Listeria Monocytogenes Contamination Research Articles

The quantitative assessment of Listeria monocytogenes growth in a laboratory ensiling system allowing limited aerobic spoilage

Listeria monocytogenes contamination is associated with silage that has been subjected to aerobic deterioration. A laboratory ensiling system based on plastic tube silos, which permit aerobic spoilage to take place in a controlled manner, is described. This system gives a high degree of consistency between replicates. Parameters measured over a 90‐d ensiling period were pH, L. monocytogenes and Enterobacteriaceae counts and percentage spoilage. The system may be suitable for assessing the effectiveness of additives on parameters, such as aerobic spoilage and L. monocytogenes, which are not easily measurable in field studies. The study confirmed the relationship between the presence of Listeria spp. and Enterobacteriaceae in aerobically spoiled silage, showing that the latter group of bacteria may be useful as indicators of the potential risk of L. monocytogenes contamination of silage.

Possible In-Plant Sources of Contamination of Lobster and Snow Crab by Listeria

The present study was undertaken to determine potential sources of Listeria monocytogenes contamination through statistical analysis of L. monocytogenes results from lobster and snow crab samples and Listeria spp. results from environmental swabs. The objectives were (1) to compare crustacean analysis records for relationships between presence/absence of L. monocytogenes and crustacean species, year, total aerobes, staphylococci and fecal coli-form; and (2) to identify sites prone to Listeria spp. contamination. There were no significant differences in L. monocytogenes incidence between lobster and snow crab nor between five consecutive years of survey. Total aerobes and staphylococci counts were significantly associated with incidence of L. monocytogenes; fecal coliform counts were not. On surfaces in contact with the processed product, Listeria spp. were mostly found within crab crushers, drum meat separators, shucking machines, on aprons, gloves, work tables and knives. Higher incidence of Listeria spp. were found on surfaces not in direct contact with the processed product such as truck boxes, foot stools, floors, drains and employee boots and pants.

Prevalence and growth of Listeria monocytogenes in naturally contaminated seafood

Listeria monocytogenes contamination of seafood varies with product category. The highest prevalence was found in cold-smoked fish (34–60%), while the lowest was found in heat-treated and cured seafood (4–12%). The prevalence of L. monocytogenes differed greatly in cold-smoked salmon between production sites, ranging from <1.4% (nil out of 70 samples) to 100%. The prevalence at the individual production sites was reproducible at repeated sampling. The results indicate that it is possible to produce cold-smoked salmon with a low prevalence of L. monocytogenes. The organism showed moderate growth in naturally contaminated cold-smoked, and `gravad', fish while the growth appeared faster in hot smoked fish. Thus L. monocytogenes is not under control in these products. Finally, the prevalence and growth of L. monocytogenes in naturally contaminated cold-smoked salmon are discussed in relation to controlling this risk.

The incidence and level of Listeria monocytogenes contamination of food sources at primary production and initial processing.

Listeria monocytogenes was isolated in low numbers from a variety of environmental samples associated with the primary production of food, including vegetation, faeces and meat. The organism was rarely detected on growing grass and vegetables prior to processing. The excretion of L. monocytogenes by farm animals was linked to their diet, with animals fed entirely on hay or manufactured diets not excreting detectable levels of Listeria (i.e. absence in 25 g). However, animals fed on silage, which is frequently contaminated with L. monocytogenes, commonly excreted the organism. Transport of live animals over long distances (> 100 km) significantly increased the level of excretion of Listeria, but the contamination of carcasses of sheep and cattle was not high. Pigs and poultry faeces were free of Listeria prior to slaughter and pig carcasses were not found to have Listeria present. Frozen and chilled chicken did show detectable levels reflecting the greater potential for contamination during poultry processing. Samples of minced beef were tested and 21 of 23 samples were positive for L. monocytogenes, demonstrating that processing significantly increases the level of contamination compared to whole carcasses. Multilocus enzyme electrophoresis of a representative selection of the isolates showed that there was a wide range of electrophoretic types present in the primary production environment, relatively few of which have been linked to cases of human listeriosis. However, these types do arise on farms and occasional contamination of food raw material by potentially virulent strains may be sufficient to allow adaptable strains to become established in the processing environment and thus be responsible for more widespread contamination of the food available to the consumer.

