Mutations in gyrA and parC Genes and Plasmid-Mediated Quinolone Resistance in Non-typhoid Salmonella Isolated from Pediatric Patients with Diarrhea in Seoul

A total of 91 non-typhoid Salmonella isolated from pediatric patients with diarrhea in Seoul from 2003 to 2009 was tested for antimicrobial susceptibility of nalidixic acid (NA). Forty strains of NA resistance or intermediate susceptible non-typhoid Salmonella were identified and their minimum inhibitory concentrations (MICs) of NA, ciprofloxacin (CIP), and norfloxacin (NOR) were determined. Of the 40 isolates, 26 were resistant to NA (MIC >256 μg/ml). Only one isolate was high-level resistant to CIP (12 μg/ml) and NOR (48 μg/ml). Mutations in gyrA and parC genes were studied by PCR and sequencing. All NA-resistant isolates carried point mutations in the gyrA quinolone resistance determining regions (QRDR) at codon 83 or 87 (MICs of NA, >256 μg/ml; MICs of CIP, 0.047~0.25 μg/ml; MICs of NOR, 0.38~1.5 μg/ml). A double change in GyrA was found in one Salmonella Enteritidis (MIC of CIP, 12 μg/ml; MIC of NOR, 48 μg/ml). In respect of the ParC protein, a single change at Thr57→Ser was found in 3 isolates (MICs of NA, >256 μg/ml; MICs of CIP, 0.19~0.25 μg/ml; MICs of NOR, 1 μg/ml). At the same time, these strains changed from Ser83 to Tyr in the gyrA. The result of the investigation for the prevalence of plasmid-mediated quinolone resistance (PMQR) genes, 14 isolates harbored qnr gene among 40 isolates. All of 14 isolates showed decreased susceptibility at NA (MICs 4~16 μg/ml) and except one strain, all of qnr genes were identified as qnrB. Mutations in the gyrA gene and production of PMQR determinants were critical for quinolone resistance and decreased susceptibility to fluoroquinolone in these isolates.