In vitro antifungal activity of eucalyptol and its interaction with antifungal drugs against clinical dermatophyte isolates including Trichophyton indotineae

Dermatophytosis, a prevalent fungal infection, often exhibits treatment failure. It poses ongoing public health concerns, urging exploration of alternative treatment strategies. This study examines eucalyptol’s in vitro activity and its interaction with antifungal agents against dermatophyte isolates. Overall, 489 patients clinically suspected of dermatophytosis were investigated, and the causative agents were molecularly identified. The antifungal activity of eucalyptol, itraconazole, terbinafine, and griseofulvin was assessed according to the guideline of the Clinical and Laboratory Standards Institute (CLSI M38 ed3). The interaction between eucalyptol and the aforementioned antifungals was determined using a checkerboard method. Dermatophytosis was confirmed in 30 out of 489 (6.13%) patients, with a female-to-male ratio of 3:2 and an age range of 8–67 years. The most commonly observed clinical manifestation was tinea corporis (34.21%), and Trichophyton indotineae (n = 14, 46%) was the most common causative agent. Antifungal susceptibility testing revealed that eucalyptol exhibited antidermatophyte properties with minimum inhibitory concentrations (MIC) ranging from 0.78 to 25 mg/mL. Itraconazole demonstrated the lowest geometric mean (GM) value (MIC range: 0.0019–0.25 µg/mL, GM: 0.015 µg/mL), while griseofulvin exhibited the highest GM value (MIC range: 0.125–8 µg/mL, GM: 2.37 µg/mL). The in vitro interaction of eucalyptol with antifungal drugs, except for its combination with terbinafine against two Trichophyton tonsurans isolates resulting in synergistic effects, showed indifference (n = 70, 77.77%) and antagonistic types (n = 18, 20%). Among the evaluated antifungals, itraconazole demonstrated the highest effectiveness against clinical isolates, while eucalyptol alone exhibited a more pronounced effect than when combined with antifungal agents.