Prevalence of quinolone resistance and mutations in the topoisomerase genes in Salmonella enterica serotype Enteritidis isolates from Serbia

Abstract

The prevalence of quinolone resistance was studied in Salmonella enterica serotype Enteritidis isolates collected during 2005–2010 in Southern Baˇcka County, Serbia. A total of 878 clinical isolates were examined, among which 19 (2.2%) nalidixic acid (NAL)-resistant S. Enteritidis were detected by selection on agar plates containing 64 mg/L NAL. Antimicrobial susceptibility of the isolates was tested by the agar dilution method. According to Clinical and Laboratory Standards Institute (CLSI) breakpoints, ciprofloxacin (CIP) resistance was not present in the strains. Multiple drug resistance was rare, and resistance to NAL was most often present as a single resistance property. All but one NAL-resistant S. Enteritidis showed reduced susceptibility to CIP [i.e. minimum inhibitory concentration (MIC) ≥ 0.125 mg/L]. This isolate of human origin had a CIP MIC of 0.064 mg/L and DNA sequencing revealed that it contained an Asp87Gly gyrA mutation. Most of the remaining isolates had MICs for NAL and CIP of 256 mg/L and 0.256 mg/L, respectively. Mutations in the Asp87 codon resulted in substitutions to Asn in most of the isolates, but Asp87Gly and Ser83Phe exchanges were also detected. No mutations were present in the gyrB, parC or parE genes. Although CIP resistance was absent, reduced susceptibility characterised by mutations in gyrA was apparent among S. Enteritidis isolates from Serbia.