In vivo selection of aac(6')-Ib-cr and mutations in the gyrA gene in a clinical qnrS1-positive Salmonella enterica serovar Typhimurium DT104B strain recovered after fluoroquinolone treatment

To characterize the mechanisms implicated in the in vivo selection of quinolone and aminoglycoside resistance in a faecal Salmonella enterica serovar Typhimurium DT104B strain recovered after ciprofloxacin treatment of a hospitalized elderly patient with acute gastroenteritis. Two Salmonella Typhimurium isolates were obtained before (Se6) and after (Se20) treatment and they were typed by PFGE and multilocus sequence typing (MLST). Antimicrobial susceptibility was determined by disc diffusion and agar dilution methods. Class 1, 2 and 3 integrons and resistance mechanisms were studied by PCR and sequencing. Plasmids were typed. Both Salmonella Typhimurium strains were resistant to tetracycline, streptomycin and sulphonamides, while Se20 was also resistant to nalidixic acid, ciprofloxacin, norfloxacin, levofloxacin, ofloxacin, amikacin, tobramycin, kanamycin and trimethoprim. PFGE and MLST showed a clonal relationship between the strains, which belonged to the sequence type ST36. Both strains contained the repC-sul2-strA-strB structure and tet(A) and qnrS1 genes, and strain Se20 also contained the aac(6')-Ib-cr gene, the Ser83 -> Tyr substitution in GyrA and one class 1 integron with the dfrA17 + aadA5 gene cassette arrangement lacking qacE delta 1 + sul1. Two different transconjugants from Salmonella Se20 (TCSe20B and TCSe20L) harboured qnrS1 and sul2 genes and the class 1 integron. The TCSe20B strain also acquired the aac(6')-Ib-cr gene located on a non-typeable plasmid. qnrS1 was identified on a ColE-type plasmid and the class 1 integron on an IncI1-type plasmid. This is the first report of in vivo selection of the aac(6')-Ib-cr gene and the Ser83 -> Tyr change in GyrA in a qnrS1-positive Salmonella Typhimurium strain after ciprofloxacin treatment; the in vitro transfer of both plasmid-mediated quinolone resistance genes was also demonstrated.