Mutations in gyrase and topoisomerase genes associated with fluoroquinolone resistance in Salmonella serovars from retail meats

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 0.5 μg/ml except 1 with 16 μg/ml. Three isolates had both a gyrA mutation (Ser83 → Tyr or Ser83 → Phe) and a parC mutation (Ser80 → Arg) (Cip MICs, 8 to 16 μg/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 ≥ 16 μg/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 ≥ 16 μg/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 ≥ 16 μg/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.