Abstract
The exchange of antimicrobial resistance (AMR) genes between aquaculture and terrestrial microbial populations has emerged as a serious public health concern. However, the nature of the mobile genetic elements in marine bacteria is poorly documented. To gain insight into the genetic mechanisms underlying AMR gene transfer from marine bacteria, we mated a multi-drug resistant Vibrio alfacsensis strain with an Escherichia coli strain, and then determined the complete genome sequences of the donor strain and multidrug-resistant transconjugants. Sequence analysis revealed a conjugative plasmid of the MOBH family in the donor strain, which was integrated into the chromosome of the recipient. The plasmid backbone in the transconjugant chromosome was flanked by two copies of a 7.1 kb integrative element, designated Tn6945, harboring a beta-lactamase gene that conferred ampicillin resistance to the host cell. Use of a recA mutant E. coli strain as the recipient yielded a transconjugant showing ampicillin resistance but not multidrug resistance, suggesting the involvement of homologous recombination in plasmid integration into the chromosome. Polymerase chain reaction experiments revealed that Tn6945 generates a circular copy without generating an empty donor site, suggesting that it moves via a copy-out-paste-in mode, as previously reported for Tn6283. Transposition of the integrative element into multiple loci in the recipient chromosome increased the resistance level of the transconjugants. Overall, these results suggest that Tn6283-like copy-out integrative elements and conjugative plasmids additively spread AMR genes among marine bacteria and contribute to the emergence of isolates with high-level resistance through amplification of AMR genes.
Competing Interest Statement
The authors have declared no competing interest.