Prevalence and antimicrobial resistance profile of Listeria spp. isolated from raw fish

Listeria spp. contamination in food, particularly antimicrobial-resistant strains, poses an escalating concern for public health. This study investigated the prevalence and antimicrobial resistance profiles of Listeria spp. isolated from raw fish collected from diverse sources in Northern Poland.

A total of 750 raw fish samples were collected from standing freshwater tanks, flowing freshwater reservoirs, and a saltwater reservoir. Isolation was performed following the standard protocol, which describes the horizontal method for detecting Listeria spp., including Listeria monocytogenes. The antimicrobial resistance profiles of isolated strains were performed using the disk diffusion method. Antimicrobial resistance genes were identified using PCR, targeting 11 genes associated with resistance to β-lactams, macrolides, glycopeptides, and sulfonamides. Statistical analyses included Phi correlation coefficients, hierarchical clustering, and logistic regression to examine associations between phenotypic resistance patterns and antimicrobial resistance genes.

Listeria spp. was isolated from 13.9% of samples (104 positive samples), comprising L. seeligeri (34.6%), L. welshimeri (28.8%), L. monocytogenes (23.1%), and L. innocua (13.5%). Phenotypic antimicrobial susceptibility testing revealed universal resistance to oxacillin (100%) across all Listeria spp. isolates. High resistance levels were also observed for cefotaxime (97.1%), cefoxitin (92.3%), rifampicin (92.3%), clindamycin (96.2%), and trimethoprim-sulfamethoxazole (91.3%). Alarmingly, 98.1% of all Listeria spp. isolates exhibited multidrug resistance (MDR), reaching 100% MDR among L. monocytogenes isolates. Specifically, L. monocytogenes isolates exhibited complete resistance to meropenem, cefoxitin, cefotaxime, rifampicin, and trimethoprim-sulfamethoxazole, and significant resistance to ciprofloxacin (91.7%), clindamycin (83.3%), tetracycline (75.0%), erythromycin (75.0%), benzylpenicillin (70.8%), and nitrofurantoin (70.8%). Molecular analysis identified blaTEM (100%), ampC (37.5%), and ereB (37.5%) as the most prevalent antimicrobial resistance genes in L. monocytogenes.

The exceptionally high prevalence of multidrug-resistant Listeria spp., particularly L. monocytogenes, in raw fish underscores a critical public health risk, suggesting the urgent need for ongoing surveillance and robust risk mitigation strategies in aquaculture and seafood processing. The elevated antimicrobial resistance levels may also indicate aquatic environmental contamination, warranting further investigation into the sources and broader ecological implications of antimicrobial resistance in these ecosystems.