Amino Acid Substitution Ser83Ile in GyrA of DNA Gyrases Confers High-Level Quinolone Resistance to Nontyphoidal Salmonella without Loss of Supercoiling Activity

Aims: Quinolone-resistant nontyphoidal Salmonella having serine replaced by isoleucine at the 83rd amino acid in GyrA (GyrA-Ser83Ile) has recently been found in Asian countries. In this study, we aimed to examine the direct effect of substitution Ser83Ile on DNA gyrase activity and/or resistance to quinolones. Materials and Methods: Using 50% of the maximal inhibitory concentrations (IC50s) of quinolones, recombinant wild type (WT) and seven mutant DNA gyrases having amino acid substitutions, including Ser83Ile, were screened for enzymatic activity that causes supercoils in relaxed plasmid DNA and resistance to quinolones. Results: Little differences in supercoiling activity were observed between WT and mutant DNA gyrases. By contrast, the IC50s of ciprofloxacin and norfloxacin against GyrA-Ser83Ile/GyrB-WT were 11.6 and 73.3 μg/mL, respectively, which were the highest used against the DNA gyrases examined in this study. Conclusion: Ser83Ile in GyrA was shown to confer high-level quinolone resistance to DNA gyrases of nontyphoidal Salmonella, with no loss of supercoiling activity. Salmonella strain carrying GyrA with Ser83Ile may emerge under a high-concentration pressure of quinolones and easily spread even with no selection bias by quinolones. Hence, avoiding the overuse of quinolones is needed to prevent the spread of Salmonella with Ser83Ile in GyrA.