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

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. © 2003 Elsevier Ltd. All rights reserved.