Identification of the plasmid-borne quinolone resistance gene qnrS in Salmonella enterica serovar Infantis

Objectives: A Salmonella enterica serovar Infantis isolate of avian origin was investigated for the presence of the gene qnrS, its transferability and its association with other resistance genes. Methods: The Salmonella Infantis isolate was investigated for its susceptibility to antimicrobial agents and its plasmid content. Hybridization experiments and PCR assays were performed to identify the resistance genes while transformation and conjugation studies were conducted to show their transferability. The quinolone resistance-determining regions of the genes gyrA, gyrB, parC and parE were sequenced. Moreover, extended sequence analysis was performed to gain insight into the structure and organization of the qnrS gene area. Results: The Salmonella Infantis isolate exhibited the Asp(87)-> Tyr(87) mutation in gyrA, but no resistance-mediating mutations in the other target genes. It carried a conjugative plasmid, pINF5, on which a qnrSgene was detected in close proximity to a Tn3-like, bla(TEM-1)-carrying transposon. Homology to the qnrS-carrying plasmid pAH0376 of Shigelia flexneri was limited to the Tn3-qnrS region. Sequence analysis of an similar to 13.4 kb region of pINF5 identified truncated insertion sequences of types IS26 and IS2 as well as an internal segment of the CS12 fimbrial gene cluster of Escherichia coli up- and downstream of the qnrS gene. Conclusions: This is to the best of our knowledge the first report of a qnrS gene in a Salmonella isolate. The analysis of the regions flanking the qnrS gene suggested that this region developed in multiple steps that included not only the integration of insertion sequences and a Tn3-like transposon, but also interplasmid recombination events.