Multicenter study reveals alarming terbinafine resistance in Trichophyton indotineae isolates from Iran

Trichophyton indotineae has recently emerged as a major etiological agent of treatment-resistant dermatophytosis, particularly in India and Iran. Resistance to terbinafine is predominantly associated with point mutations in the squalene epoxidase (SQLE) gene. This study investigated the antifungal susceptibility of clinical dermatophyte isolates from Iran and characterized SQLE mutations in terbinafine-resistant strains. Over a 6-month period, 176 clinical dermatophyte isolates comprising 134 T. indotineae and 42 non-indotineae species were collected and identified by internal transcribed spacer (ITS) sequencing. In vitro antifungal susceptibility testing to eight antifungal agents was assessed according to CLSI M38 guidelines. Minimum inhibitory concentration (MIC) ranges, MIC₅₀, MIC₉₀, and geometric mean MICs were calculated. SQLE gene sequencing was performed for isolates exhibiting reduced terbinafine susceptibility. Out of 176 dermatophyte isolates, 66 (37.5%) demonstrated terbinafine MICs ≥ 0.5 µg/ml. Among these 66 isolates, 64 were T. indotineae (out of 134 total T. indotineae isolates, 47.76%), and 2 were T. tonsurans (out of 21 total T. tonsurans isolates, 9.52%). High-level terbinafine resistance (MIC ≥ 16 µg/ml) was observed in 50% (32/64) of T. indotineae isolates. Among the 66 resistant isolates, only 3 were wild-type: one T. indotineae and two T. tonsurans. The most prevalent SQLE alterations were the double mutation Phe397Leu + Ala448Thr (34.38%) and the single mutation Leu393Ser (32.81%), followed by the 1192 C→A substitution (10.94%). Notably, Ala448Thr also occurred in phenotypically susceptible isolates, yet correlated with increased MICs of itraconazole and voriconazole. Fluconazole showed poor activity (geometric mean MIC: 43.5072 µg/ml), whereas posaconazole exhibited the highest in vitro activity. Terbinafine remained effective for non-T. indotineae species, but is ineffective for T. indotineae-associated dermatophytosis due to widespread resistance. None of the isolates identified as T. interdigitale/mentagrophytes, T. rubrum, Microsporum canis, and Trichophyton benhamiae were resistant to TRB. This multicenter survey reveals high terbinafine resistance in Iranian T. indotineae isolates, linked to SQLE mutations and azole cross-resistance. Routine antifungal susceptibility testing, molecular screening, and alternative therapies are essential, prioritizing non-terbinafine options for T. indotineae (while it remains effective against other dermatophytes). Continued surveillance, accurate species identification, and tailored management are critical to control recalcitrant dermatophytosis.