Prevalence, genetic diversity and antimicrobial resistance of Listeria monocytogenes isolated from fresh and smoked fish in Poland

A total of 57 out of 301 (18.9%) fresh and smoked fish samples in Poland were positive for Listeria monocytotgenes. The bacteria were most frequently identified in fresh and smoked salmon (32.0% and 33.8% respectively) as well as in fresh cod (31.8%). Only three samples of smoked salmon were contaminated with the bacteria above 100 CFU/g. Four molecular serogroups were identified and the most prevalent, 1/2a-3a (40 isolates; 70.2%), was present in samples from both marine (33 strains; 71.7%) and freshwater fish (7 isolates; 63.6%). Similar duality of prevalence was observed only for L. monocytogenes of 1/2b-3b-7 serogroup (14 strains; 24.6%), which was identified in 11 (23.9%) marine and 3 (27.3%) freshwater fish. All isolates harboured 10 virulence-associated genes (inlA, inlB, inlC, inlJ, lmo2672, plcA, plcB, hlyA, actA, and mpl) and most of them (56; 98.2%) also possessed the flaA marker. Several strains displayed resistance to oxacillin (33; 57.9%), ceftriaxone (18; 31.6%), or clindamycin (5; 8.8%), and two isolates of serogroup 1/2a-3a showed multiresistance to all three. Genetic subtyping showed the presence of different pulsotypes belonging to six PFGE clusters. The obtained results provide useful information regarding fish contamination with L. monocytogenes which may have implications for public health.