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

Abstract

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 i0-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.Antifungal imidazole-containing drugs can inhibit biosynthesis of essential fungal cell membrane components (10,12,(15)(16)(17) and exert direct physicochemical cell membrane damage unrelated to any metabolic event (5,8, 11,14,18). Each effect has been offered as the primary mechanism by which imidazoles disrupt membrane permeability. A possible reconciliation of these hypotheses is seen in some recent studies with Saccharomyces cerevisiae (12, 13). Low levels of miconazole (MCZ) inhibited synthesis of membrane components, and this was correlated with fungistatic activity. At relatively high concentrations the drug exerted direct membrane damage and was fungicidal. However, in parallel studies with ketoconazole (KCZ), the most promising of the newer imidazoles, a striking difference was noted (13). Although KCZ inhibited synthesis of membrane components and was fungistatic, it failed to show a capacity for direct membrane damage. Thus, at relatively high concentrations, MCZ exerted much greater antifungal activity than KCZ. From recent experiments with strains of Candida albicans and Candida parapsilosis, we obtained results supporting the existence of a basic difference between the drugs with respect to direct membrane damage potential (2, 3). However, we were unable to show that MCZ is more active than an equimolar concentration of KCZ against growing cultures (1, 3). In view of studies showing that in stationary phase C. albicans develops phenotypic resistance to the lethal action of MCZ (5, 9), the physiological state of our inoculum cells might explain the apparent inability of MCZ to cause direct membrane damage and death of C. albicans in a growth environment (1). This question was examined. MATE...

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