Microplate Alamar blue assay for susceptibility testing of Candida albicans biofilms

Although biofilm-based fungal infections are an important cause of morbidity and mortality in patients, there is no standardized method for the in vitro evaluation of the drug susceptibility of biofilms. We investigated a high-throughput method for determining the susceptibility of Candida albicans biofilms that uses the oxidation reduction indicator Alamar blue (AB). Biofilms from the tested Candida albicans strains were markedly resistant to amphotericin B (AMB), nystatin (NYT), fluconazole (FLC) and 5-fluorouracil (5FC), but susceptible to Conflikt disinfectant. The latter was used in comparative studies of AB reduction with two other methods for assessing in vitro drug susceptibility i.e., 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide (XTT) reduction and enumeration of viable colony counts (CFU/ml). AB results correlated well with XTT (r = 0.88-0.93) and CFU/ml (r = 0.93-0.99) for all four C. albicans test strains. This simple, reproducible method for determining in vitro drug susceptibility should facilitate discovery of antifungals active against Candida biofilms.