First description of ColE-type plasmid in Aeromonas spp. carrying quinolone resistance (qnrS2) gene

Aims: Isolation and full sequence analysis of ColE-type plasmid, which carries the qnrS2 gene. Methods and Results: Quinolone resistance (qnrS2) gene-carrying plasmids were isolated from Aeromonas sobria and Aeromonas hydrophila strains, and plasmid sequencing was achieved by a primer-walking approach. The total sizes of these plasmids (pAQ2-1 and pAQ2-2) were 6900 bp and 6903 bp, respectively, and they were 99.1% identical to each other. The genes (oriV and repA) for plasmid replication were organized similar to the corresponding genes in the ColE2-type plasmids, pAsa3 and pAsa1, isolated from Aeromonas salmonicida subsp. salmonicida, but the gene (mobA) for mobilization was homologue to ColE1-type plasmid (pAsa2) from Aer. salmonicida subsp. salmonicida. Additionally, the qnrS2 gene was part of a mobile insertion cassette element in the plasmid. Conclusions: Two plasmids were assumed to be the same plasmid, and this identification of a plasmid-mediated qnrS2 gene from the two different strains underlines a possible diffusion of these resistance determinants in an aquaculture system. Significance and Impact of the Study: This is the first finding of the ColE-type plasmid carrying the qnrS2 gene.