Non-target gene mutations in the development of fluoroquinolone resistancein Escherichia coli

Mutations in loci other than genes for the target topoisomerases of fluoroq uinolones, gyrA and parC, may play a role in the development of fluoroquino lone resistance in Escherichia coli. A series of mutants with increasing re sistance to ofloxacin was obtained from an E. coli K-12 strain and five cli nical isolates, First-step mutants acquired a gyrA mutation. Second-step mu tants reproducibly acquired a phenotype of multiple antibiotic resistance ( Mar) and organic solvent tolerance and showed enhanced fluoroquinolone effl ux. None of the second-step mutants showed additional topoisomerase mutatio ns. All second-step mutants showed constitutive expression of marA and/or o verexpressed soxS. In some third step mutants, fluoroquinolone efflux was f urther enhanced compared to that for second-step mutants, even when the mut ant had acquired additional topoisomerase mutations. Attempts to circumvent the second-step Mar mutation by induction of the mar locus with sodium sal icylate and thus to select for pure topoisomerase mutants at the second ste p were not successful. At least in vitro, non-target gene mutations accumul ate in second- and third-step mutants upon exposure to a fluoroquinolone an d typically include, but do not appear to be limited to, mutations in the m ar or sox regulons with consequent increased drug efflux.