Mutations in Topoisomerase Genes and Expression of AcrAB Multidrug Efflux System in Fluoroquinolone-Resistant Salmonella enterica from Pork and Patients

Abstract

The aim of this study was to further examine the mechanisms of ciprofloxacin resistance in Salmonella enterica isolates from pork and patients. Twenty-four isolates including 18 ciprofloxacin- resistant isolates and six susceptible strains were used. Mutations in the quinolone resistance-determining regions (QRDRs) of gyrA, gyrB, parC and parE were examined. Expression of AcrAB-TolC efflux pump was quantitatively determined using qRT-PCR. While nucleotide modifications were found in gyrA, gyrB and parC, all mutations identified were novel. All 18 ciprofloxacin resistant isolates carried mutations in at least one target gene and all six isolates susceptible to ciprofloxacin did not carry mutations in their QRDRs. There were no associations between the number and type of mutations and ciprofloxacin resistance level among the ciprofloxacin Salmonella isolates. All the Salmonella isolates produced AcrB, ranging from 1 to 430 folds. A Salmonella strain susceptible to all antimicrobials tested overexpressed AcrB. The isolates with comparable AcrB demonstrated different susceptibility to ciprofloxacin. Therefore, the role of mutations in topoisomerase genes and the AcrAB-TolC efflux pump in decreased susceptibility to ciprofloxacin varied among the S. enterica clinical isolates.