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 <i>Trichophyton rubrum</i>, members of the <i>T. mentagrophytes</i> complex, and emerging pathogens in this infection, <i>T. schoenleinii</i> and <i>T. tonsurans</i>. Tea tree (<i>Melaleuca alternifolia</i> 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 <i>in vitro</i> study using EUCAST-AFST protocols to obtain the values of MIC (minimum inhibitory concentration) and MFC (minimum fungicidal concentration) of several commercial <i>M. alternifolia</i> 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 <i>T. rubrum</i>, which had an MIC value more than 13 times lower than the value obtained for <i>T. schoenleinii</i> (0.4% <i>v</i>/<i>v</i>), the most resistant isolate. No differences in antifungal activity were observed by the analysed EOs or between the MIC and MFC values. These <i>in vitro</i> 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.