The Effect of Fluconazole in Combination with Rifampin in vitro on Candida albicans Biofilms

Production of Candida albicans biofilms is one resistance mechanism of fluconazole in C. albicans and accordingly biofilm-related infections pose a great challenge to antifungal therapy. In this study, each of the three antibacterial agents of azithromycin, fosfomycin and rifampin, which could destroy bacteria biofilms, was respectively tested in combination with fluconazole to verify whether they had synergistic effects on C. albicans biofilms. The nature of drug interaction was interpreted by calculating fractional inhibitory concentration index (FICI) and percentages of growth difference (ΔE) from the results of checkerboard assay. In order to evaluate the potential of the three antibiotics on fluconazole penetrating the biofilms, a penetrating experiment was explored. In addition, fluorescein diacetate staining method was adopted to observe metabolic activity of the sessile cell. It was revealed that rifampin could work synergistically with fluconazole against the biofilms, while only negligible antifungal effects were observed for the other drug combinations. A potent synergistic interaction in vitro between fluconazole and rifampin against the biofilms by a fluconazole-resistant strain was demonstrated by a FICI value of 0.035 and synergistic high-interaction percentages of 1256% as interpreted by the ΔE model. Results of the penetrating experiment indicated that fluconazole combined with rifampin markedly enhanced inhibitory zone of an indicative organism. Fluorescence micrographs showed that the addition of rifampin rendered the cellular metabolism largely reduced. To sum up, rifampin could sensibilize fluconazole’ antifungal effect on C. albicans biofilms and the synergistic mechanism needs further investigating.