Background Little is known about antifungal resistance in dermatophytes despite the widespread use of oral drugs to treat patients suffering from onychomycosis. In this study we have analyzed the spontaneous resistance frequency to terbinafine of T. rubrum strains and their capacity to develop resistance to the drug in vitro. Methods To determine resistance frequency, about 1 x 109 CFU of 5 strains were plated on potato dextrose agar (PDA) medium containing 0.06 Âµg/ml terbinafine (cidal concentration) and incubated for 3 weeks at 30 Â°C. To investigate resistance induction, four strains were serially passaged at 30 Â°C either on PDA plates or in RPMI 1640 liquid medium containing increasing concentrations of terbinafine, starting at 0.002 Âµg/ml. In both cases, MIC values were determined before and after treatments, using microdilution assay in RPMI 1640 medium. Results The resistance frequency of the 5 strains was 5 x 10-9 (MIC 0.5 Âµg/ml terbinafine for the resistant isolates versus 0.004 Âµg/ml for the wild-type strains). All the 9 spontaneous terbinafine-resistant colonies were cross-resistant to other squalene epoxidase inhibitors but normally susceptible to antifungals with a different mechanism. Only 1 of the 4 passaged strains on PDA plates grew at a terbinafine concentration > 0.016 Âµg/ml (0.06 Âµg/ml, while the initial MICs were 0.004 Âµg/ml for the 4 strains). In liquid medium, after more than 9 months incubation, the same strain managed to grow in the presence of 0.125 Âµg/ml terbinafine, while the others could not grow at a concentration 0.03 Âµg/ml. The isolate able to grow in 0.125 Âµg/ml terbinafine was also less susceptible ( 16x ) to other squalene epoxidase inhibitors and azoles. Conclusion Naturally occurring mutant resistants to terbinafine in a population of T. rubrum are very rare. Their selective cross-resistance to other squalene epoxidase inhibitors suggests an alteration of the target. T. rubrum does not appear to be able to develop either rapid or extensive resistance to terbinafine in vitro.
Full conference title:
42nd Interscience Conference on Antimicrobial Agents and Chemotherapy
- ICAAC 42nd