In vivo selection during pefloxacin therapy of a mutant of Staphylococcus aureus with two mechanisms of fluoroquinolone resistance.

Staphylococcus aureus BM4626 (ciprofloxacin MIC, 0.5 microgram/ml) and BM4627 (ciprofloxacin MIC, 32 microgram/ml) were isolated from the same patient before and during pefloxcin therapy for septic tibial nonunion, respectively. The two strains had similar serotypes and indistinguishable phage types and SmaI-generated restriction fragment length polymorphisms. Portions of the gyrA (codons 60 to 120) and the gyrB (codons 420 to 480) genes of each clinical isolate were amplified by PCR and sequenced. Strain BM4627 had a serine-to-leucine substitution resulting from a cytosine-to-thymidine mutation at codon 84 of gyrA relative to the sequence of the gyrA gene of BM4626. Norfloxacin accumulation, measured in a whole-cell uptake assay, was significantly lower in BM4627 than BM4626. These data indicate that double mutants can be selected in vivo under fluoroquinolone therapy.