In Vitro Effect of qnrA1, qnrB1, and qnrS1 Genes on Fluoroquinolone Activity against Isogenic Escherichia coli Isolates with Mutations in gyrA and parC

This article provides an analysis of the in vitro effect of qnrA1, qnrB1, and qnrS1 genes, combined with quinolone-resistant Ser83Leu substitutions in GyrA and/or Ser80Arg in ParC, on fluoroquinolone (FQ) resistance in isogenic Escherichia coli strains. The association of Ser83Leu substitution in GyrA, Ser80Arg substitution in ParC, and qnr gene expression increased the MIC of ciprofloxacin to 2 mu g/ml. qnr genes present in E. coli that harbored a Ser83Leu substitution in GyrA increased mutant prevention concentration (MPC) values to 8 to 32 mu g/ml. qnr gene expression in E. coli may play an important role in selecting for one-step FQ-resistant mutants.