Antifungal Activity of Tea Tree (Melaleuca alternifolia Cheel) Essential Oils against the Main Onychomycosis-Causing Dermatophytes

Onychomycosis is a common fungal infection that affects the nails and accounts for approximately 50% of all nail diseases. The main pathogens involved include dermatophytes, such as Trichophyton rubrum, members of the T. mentagrophytes complex, and emerging pathogens in this infection, T. schoenleinii and T. tonsurans. Tea tree (Melaleuca alternifolia Cheel) essential oil (EO) has been proposed as a promising natural alternative to traditional treatments due to its antimicrobial properties. Among its more than 100 compounds, terpinen-4-ol is one of the main contributors to the antifungal action of this EO. To determine the antifungal activity of tea tree EO against dermatophytes, we designed an in vitro study using EUCAST-AFST protocols to obtain the values of MIC (minimum inhibitory concentration) and MFC (minimum fungicidal concentration) of several commercial M. alternifolia Cheel EOs against three species of dermatophytes isolated from clinical samples with suspected toenail onychomycosis. The results showed that the microorganism most sensitive to the action of the EO was T. rubrum, which had an MIC value more than 13 times lower than the value obtained for T. schoenleinii (0.4% v/v), the most resistant isolate. No differences in antifungal activity were observed by the analysed EOs or between the MIC and MFC values. These in vitro results suggest that tea tree EO is a viable option for the alternative treatment of onychomycosis, although clinical studies are needed to confirm the long-term antifungal activity, safety and efficacy of the oils studied in a clinical context.