Characteristics of quinolone-induced small colony variants in Staphylococcus aureus

Exposure of Staphylococcus aureus to 1 x MIC of the quinolone antibiotic pazufloxacin for 24 h, followed by plating on drug-free media, led to the emergence of small colony variants (SCVs) in addition to large colony variants (LCVs). However, following incubation with 0.25 or 4 x MIC of pazufloxacin, only LCVs were obtained. The SCVs were half as susceptible to pazufloxacin or ciprofloxacin as wild-type S. aureus, while the susceptibilities of LCVs were essentially unchanged. The reduced susceptibilities of SCVs did not result from mutations in the quinolone-resistance-determining regions of DNA gyrase and topoisomerase IV, since the sequences of these genes were identical to those of the wild-type. However, the SCVs accumulated pazufloxacin and ciprofloxacin to a lesser degree than did wild-type. Furthermore, their susceptibility to quinolones was almost unaffected by reserpine or verapamil, suggesting that the reduced uptake resulted from decreased permeability, rather than from an active efflux pump. The ability of various quinolones to induce emergence of SCVs in S. aureus, correlated with the presence of carbon-bonded substituents at the C-7 position of a quinoline or naphthyridine nucleus, or with the presence of a benzoxazine nucleus. In conclusion, pazufloxacin-induced SCVs represent a mutant that one might expect to be rapidly eliminated in vivo and, hence, not to survive as a quinolone-resistant pathogen. This finding suggests a novel approach for development of future quinolones.