The incidence and level of Listeria monocytogenes contamination of food sources at primary production and initial processing

The incidence and level of Listeria monocytogenes contamination of food sources at primary production and initial processing

Pathways of Listeria monocytogenes contamination in the meat processing industry

One hundred and thirty-three isolates of Listeria monocytogenes from deboned fresh meat, production environment, cold cuts from five meat processing plants and from one plant producing cured dried sausages, were characterized using multilocus enzyme electrophoresis. On the basis of electrophoretically demonstrable allelic variation at 21 enzyme loci, 21 electrophoretic types (ETs) were distinguished. Analysis of the genetic relationships among the 21 ETs revealed two distinct clusters: Cluster A and Cluster B. With the exception of two isolates from one plant, all isolates from deboned fresh meat belonged to Cluster B. During processing of cold cuts, however, isolates belonging to Cluster A became more frequent, and only one of the 37 isolates from cold cuts belonged to Cluster B. In contrast, six of the nine isolates from cured dried sausages had ETs in Cluster B. One clone of Cluster A, ET-6 was isolated from cold cuts in four of six plants. This is one of the ETs most frequently recovered from patients in Norway. Isolates of ET-6 were further characterized using restriction fragment length polymorphism (RFLP) analysis of chromosomal DNA. Six distinct restriction patterns were distinguished among the 44 ET-6 strains. In one plant, four different RFLP patterns could be identified. Two clone variants seemed to have colonized different areas in this plant for at least four years. However, in each of the other plants, all ET-6 isolates had the same RFLP patterns. In only one of the five plants producing cold cuts was the same ET found both in the fresh meat and along the processing chain including the end-product. This indicates that the potential risk for contamination of the final end-product (cold cuts) posed by L. monocytogenrs in the fresh meat might have been overestimated. It is possible that personal and general hygiene in the packing room are more significant with regard to L. monocytogenes contamination of cold cuts. Good manufacturing practices and attention to critical control points during processing in the meat plants are probably the most important prophylactic measures to avoid L. monocytogenrs in cold cuts.

Use of molecular typing methods to trace the dissemination of Listeria monocytogenes in a shrimp processing plant.

Molecular typing of bacteria has been widely used in epidemiological studies but not as extensively for tracing the transmission of pathogenic bacteria in food plants. This study was conducted to examine the potential use of two molecular typing methods, random amplified polymorphic DNA (RAPD) analysis and pulsed-field gel electrophoresis (PFGE), to trace Listeria monocytogenes contamination in a shrimp processing plant. Ribotyping and phase typing were also performed on a select number of strains. One hundred fifteen strains of L. monocytogenes collected in different areas of a shrimp processing plant were first serotyped and then subtyped by molecular typing. RAPD and PFGE showed great promise for typing L. monocytogenes isolates since distinguishable and reproducible DNA polymorphisms were obtained. When the composite profile from both (RAPD and PFGE) methods was generated, there was an increase in the discriminatory power to discern differences between strains of L. monocytogenes. The results indicated that environmental strains all fell into composite profile groupings unique to the environment, while strains from both water and utensils shared another composite profile group. L. monocytogenes fresh shrimp isolates belonging to one profile group were found in different areas of the processing line. This same profile group was also present in food handlers from the processing and packaging areas of the plant.

Study of Listeria monocytogenes contamination in a dairy plant and characterization of the strains isolated

From 1988 to 1990, a dairy plant was examined for Listeria contamination. Three hundred and forty samples were collected and analysed for the presence of Listeria. Sixty-one Listeria strains ( L. monocytogenes, 44, and L. Innocua, 17) were isolated from four varieties of cheese, cheese brines, process equipment and plant environment. The L. monocytogenes strains were recovered during the ripening and rind washing stages and not before, so it is likely that cheese contamination occurred at these points in the manufacturing process. The characterization of the isolated L. monocytogenes strains by serotyping and phage typing showed different serovars and phagovars. Some strains with the same serovar and the same phagovar were isolated from cheeses and process equipment (shelves) indicating that cheese contamination occurs during ripening. Only one profile was found from the analysis of the ribosomal ribonucleic acid (rRNA) gene restriction patterns of 38 L. monocytogenes strains with different serovars and phagovars. This suggests that all L. monocytogenes strains isolated in the dairy plant could have been derived from a single ancestral group.

