The problem of assessing the occurrence of the food-borne pathogen Listeria monocytogenes in the food chain, and therefore the risk of exposure of the human population, is often challenging because of the limited scope of some studies.
In this study the occurrence of L. monocytogenes in food from four major food groups, dairy products,
meats, seafood and vegetables, and associated food processing environments in Ireland was studied over a three-year period. Fifty-four small food businesses participated in the study and sent both food and environmental samples every 2 months between 2013 and 2015. L.monocytogenes was isolated using the ISO11290 standard method.
Confirmation of L. monocytogenes and identification of serogroups were achieved using a multiplex PCR
assay, and for some isolates serotype was determined using commercial antisera. Pulsed- field gel electrophoresis (PFGE) analysis was performed on all isolates allowing the relatedness of isolates from different food businesses
to be compared nationwide. In total, 86 distinct pulsotypes were identified. The overall occurrence of L.
monocytogenes in food samples was 4.2%, while in environmental samples it was 3.8%. In general, the occurrence of L. monocytogenes in food businesses decreased over the course of the study, presumably reflecting increased awareness and vigilance.
The majority of the pulsotypes detected were unique to a particular food group (63/
86), while only three pulsotypes were found in all four food groups investigated. The highest occurrence in food was found in the meat category (7.5%) while seafood had the lowest rate of occurrence (1.8%). Seventeen of the pulsotypes detected in the study were persistent, where persistence was defined as repeated isolation from a single facility with a minimum time interval of 6 months. Using PFGE, 11 of the pulsotypes identified in this study were indistinguishable from those of 11 clinical isolates obtained from patients in Ireland over the last 4 years, highlighting the fact that these pulsotypes are capable of causing disease.
Overall, the study shows the diversity of L. monocytogenes strains in the Irish food chain and highlights the ability of many of these strains to persist in food processing environments. The finding that a significant proportion of these pulsotypes are also found in clinical settings highlights the need for continued vigilance by food producers, including frequent sampling and typing of isolates detected.