Characteristics of quinolone-induced small colony variants in Staphylococcus aureus

Abstract

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.

Antimicrobial susceptibility patterns of Staphylococcus aureus and coagulase negative staphylococci isolated from humans in Nairobi, Kenya

“…Because isolates in this study did not show resistance to vancomycin, it is advisable to limit the use of glycopeptides to treat only MRSA infections so as to reduce the selective pressure and likely emergence of resistance to this class of antibiotics. Decreased staphylococcal susceptibility to vancomycin is not due to transfer of vanR genes from vancomycin-resistant enterococci (VRE) or to small colony variants, as noted in staphylococci for other antimicrobial agents (Mitsuyama et al, 1997) but appears to be a gradual selection process due to treatment pressure. Glycopeptideresistant mutants of S. aureus have been experimentally selected by increasing the levels of vancomycin present during in vitro growth (Daum et al, 1992;Sieradzki and Tomasz, 1997).…”

Comparative Mutant Prevention Concentrations of Pradofloxacin and Other Veterinary Fluoroquinolones Indicate Differing Potentials in Preventing Selection of Resistance

“…SCVs were carefully recorded because of their undefined relevance in the selection of FQ resistance (33,35). Generally, SCVs are thought to exhibit a resistance phenotype caused by impaired heme or quinone biosynthesis (29,31). The resulting ATP deficiency, however, may also be characteristic of SCVs appearing under drug selective pressure (33).…”

Small-Colony Mutants of Staphylococcus aureus Allow Selection of Gyrase-Mediated Resistance to Dual-Target Fluoroquinolones

“…Although SCVs have altered accumulation of drugs, including quinolones (30,37), it is difficult to envisage how changes in efflux could differentially affect drug targeting. Instead, we envisage that small-colony mutations either alter quinolone-target interactions at the level of cleavable-complex formation with gyrase and topoisomerase IV or, alternatively, influence the processing of such complexes to give the lethal lesion.…”

“…It is interesting to ask whether small-colony mutants can be selected by quinolones other than NSFQ-105 and sparfloxacin and acquire further mutational resistance. It has been reported that small-colony mutants are induced in liquid culture at the MIC by pazufloxacin, a putative dual-target agent (42), and by nalidixic acid, which is thought to target gyrase, but not by ciprofloxacin, norfloxacin, fleroxacin, enoxacin, pipemidic acid, or tosufloxacin, which act through topoisomerase IV (30). More work will be needed to establish whether failure to recover small-colony mutants in vitro is due to a chemical feature of the quinolone or, more likely, a narrow selection window given their low-level (twofold) resistance.…”