Three strains each of Streptococcus pneumoniae, Lancefield group A streptococci, Lancefield group B streptococci and Lancefield group D streptococci were examined for the frequency of spontaneous mutation to give decreased susceptibility at two, four and six times the minimum inhibitory concentration of ofloxacin, ciprofloxacin, enoxacin and norfloxacin. Any putative mutants were examined for stability of resistance and susceptibility to all quinolones in the study, erythromycin and benzyl penicillin. All strains yielded mutants to twice the MIC of each quinolone at a frequency associated with a mutation at a single gene. Each species in the study responded differently at four and six times the MIC of each quinolone. Group B streptococci yielded most mutants and resistant Str. pneumoniae were the most difficult to select. Most mutants were cross-resistant to quinolones only, probably owing to an altered DNA gyrase. Fewer resistant mutants of Str. pneumoniae were selected using ofloxacin compared to ciprofloxacin.
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