Listeria monocytogenes from Marine Fish and the Seafood Market Environment in Northern Greece: Prevalence, Molecular Characterization, and Antibiotic Resistance

The occurrence of Listeria monocytogenes in marine fish and fish market areas was investigated. Two hundred and eighty-eight samples (123 environmental samples—siphons, knives, cutting boards, floor, sinks, water, and ice—and 165 marine fish samples) were examined. Twenty-four isolates were characterized as Listeria monocytogenes (five from environmental samples (4.0%) and 19 from fish samples (11.5%)). The strains were further characterized according to their antibiotic resistance, pathogenicity, and biofilm formation ability. They were molecularly serotyped as IIc (n = 22) and IVb (n = 2) and possessed all the virulence genes tested (inlA, inlB, inlC, inlJ, actA, hlyA, iap, plcA, and prfA), except for two strains lacking the hlyA and iap genes, respectively. All strains showed strong (41.7%) or moderate biofilm-producing ability (58.3%) and almost all showed resistance to at least one antibiotic, with the highest rates being observed against clindamycin and vancomycin. The proteomic analysis by MALDI-TOF revealed two distinct clusters that involved strains from fish only and those from both fish and the environment. The presence of Listeria monocytogenes in the fish-market environment and marine fish, along with the pathogenicity and persistence characteristics of the seafood-related strains, emphasize the need for vigilance concerning the spread of this notorious foodborne pathogen.