Use of an oxidation-fermentation medium in the identification of yeasts.

FOR MANY YEARS, differentiating Candida albicans and Cryptococcus neoformanns from other yeasts has been considered adequate characterization of clinical specimens. It has become increasingly obvious in recent years, however, that other species of yeasts are also implicated in pathological conditions and that a more definitive characterization of yeasts as etiological agents is necessary. The identification of yeasts requires consideration of both morphological and physiological characteristics. The physiological characterization is achieved by differentiating the organisms according to the activity of urease, the assimilation of nitrate, and the fermentations and assimilations of carbohydrates. The assimilation of carbohydrates is determined by the classic Wickerham broth procedure (1) or the agar auxanogram procedures. It is generally conceded that the Wickerham procedure is more sensitive than the comparable agar auxanogram techniques. Preliminary studies in the laboratory of the Proficiency Testing Section have revealed that the differential carbohydrate assimilation patterns achieved by the Wickerham broth technique can be reproduced identically in an oxidation-fermentation (O-F) medium incubated under aerobic conditions at 250C. In contrast to the classic assimilation tests, growth from the pure culture of yeasts was stabbed into the semisolid O-F medium with a straight wire without the requirement for starving the inoculum or tedious dilution and adjustment procedures. Inoculation of the carbohydrate-free basal medium failed to show that acid production from a carryover of a carbohydrate substrate in the inoculum was a problem.