Are Essential Oils Effective Against Human Skin Yeasts? Experimental Evidence from In Vitro Assays

Skin-associated yeasts are involved in several skin disorders, while increasing resistance to conventional antifungal agents highlights the need for alternative therapeutic approaches. Essential oils (EOs) have attracted increasing scientific interest because of their broad antimicrobial properties and potential as plant-based antifungal agents. However, their activity against clinically relevant skin-associated yeasts, particularly Malassezia spp., remains insufficiently explored. The antifungal activity of six EOs—rosemary, eucalyptus, lavender, basil, sage, and thyme—was evaluated against Candida albicans (C. albicans), Candida tropicalis (C. tropicalis), and Malassezia furfur (M. furfur). Antifungal activity was assessed using disk diffusion, minimum inhibitory and fungicidal concentrations (MIC and MFC), growth inhibition curves, and biofilm formation assays. The effects of selected EOs on yeast viability and membrane integrity were further investigated by flow cytometry and fluorescence microscopy. All EOs exhibited antifungal activity, although their efficacy varied according to both EO and yeast species. Thyme EO consistently showed the strongest activity, producing the largest inhibition zones and the lowest MIC values (0.539–1.404 mg/mL), together with pronounced fungicidal and antibiofilm effects. Flow cytometry further confirmed its strong antifungal activity through extensive membrane damage. C. albicans was the most resistant yeast, whereas C. tropicalis was the most susceptible. All EOs also exhibited antifungal activity against M. furfur. Fluorescence microscopy provided visual evidence of viable M. furfur cells despite the absence of thiazole orange fluorescence observed by flow cytometry. Overall, the findings of this study support the potential of EOs, particularly thyme EO, as promising plant-based candidates for the management of skin-associated yeast infections.