Resistance to fluoroquinolones linked to gyrA and parC mutations and overexpression of acrAB efflux pump in Salmonella enterica serotype choleraesuis

Between 2000 and 2002, 60 clinical isolates of Salmonella enterica serotype Choleraesuis were collected to investigate the mechanism of fluoroquinolone resistance. PCR and sequencing were performed to identify mutations in gyrA, gyrB, parC, the AcrAB-TolC efflux pump regulator, acrR, and the global regulons marRAB and soxRS. All resistant strains showed mutations in the target genes leading to amino acid changes of Ser83Phe and Asp87Asn in GyrA and Ser80Ile in ParC. A mutation in gyrB was linked to the serotype genetic diversity but not to fluoroquinolone resistance. An efflux pump inhibitor, Phe-Arg-β- naphthylamide, caused four-fold lower MIC of ciprofloxacin in the resistant isolates, indicating that efflux systems are involved in fluoroquinolone resistance. Western blot analysis showed moderate overproduction of AcrA in fluoroquinolone-resistant isolates. A mutation in acrR gave rise to an internal stop codon in both ciprofloxacin-resistant and -susceptible isolates, suggesting another serotype genetic diversity. No mutations were detected in marRAB and soxRS among the isolates examined. Cross-resistance to three fluoroquinolones was observed, but gatifloxacin demonstrated relatively lower MICs than those of ciprofloxacin and levofloxacin. Fluoroquinolone resistance in S. Choleraesuis appears to be the combination effect of multiple mutations in various target genes and overexpression of the AcrAB-TolC efflux pump.