Comparisons of the susceptibilities of planktonic and adherent Candida albicans to antifungal agents: a modified XTT tetrazolium assay using synchronised C. albicans cells.

Adhesion of synchronised yeast-phase Candida albicans cells to tissue culture plastic, and the susceptibility of planktonic and adherent cells to antifungal agents, was investigated using a modified tetrazolium (XTT) assay. MIC data demonstrated that ketoconazole and amphotericin B were highly active against planktonic C. albicans yeast-phase cells. XTT tetrazolium assays permitted comparisons of MIC values with XTT formazan IC50 and IC80 (percentage inhibitory concentrations); IC50 and IC80 values for amphotericin B and ketoconazole were similar. Furthermore, IC50 and IC80 values for 24 h incubation with antifungal agent were typically higher than corresponding IC50 and IC80 values for 48 h incubation. Furthermore, in comparison to values for planktonic Candida cells, adherent cells were typically less susceptible to amphotericin B and ketoconazole. For example, with increasing incubation time following the initial adhesion period, cells became progressively less susceptible to amphotericin B and ketoconazole: 24 h (P < 0.05) and 48 h (P < 0.001). Furthermore, other azoles showed the same activities compared with ketoconazole against both planktonic and adherent cells. Overall, the data demonstrate the usefulness of the XTT tetrazolium assay in describing comparisons of the susceptibility profiles for both planktonic and adherent synchronous yeast phase C. albicans in vitro.