Molecular characterization of ciprofloxacin-resistant Salmonella isolates from sick animals

A total of 13 non-duplicate Salmonella isolates resistant to ciprofloxacin were collected from sick food-producing animals subjected to phenotypic characterization including the amplification of plasmid-mediated quinolone resistance(PMQR) genes,as well as β-lactamases encoding genes only in PMQR positive isolates by PCR,the screening of mutations in the quinolone resistance-determining regions of gyrA and parC by PCR.The qnr,aac(6′)-Ib-cr and oqxA genes were detected in 13 of the isolates and were frequently co-prevalent.Seven isolates demonstrated reducing susceptible or low resistant for ciprofloxacin with minimal inhibitory concentration(MIC) from 0.125 to 4 μg/mL.These isolates had either single mutation in gyrA or no mutation in any target genes as well the active efflux.Five isolates except one presented double mutations in both gyrA and parC and the active efflux conferring high resistant level to ciprofloxacin(MIC,32-128 μg/mL).Only blaCTX-M type ESBLs were detected in 4 isolates with PMQR presenting high level of resistance to ciprofloxacin.Reduced ciprofloxacin susceptibility was arisen from either single mutation in gyrA or the presence of PMQR genes,irrelevant to the active efflux,while high level of resis-tance to ciprofloxacin was conferred by chromosomal mutations in target genes combining with active efflux.