Abstract

Mutations in gyrase and topoisomerase genes (gyrA, gyrB, parC and parE) were examined among 30 multidrug-resistant (MDR) Salmonella isolates recovered from retail meats using PCR and DNA sequencing analysis. Six isolates had a single gyrA mutation (Ser83→Phe or Ser83→Tyr or Asp87→Asn) with ciprofloxacin (Cip) minimum inhibitory concentrations (MICs) ranging from 0.125 to 0.5μg/ml except 1 with 16μg/ml. Three isolates had both a gyrA mutation (Ser83→Tyr or Ser83→Phe) and a parC mutation (Ser80→Arg) (Cip MICs, 8 to 16μg/ml). Fifteen isolates had both double mutations in gyrA (Ser83→Phe and Asp87→Gly or Asp87→Asn) and a single mutation (Ser80→Arg) in parC (Cip MICs, 8 to ≥16μg/ml). Three had double gyrA mutations (Ser83→Phe, Asp87→Gly or Asp87→Asn), one parC mutation (Ser80→Arg), and 1 parE mutation (Lys428→Gln or Gly442→Ser) (Cip MICs, 8 to ≥16μg/ml). One had double gyrA mutations (Ser83→Phe, Asp87→Gly or Asp87→Asn), one parC mutation (Ser80→Arg), and 2 parE mutations (Lys441→Ile and Asp494→Asn) (Cip MICs, 8 to ≥16μg/ml). Isolates with single gyrA mutations were less resistant to quinolone and fluoroquinolones than those together with additional parC and/or parE mutations in Salmonella. Our findings confirmed the importance of point mutations in gyrase and topoisomerase in reduced susceptibility to quinolone and fluoroquinolones. Mutations in gyrA confer low-level quinolone and fluoroquinolone resistance, while additional gyrA mutation(s) together with parC and/or parE mutations increase the resistance to a high level.

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