Rate of occurrence of mutations at Ser-84 in Staphylococcus aureus DNA gyrase A protein

We examined the distribution of mutations in DNA gyrase A protein gene, which is considered to be especially important in the mechanism responsible for high level quinolone resistance, in clinical isolates of Staphylococcus aureus. The rate of occurrence of mutations at serine-84 in the DNA gyrase A protein was investigated by polymerase chain reaction (PCR) and Hinf I digestion in 110 clinical isolates of Staphylococcus aureus from the Tohoku district, collected in 1992, and the following results were obtained. (1) Serine-84 mutations were very widely distributed among the isolates against which the MICs of ofloxacin were above 4 μg/ml. (2) Serine-84 mutations were observed without exception in highly resistant strains (MIC≥16 μg/ml). (3) The mutant/non-mutant strain ratios were essentially the same for ofloxacin and for 8 other quinolone agents, but the borderline MICs between the mutant and non-mutant strains varied from agent to agent. (4) There was no significant difference in the mutant/non-mutant strain ratios among MRSA, MSSA (without mecA gene), and MSSA (with mecA gene). So we concluded that there was no correlation between serine-84 mutations and methicillin resistance.