Synergistic effect of Tea Tree Oil and Amphotericin B against Candida albicans: an alternative treatment

Candidosis is the most important cause of fungal infections in humans. The yeast Candida albicans can form biofilms, and it is known that microbial biofilms play an important role in human diseases being very difficult to treat. The prolonged treatment with drugs has often resulted in failure and resistance. Due to the emergence of multidrug resistance, alternatives to conventional antimicrobial therapy are needed. The synergistic or additive effect produced by the combination between essential oils and antimicrobial drugs was referred as a strategy for counteracting microbial development (Wagner and Ulrich-Merzenich, 2009). In this work a possible synergistic anti-Candida effect of of Melaleuca alternifolia (tea tree oil, TTO) oils of two different batches (TTO_1 and TTO_2) and amphotericin B towards Candida albicans has been studied with the aim of finding minimum effective dose that maintain antifungal activity but minimize amphotericin side effects and down the onset of microbial resistance. First the minimum inhibitory concentrations (MIC) of the individual oils was determined by using the microdilution method following CLSI guidelines. The checkerboard assay was carried out as described by Lorian (2005) with slight modifications in order to evaluate the synergy between TTOs and amphotericin B (AppliChem GmbH, Germany) against C. albicans ATCC 48274: every possible combinations of seven serial two fold dilution of TTO (from 0,06% v/v to 4% v/v) and antibiotic (from 0,125μg/ml to 4 μg/ml) were tested. All experiments were conducted in triplicate. The components of the TTO oils have been previously characterized by chromatographic analyses. The results obtained showed the occurrence of pronounced fungicidal synergism [FFC (fractional fungicidal concentration) = 0.32 for TTO_1 and FFC= 0.28 for TTO_2] in which a low concentration of TTO (0.06% v/v) enhances the action of amphotericin B at a lower dose (0.125 μg/ml) compared to amphotericin B used alone (1μg/ml) against C. albicans. The in vitro data of the present study suggest the possibility to reduce the effective dose of antibiotic for treatment in the clinical setting decreasing the adverse effects of antibiotics. Further studies are needed to evaluate the toxicology of single components and their combination and to investigate the efficacy of this combination in animal models. Wagner H., Ulrich-Merzenich G. (2009) Synergy research: approaching a new generation of phytopharmaceuticals. Phytomedicine. 16:97-110. Lorian, V. (2005). Antibiotics in laboratory medicine 5th ed, Lippincott Williams & Wilkins, ISBN 978-078-1749-83-1, Philadelphia, US