Purpose: The aim of this study was to determine the prevalence of L. monocytogenes in fish and fish products as well as molecular characterization and antimicrobial resistance of the bacterial isolates.
Methods: A total of 100 samples of fresh fish (n = 26) and fish products (n = 74) purchased in local retail shops and investigated for the presence of L. monocytogenes using the EN-ISO 11290 standard. The fresh fish represented 9 species (cod, flounder, halibut, crucian, bream, salmon, panga, roach, trout) whereas the samples of fish products (smoked fish) were cod, halibut, bream, salmon, mackerel, sprat, and herring. Molecular serotyping and virulence genes (inlA, inlC, inlJ, lmo2672, llsX) were identified with PCR. Antimicrobial susceptibility of the isolates to 17 antimicrobials was determined using the MIC (Minimal Inhibitory Concentration) method.
Results: L. monocytogenes was found in 5 (19.2%) samples of fish and in 11 (14.9%) fish product samples. In all samples the number of L. monocytogenes was below 100 cfu/g. In all strains the inlA, inlC, inlJ, lmo2672, plcA, hly, mpl, actA, plcB, inlB virulence markers were identified; none of the L. monocytogenes strains were positive llsX. The presence of the flaA gene was identified only in strains of the 1/2a serotype. It was found that the most contaminated species was salmon, especially smoked salmon. The antibiotic resistance analyses revealed that L. monocytogenes were sensitive to most of the antibiotics used. Single isolates were only resistant to ceftriaxone (1 strain) and to oxacillin (2 strains); some strains showed an intermediate resistance to clindamycin (9 strains), ceftriaxone (6 strains) or ciprofloxacin (3 strains).
Significance: The data show that some fresh and smoked fish available on the Polish market were contaminated with L. monocytogenes. The isolates possessed virulence markers that make them potentially pathogenic for humans.