The Use of Bacteriocin-Producing Pediococcus acidilactici to Control Postprocessing Listeria monocytogenes Contamination of Frankfurters

The ability of a bacteriocin-producing Pediococcus acidilactici to control postprocessing Listeria monocytogenes contamination of frankfurters was examined. Bacteriocin-producing P. acidilactici JD1–23 or its plasmid-cured derivative JD-M and a five-strain composite of L. monocytogenes were inoculated onto fully processed frankfurters. Under normal storage conditions at 4°C under vacuum, L. monocytogenes without added pediococci grew from an initial level of 104 CFU/g to a final level of 106 CFU/g over 60 d, with a lag time of 20–30 d. Under the same conditions, high levels (107 CFU/g) of either pediococcal strain inhibited growth of L. monocytogenes up to 60 d, although no reduction of cells occurred. With low levels of pediococci (103–104 CFU/g), Listeria grew, although the lag time was increased, and a bacteriocin effect was observed on frankfurters inoculated with JD1–23.In additional experiments done at 4°C and 15°C under aerobic and anaerobic conditions, levels of 107–108 CFU/g of either pediococci were observed to control Listeria growth on frankfurters at 15°C under anaerobic conditions for up to 15 d. At 4°C under aerobic conditions, L. monocytogenes grew on frankfurters inoculated with JD-M, while JD1–23 inhibited growth of the organism up to 30 d. At 15°C under aerobic conditions, L. monocytogenes grew in the presence of either pediococci, although a bacteriocin effect was indicated. Package atmosphere was observed to affect L. monocytogenes growth on this product.

Listeria monocytogenes Contamination of Turkey Franks: Evaluation of a Production Facility

A facility which produced turkey franks that had been microbiologically linked to a case of human listeriosis was evaluated to establish prevalence of contamination and identify potential points for intervention. Listeria monocytogenes was isolated from only two of 41 environmental samples obtained in the plant. Among production line product samples analyzed by the Centers for Disease Control, 0 to 8% of samples from the production stages before the peeler-conveyor belt apparatus were positive for the case strain of L. monocytogenes, whereas 12 of 14 (86%) samples collected from this apparatus were positive (p <0.001). The most probable number (MPN) of L. monocytogenes in finished product purchased from a retail outlet was less than 0.3 per gram; however, the opened package of franks from the case patient's refrigerator had an MPN of >1100 per gram. These data suggest that systematic culturing and analysis of products and production facilities may help identify appropriate interventions to reduce L. monocytogenes contamination in food processing plants and contribute to control of L. monocytogenes in ready-to-eat meat products.

Isolation of Listeria monocytogenes from Raw Milk and Its Environment at Dairy Farms in Japan

The incidence of Listeria monocytogenes contamination in raw milk from farm bulk tanks and silage was investigated monthly at 20 dairy farms in Aomori prefecture, Japan, from January to July in 1989. Listeria sp. was isolated from one sample (5%) of raw milk collected in May, but L. monocytogenes was not isolated from raw milk. L. monocytogenes was isolated from two samples of silage (10%) collected in June. Silage, cow feces, sawdust bedding, dust and soil in these farms were tested for listerias, and we confirmed listerias contamination of the environment in the dairy farms.

The relationship between spoilage and Listeria monocytogenes contamination in bagged and wrapped big bale silage

AbstractA farm‐scale trial to compare the effectiveness of polythene bags and wrapping of big bale silage was monitored for Listeria monocytogenes contamination. There was no significant difference between the level of L. monocytogenes found in wrapped and bagged big bale silage, but the work confirmed that big bale silage is prone to contamination by Listeria organisms, and that the degree of Listeria contamination could be significantly reduced by the removal of obviously spoiled material prior to feeding.