The Synergetic Efficacy of the Combination of Amphotericin B and Certain Essential Oils against Selected Fungal Clinical Isolates

The synergistic effect of five potential essential oils from each of the following plant species: Thymus vulgaris, Lamiaceae, Cinnamomum zeylanicum, Lauraceae, Eugenia aromatica, Myrtaceae, Eucalyptus globulus, Myrtaceae and Zingiber officinalis, Zingiberaceae was assessed in combination with the antifungal agent amphotericin B. The oils were analysed and their main components identified using GC/FID and GC/MS. The antifungal activities of the oils were investigated against two clinical isolates of the fungal species; Aspergillus niger and Candida albicans. The minimum inhibitory concentrations of the individual oils were evaluated using the agar disc diffusion assay while the synergetic effects between each of the oils and amphotericin B were evaluated using the agar dilution checkerboard micro titer test. The results indicated significant synergetic effects between thyme oil and amphotericin B against C. albicans and A. niger with FIC indices of 0.25 and 0.13 respectively. Moreover, cinnamon oil acted synergistically with amphotericin B against C. albicans with an FICI value of 0.28, however when tested against A. niger, an additive effect was observed (FICI = 1). The combination of thyme oil and to a less extent, cinnamon oil with amphotericin B is suggested for the treatment of infections caused by C. albicans and A. niger.