the Probelia polymerase chain reaction system

A validation was conducted on the performance of a commercially available polymerase chain reaction (PCR) kit (Probelia) in comparison with International Organization for Standardization (ISO) method 11290-1 (adopted as an Australian New Zealand Standard Method, AS/NZS 1766.2.16.1:1998) for the detection of Listeria monocytogenes in salmon samples. The validation was conducted following the guidelines of an Australian New Zealand Standard (Guide to Determining the Equivalence of Food Microbiology Test Methods, Part 1, Qualitative Tests, AS/NZS 4659.1:1999), which adopts an approach similar to that recommended by the Association of Analytical Communities Microbiology Method Validation Program for Performance Tested and Peer Verified Methods. The validation study involved the use of five cultures of L. monocytogenes, each challenged at a single level of inoculation into five different types of salmon samples. A total of 60 salmon samples (30 unchallenged and 30 challenged) were tested using both the PCR method and the ISO method. Results from this study indicated that the Probelia PCR method is equivalent to the ISO method. In addition, the detection sensitivity of the Probelia PCR system was determined as approximately 0.5 CFU per PCR assay (equivalent to 20 CFU/ml broth culture) for a pure culture of L. monocytogenes. The Probelia PCR method offers the advantage of detecting L. monocytogenes to genetic specificity within 48 to 50 h, whereas the ISO method requires 5 days for negative results with additional days for confirmed positive results by the use of other biochemical and cultural tests.

Listeria monocytogenes contamination patterns for the smoked fish processing environment and for raw fish

Reliable data on the sources of Listeria monocytogenes contamination in cold-smoked fish processing are crucial in designing effective intervention strategies. Environmental samples (n = 512) and raw fish samples (n = 315) from two smoked fish processing facilities were screened for L. monocytogenes, and all isolates were subtyped by automated ribotyping to examine the relationship between L. monocytogenes contamination from raw materials and that from environmental sites. Samples were collected over two 8-week periods in early spring and summer. The five types of raw fish tested included lake whitefish, sablefish, farm-raised Norwegian salmon, farm-raised Chilean salmon, and feral (wild-caught) salmon from the U.S. West Coast. One hundred fifteen environmental samples and 46 raw fish samples tested positive for L. monocytogenes. Prevalence values for environmental samples varied significantly (P < 0.0001) between the two plants; plant A had a prevalence value of 43.8% (112 of 256 samples), and plant B had a value of 1.2% (3 of 256 samples). For plant A, 62.5% of drain samples tested positive for L. monocytogenes, compared with 32.3% of samples collected from other environmental sites and 3.1% of samples collected from food contact surfaces. Ribotyping identified 11 subtypes present in the plant environments. Multiple subtypes, including four subtypes not found on any raw fish, were found to persist in plant A throughout the study. Contamination prevalence values for raw fish varied from 3.6% (sablefish) to 29.5% (U.S. West Coast salmon), with an average overall prevalence of 14.6%. Sixteen separate L. monocytogenes subtypes were present on raw fish, including nine that were not found in the plant environment. Our results indicate a disparity between the subtypes found on raw fish and those found in the processing environment. We thus conclude that environmental contamination is largely separate from that of incoming raw materials and includes strains persisting, possibly for years, within the plant. Operational and sanitation procedures appear to have a significant impact on environmental contamination, with both plants having similar prevalence values for raw materials but disparate contamination prevalence values for the environmental sites. We also conclude that regular L. monocyrogenes testing of drains, combined with molecular subtyping of the isolates obtained, allows for efficient monitoring of persistent L. monocytogenes contamination in a processing plant.