Turbidimetric studies of growth inhibition of yeasts with three drugs: inquiry into inoculum-dependent susceptibility testing, time of onset of drug effect, and implications for current and newer methods.

Susceptibility testing with the broth-dilution visual end-point method is inoculum dependent with miconazole and 5-fluorocystosine, but not with amphotericin B. Turbidimetric measurements of yeast growth in the presence of antifungal drugs were therefore performed. With miconazole and 5-fluorocytosine, over the range of concentrations studied, growth occurred until a plateau phase was reached. With drug present prior to log phase growth, a delayed onset of effect was noted which was proportional to the generation time of the organism. With amphotericin B, in contrast, there was sharp transition with increasing drug concentration from no inhibition to complete arrest of growth, and no relation of onset of effect to generation time. These findings provide a possible explanation of inoculum dependence; i.e., at higher inocula, partially inhibited but growing yeasts become visible at higher drug concentrations. Supporting evidence derives from observations with different starting inocula, varying time of reading end points, and other methods of studying growth inhibition. The delay in miconazole and 5-fluorocytosine effect described suggests that rapid methods of susceptibility testing should be utilized with cultures already in log phase.