A risk assessment of the environmental and clinical implications of aquaculture-associated, multidrug-resistant Aeromonas veronii.

The emergence and spread of antibiotic resistance genes (ARGs) mediated by mobile genetic elements (MGEs) pose a significant threat to public and environmental health. While aquaculture ecosystems are recognised as critical reservoirs for ARGs, the genomic architecture of MGEs that facilitate resistance dissemination in aquatic pathogens - particularly Aeromonas veronii (A. veronii) - remains underexplored. This study isolated a highly virulent, multidrug-resistant strain of A. veronii (Y6) from diseased largemouth bass, underscoring its potential dual threat to aquaculture and public health. Whole-genome sequencing revealed that strain Y6 harbours a chromosomal genomic island, GI22, containing a integron 1, which encodes 15 ARGs. Embedded within this island is a novel composite transposon, TnY6-1, which carries five of these ARGs and has the potential to facilitate horizontal gene transfer. Phylogenetic analysis revealed that strain Y6 clusters closely with human clinical isolates, underscoring its zoonotic potential. Comparative genomic analysis revealed that the resistance gene cassettes in integron 1 of strain Y6 are highly conserved across fish, human, and environmental isolates, suggesting the possibility of ARG transmission between different ecosystems. To quantify the environmental threat posed by TnY6-1, we applied a multidimensional risk assessment framework adapted from the PIPdb database. This marks the first classification of TnY6-1 as a Level IV high-risk MGE. These findings highlight the importance of implementing risk-based surveillance systems targeting hazardous MGEs in aquaculture to reduce the environmental-to-clinical spillover of antimicrobial resistance.