Genetic characterization of plasmid-mediated quinolone resistance gene qnrS2 in Pseudoalteromonas and Shewanella isolates from seawater.

Three qnrS2-containing isolates of Pseudoalteromonas and Shewanella were collected from the seawater samples of Qingdao in China during 2014. They displayed resistance to ampicillin, ciprofloxacin, kanamycin, nalidixic acid and sulfamethoxazole. The qnrS2 genes were identified in the chromosomes of Pseudoalteromonas strains E8 and S16, and in a 140-kb plasmid in Shewanella strain S14, respectively. In addition, two copies of qnrS2 were identified in the strain E8. Sequence analyses revealed that there was an identical DNA segment located in the downstream of qnrS2 in strain S14 and E8, coding for a TetR transcriptional regulator, two putative integrases and a hypothetical protein. However, different genetic structures were identified in the upstream sequences: the terB gene associated with tellurite resistance in the strain S14, and a putative integron with dfrA6 and aadA13 gene cassettes or the Tn7-related gene complex tnsABC in the strain E8. In Pseudoalteromonas strain S16, qnrS2 was bracketed by the endonuclease I and III genes, and the electron transport complex rsxCDGE was located in the upstream sequences. This is the first report of two copies of the qnrS2 gene existing in one bacterial chromosome, and also the first identification of qnrS2 in Shewanella.