Abstract
The objective of this study was to investigate the molecular characterization of Listeria species isolated from frozen raw fish. A total of 219 samples consisting of 104 mackerel, 52 horse mackerel, 51 catfish and 12 herring were collected and analyzed by bacteriological, serological, antimicrobial and molecular methods. Overall, 29(56.9%) and 1(0.96%) of catfish samples and mackerel samples respectively were positive for Listeria spp. No Listeria was detected in herring and horse mackerel. In catfish, L. welshimeri (13.7 %) was the most commonly isolated species followed by L. monocytogenes (11.8 %), L. innocua (9.8 %), L. grayi subsp. murrayi (9.8 %), L. grayi subsp. grayi (7.8 %), and L. ivanovii (3.9 %). In mackerel, only L. monocytogenes was detected in one sample. L. monocytogenes isolates serotyped as type 1 and type 4 (3 isolates each) and one non-typeable. Antimicrobial resistance profiling showed all L. monocytogenes isolates were resistant to ampicillin and tetracycline. Two were resistant to erythromycin. However, they were susceptible to rifampicin, vancomycin, chloramphenicol and streptomycin. Four virulence-associated genes (prfA, hlyA, actA and inlA) in addition to the genus gene (prs) were investigated using multiplex PCR. All the isolates were positive for prs gene but, only L. monocytogenes isolates were positive for all tested virulence genes. Our study indicates that imported raw catfish can represent a significant source of L. monocytogenes and potential health risk for listeriosis.

Key words: Listeria monocytogenes Prevalence Antibiotic resistance Virulence genes Basa catfish