Comparison of the mechanism of action and resistance of two new fluoroquinolones, rufloxacin and MF961 with those of ofloxacin and fleroxacin in gram-negative and gram-positive bacteria.

For rufloxacin MF961, ofloxacin and fleroxacin the inhibition of DNA synthesis, intracellular accumulation, optimum bactericidal concentration (OBC) and killing kinetics at the OBC for Enterobacteriaceae, Pseudomonas aeruginosa and staphylococci and induction of recA in Escherichia coli were determined. All agents had good activity against all the strains and inhibited DNA synthesis by 50% at concentrations correlating with the MIC. The maximum recA inducing concentrations after 60 min exposure to the quinolones in E. coli were 0.5 mg/L of rufloxacin, MF961, ofloxacin and 0.05 mg/L of fleroxacin. Accumulation of all quinolones was rapid; however, higher concentrations of all agents were accumulated within staphylococci than in Gram-negative bacteria. Rufloxacin was accumulated to higher concentrations in all bacteria than the other three agents. Laboratory mutants with decreased susceptibility to the four drugs were selected from each strain. All agents selected mutants with decreased susceptibility to quinolones alone, with phenotypes suggesting mutations in gyrA. Multiply-resistant mutants were also selected; however, as few had decreased expression of OmpF, the mutated gene is unlikely to be an allele of marA. All mutants had MICs > or = 2 mg/L, a typical breakpoint concentration for most quinolones.