A novel locus conferring fluoroquinolone resistance in Staphylococcus aureus

Fluoroquinolones such as ciprofloxacin and ofloxacin are potent antimicrobial agents that antagonize the A subunit of DNA gyrase. We selected and mapped a novel fluoroquinolone resistance gene on the Staphylococcus aureus chromosome. Resistant mutants were selected with ciprofloxacin or ofloxacin and were uniformly localized to the A fragment of chromosomal DNA digested with SmaI and arrayed by pulsed-field gel electrophoresis. Several mutants (cfxB, ofxC) were genetically mapped between the thr and trp loci in the A fragment. A majority of A fragment fluoroquinolone resistance mutations were associated with reduced susceptibility to novobiocin, an antagonist of the B subunit of DNA gyrase. Two genes previously associated with fluoroquinolone resistance, the gyrA gene of DNA gyrase and the norA gene (associated with decreased drug accumulation), were localized to the G and D fragments, respectively. Thus, the fluoroquinolone resistance mutations in the A fragment are distinct from previously identified fluoroquinolone resistance mutations in gyrA and norA. Whether mutations in the A fragment after a second topoisomerase or another gene controlling supercoiling or affect drug permeation is unknown.