Microbiological quality, antibiotic resistant bacteria and relevant resistance genes in ready-to-eat Pacific oysters (Magallana gigas).

Oysters are a highly valued seafood but can endanger public health, if they are eaten raw or barely cooked. We evaluated the microbiological quality of Pacific oysters (Magallana gigas) by international standard methods in four groups (each with four to five animals) acquired from supermarkets and directly from a farm producer. Most of the groups presented satisfactory microbiological quality. In two groups of oysters, 'questionable' or 'unsatisfactory' quality was observed for the coagulase-positive Staphylococcus parameter. Culture-based methods did not detect Salmonella spp. or enteropathogenic Vibrio spp., but Vibrio alginolyticus, a potential foodborne pathogen, was identified by molecular analysis. Fifty strains, belonging to 19 species, were isolated in antibiotic-supplemented media, and their antibiotic susceptibility profile was evaluated. Genes coding for β-lactamases were searched by PCR in bacteria showing resistance phenotype. Decreased susceptibility or resistance to distinct antibiotics were observed for bacteria from depurated and non-depurated oysters. The blaTEM gene was identified in Escherichia fergusonii and Shigella dysenteriae strains, which showed multidrug-resistant phenotypes. The possibility that oysters might be a source of antibiotic-resistant bacteria/antibiotic resistance genes is of great concern and highlights the need for stricter controls and preventative measures to mitigate and counteract the dissemination of antibiotic resistance across the food chain.