The morphological identification of pathogenic yeasts using carbohydrate media.

Eight isolates of C. albicans were used to determine the frequency with which germ tube formation occurred: on rice extract -Tween 80 agar, on its components, and on 1% bactopeptone agar after three hr at 37 degrees C; in 0.5% aqueous solution of various carbohydrates; in various concentrations of glucose; on 0.5 and 0.1% glucose agar and on various types of agar alone. Subsequently 250 isolates of yeast of the genera Candida, Torulopsis, Trichosporon, Cryptococcus, and Saccharomyces, which were obtained from a clinical laboratory, were spread on rice extract -Tween 80 agar and on 0.1% glucose agar and covered with coverslips. Direct microscopic examination after incubation for three hours at 37 degrees C demonstrated germ tube formation by all 140 isolates of C. albicans, but by none of the other yeasts. The characteristic features of the pseudomycelia of isolates of Candida and Trichosporon were evident on reexamination after a further 45 to 69 hours at room temperature (22 degrees C). These morphological observations suggested the identity of the isolates of Torulopsis, Cryptococcus, and Saccharomyces but identified virtually all (98.2%) of those of the genera which formed pseudomycelia. Of the latter group only four isolates required fermentation and assimilation tests to determine whether they were C. parapsilosis (1) or C. guilliermondii (3).