Evaluation of the antifungal activity of selected essential oils on Aspergillus spp. and Candida spp.

Microorganisms have a remarkable ability to develop resistance to antimicrobial agents, threatening the effectiveness of drugs and becoming a serious public health problem. This is more evident for fungal infections that can therefore sometimes be difficult to treat. Currently, new trend in drug discovery to overcome this problem is represented by natural products from plants, or their extracts. Particularly, there is a great interest in essential oils (EOs) recognized for their antimicrobial role towards bacteria, fungi and viruses. In this study, we evaluated the antifungal activity of several commercial EOs (Flora, Italy) in comparison with fluconazole (FLC), itraconazole (ITZ), voriconazole (VRZ) and amphotericin B (AMB) against Candida krusei, C. valida, Aspergillus fumigatus and A. niger, clinical isolates that continue to increase as the population of immunocompromised patients increases. Commercial EOs of clove (Eugenia caryophyllata), geranium (Pelargonium asperum), oregano (Origanum vulgare), lavender (Lavandula vera) and thyme red (Thymus vulgaris) have been screened for antifungal activity against 6 A. fumigatus, 5 A. niger, 6 C. krusei and 6 C. valida strains. Antifungal activity was determined by broth microdilution method, and, for A. fumigatus and A. niger strains, also by aromatogram. FLC, VRC, ITZ and AMB were used as positive controls. Minimum Inhibitory Concentration (MIC) and the Minimum Fungicidal Concentration (MFC) of EOs and drugs were evaluated according to the CLSI, with some modifications for EOs. The final EOs concentrations ranged from 1-0.078% (v/v). All aspergilli tested were resistant to ITZ and/or AMB (breakpoint R>2µg/ml). All C. krusei strains were resistant to VRC, while on the contrary all C. valida strains were resistant to FLC. Geranium and clove EOs showed the highest antifungal activity against all tested strains with geranium’s MIC ranges of 0.06-0.25% (v/v) and clove’s MIC ranges of 0.125-0.25% (v/v). As regard C. valida and C. krusei, uncommon but increasing pathogen yeasts, thyme red showed the highest activity in vitro, with MIC ranges of 0.06-0.12% (v/v). MFC was 1-2 times higher than MIC, indicating a fungistatic action of the tested OEs especially against A. fumigatus strains. Aromatogram confirmed the antifungal activity of these two OEs, showing significant growth inhibition. The data obtained indicate a good antifungal activity of some OEs against candida and aspergillus strains resistant to conventional drugs. The results of this work support the research for new alternatives or complementary therapies based on EOs.