Rapid microdilution-colorimetric assay for yeast susceptibility to fluorocytosine

Acid production by certain yeast species through the fermentation of glucose was used as the basis of an in vitro test for measuring susceptibility of these organisms to 5-fluorocytosine. Serial dilutions of 5-fluorocytosine in yeast nitrogen base broth, with bromothymol blue indicator dye, were made on microtiter plates. A fixed-concentration suspension of yeast cells was added to successive wells of the plates, and the color change from blue to yellow, indicating generation of acid, was noted. Eighteen hours after inoculation the lowest concentration of 5-fluorocytosine that completely inhibited the production of acid was recorded as the minimum inhibitory concentration. The results were reproducible in multiple trials with organisms of the genera Candida, Torulopsis, and Saccharomyces. This test is a rapid, inexpensive alternative to current 48- to 72-h methods in which broth turbidity is used as the end point.