Antibiotic use in oyster hatcheries promotes rapid spread of a highly transferable and modular resistance plasmid in Vibrio.

Plasmids play a key role in the horizontal gene transfer of antibiotic resistance genes (ARGs), particularly in aquaculture where ARG-carrying Vibrio bacteria are frequently detected. Given the expansion of global aquaculture and its reliance on antibiotics, we investigated how these practices influence the emergence, dynamics, and spread of ARGs, focusing on Magallana gigas hatcheries-the world's most widely farmed shellfish. Among the three antibiotics tested, only chloramphenicol (CHL) led to a pronounced selection and dissemination of CHL-resistant Vibrio isolates. Resistance was mediated by catA2, located in a highly modular, insertion sequence- and transposase-rich region of a conjugative plasmid, alongside tet(B). This plasmid was closely related to emerging pAQU-type plasmids unreported in Europe. pAQU-MAN, derived from Marine ANtimicrobial resistance, is a low-copy, highly transferable plasmid that rapidly spread throughout the hatchery following CHL treatment. Though naturally found in commensal Vibrio, it exhibited a broad host range, transferring efficiently to both oyster- and human-pathogenic Vibrio strains, as well as to Escherichia coli, with high conjugation rates. Additionally, it remained stable in Vibrio hosts and was transmitted from oyster parents to progenies, even in the absence of antibiotic. It eventually disappeared from the microbial community associated to adults. Our findings highlight that antibiotic use in oyster hatcheries can select for highly modular and transferable multidrug-resistant plasmids, posing a risk of environmental dissemination, although their limited persistence in juvenile oyster reduces the likelihood of transmission to humans. We discuss the human and ecological factors driving pAQU-MAN spread and control in aquaculture settings.