Use of a microtiter plate assay to detect the rate of killing of adherent Candida albicans by antifungal agents

Objectives. Candida albicans may become adherent to prosthetic devices of various kinds and thereby produce infections that are difficult to treat with standard antifungal therapy. The objective of the present work was to study the effectiveness of antifungal agents against adherent C. albicans yeast cells. Study design. A microtiter plate assay was developed to assess the time required for killing of the fungal cells by three antifungal agents. Results. The assay initially was validated by demonstrating that the percentage of organisms adhering to the test wells was relatively constant and that exposure to the antifungal agents caused only minimal dislodgement of viable organisms from the plates. In studies that used this assay to determine the time required for killing the adherent yeast cells, chlorhexidine was found to be the most effective; in fact, in comparing the minimal lethal concentrations of the agents for exposures of 2 minutes versus 4 hours, a ratio of 2.9 was obtained for chlorhexidine versus 1050 for amphotericin B and 556 for nystatin. Conclusion. The microtiter plate assay used in these studies may therefore be useful as a screening test to determine which antifungal agents have the most rapid fungicidal effects on adherent fungal organisms.