Prevalence and typing of listeria monocytogenes strains in retail vacuum-packed cold-smoked salmon in the republic of ireland

Nissreen Abu-Ghannam, Dublin Institute of Technology

Document Type Article

Journal of Food Safety ISSN 0149-6085


The prevalence of Listeria monocytogenes in five brands (A, B, C, D and E) of vacuum-packed pre-sliced cold-smoked salmon (n = 120) marketed in different retail outlets in the Republic of Ireland was investigated. The prevalence of L. monocytogenes in the cold-smoked salmon was 21.6%. The L. monocytogenes strains isolated from the prevalence studies were typed by using Multi-Locus Variable number tandem repeat Analysis; this method confirmed the type division among the L. monocytogenes isolates. A minimum spanning tree showed that isolates from brand A clustered together. Few isolates from brand B, C, D and E showed genetic similarities, while others were spread throughout the cladogram. The genetic distinction among subtypes may be processor-specific, while the similarities among the subtypes support the hypothesis of well-adapted clones of L. monocytogenes in the fish industry. The results highlight the need for food safety control at both pre-processing and processing stages, as there is no bactericidal process involved to eliminate any pathogen during the processing of cold-smoked salmon. PRACTICAL APPLICATIONS This study highlights the use of the Multi-Locus Variable number tandem repeat Analysis (MLVA) strategy for subtyping L. monocytogenes strains. Genomic profiles of 61 L. monocytogenes strains were compared following isolation as part of a prevalence study for vacuum-packed cold-smoked salmon in Ireland. From a food safety perspective, it is important to integrate the population genetics, evolutionary and epidemiological characteristics to trace the origin and spread of bacterial disease. MLVA is a polymerase chain reaction-based method that uses the genetic relationship amongst the isolates, thus population and evolutionary characteristics can be inferred. Based on the MLVA analysis, the results highlighted the grouping of the isolates, which were genetically similar. The study points the importance of pre-processing and processing safety control management from a risk minimization perspective. In addition to distinguishing subtypes, MLVA has the added advantage of being relatively faster and cheaper when compared to other subtyping methods.