Persistence of Listeria monocytogenes strains isolated from products in a Polish fish-processing plant over a 1-year period Original Research Article

Seventy-one presumptive Listeria monocytogenes strains were isolated over a year from 152 samples comprising raw fish (salmon, seatrout) and their products (mainly, vacuum-packed cold-smoked sliced salmon) in a selected Polish fish-processing plant. Contamination of raw materials was at the level of 4.3–15.4%, whereas final products revealed significantly higher contamination (up to 77.8%) than regarded by other studies as typical (up to 40%). Strains were identified using conventional microbiological methods (including API®LISTERIA tests) and the PCR technique (aimed at iap gene fragment detection). A random amplification polymorphic DNA (RAPD) technique was applied to analyse their intraspecies diversity. RAPD typing revealed an incidence of eight RAPD types. Three of them were isolated over 8–10 months during the plant monitoring. It suggested that they were a persistent element of ‘in-house’ microflora and the applied typing technique produced evidence that fish products could be probably contaminated at the last stages of fish processing (e.g. smoking, slicing, and/or packaging). Their occurrence was probably supported by clone selection caused by ineffective application of cleaning and sanitizing procedures. The possibility of colonization of the production environment by fish-originated L. monocytogenes was also proven. Strains that belonged to a dominant RAPD type were additionally subjected to restriction fragment length polymorphism-pulsed field gel electrophoresis (RFLP-PFGE). RFLP-PFGE confirmed intraspecies similarity of strains belonging to a dominant RAPD type. A subset of strains from salmon samples was also characterized by serotyping. Contrary to earlier reports, they belonged mainly (91.7%) to the serotype 4.