Screening for terbinafine resistance in clinical dermatophyte isolates in Australia
Onychomycosis is frequently recalcitrant to treatment. The issue of terbinafine resistance has been raised by the current epidemic of dermatophytoses in India. Laboratory identification and microdilution susceptibility testing of the causative agent is costly and time consuming. It may be more efficient to screen cultured isolates for resistance. A screening assay, covering a range of terbinafine concentrations, was developed to indicate which clinical strains might be resistant. Four 6 cm petri dishes were used per isolate. All plates contained solid PDA with: 0, 0.05 µg/ml, 0.28 µg/ml or 0.5 µg/ml terbinafine concentrations. From each isolate, 4 mm2 conidia/hyphae was scraped from the colony surface, suspended in 5 ml of sterile water and vortexed. From this suspension, 0.15 µl was spread onto each plate, incubated for up to 7 days and checked for growth. Growth on any of the three terbinafine-containing plates was considered to indicate a degree of resistance. In this way. Seventy-three isolates were tested: 59 isolates of Trichophyton interdigitale, 2 of T. rubrum, 10 of Arthroderma quadrifidum, 1 of A. melbournense and 1 of Paraphyton cookei, along with known MIC refence strains of T. rubrum and T. interdigitale. Four strains (6.8%) of T. interdigitale had good growth on all terbinafine-containing plates. Two strains (20%) of A. quadrifidum grew well on the 0.05 µg/ml plates. One A. quadrifidum isolate did not grow on the stronger concentrations and the other showed a reduction in colony count and size. These results indicate that this method may be useful in screening for resistance, giving quick information to clinicians that could guide treatment. It could also enable cost-effective testing of a large number of isolates, enabling the identification of resistant strains for further study. Routine screening would also establish data regarding resistance in regions under study. This screening assay could also be adapted for the testing of other antifungals.