Growth phase in relation to ketoconazole and miconazole susceptibilities of Candida albicans

The antifungal imidazoles miconazole and ketoconazole inhibit synthesis of essential cell membrane components. Furthermore, miconazole can exert direct physicochemical cell membrane damage at relatively high levels, but ketoconazole cannot. Experiments were designed to explain our previous observation that concentrations of miconazole capable of causing direct membrane damage were no more active against Candida albicans than equimolar levels of ketoconazole. When stationary-phase cells were inoculated into medium containing either drug at 3.8 X 10(-5) M, fungistatic effects were indistinguishable. If, however, such cultures were incubated 3 h before drug addition, differences were remarkable. After 3 h, miconazole caused a 99% reduction in CFU per milliliter within 20 min, but ketoconazole again was only fungistatic. The immediate onset, rapidity, and magnitude of the miconazole effect were indicative of direct lethal cell damage. Miconazole concentrations as low as 1.0 X 10(-5) M were similarly active. It was concluded that C. albicans undergoes phenotypic changes during the growth cycle that coincidentally confer susceptibility or resistance to the lethal direct membrane damage effect of miconazole. The fungistatic or metabolic effects of ketoconazole or low-level miconazole appeared to be independent of growth phase.