Conventional Yeast identification methods
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p780-781
2025-12-11
56
A few laboratories still prefer to use conventional methods to identify yeasts. Regardless of the type of identification system used, the germ tube test often is the first step in screening a large number of isolates, unless another screening test for C. albicans has been used (e.g., PNA FISH). As previously mentioned, approximately 75% of yeasts recovered in the clinical laboratory can be identified using the germ tube test.
CORNMEAL AGAR MORPHOLOGY
The second major step in this practical identification schema is to use cornmeal agar morphology as a means to determine whether the yeast produces blastoconidia, arthroconidia, pseudohyphae, true hyphae, and/or chlamydoconidia. Cornmeal agar morphology can be used to detect characteristic chlamydoconidia produced by C. albicans. This method currently is satisfactory for definitive identification of C. albicans when the germ tube test is negative. In other instances, microscopic morphologic features on cornmeal agar help differentiate the genera Cryptococcus, Saccharomyces, Candida, Geotrichum, and Trichosporon. Previously, it was believed that the morphologic features of the common Candida spp. were distinct enough to provide a presumptive identification. This can be accomplished for C. albicans, C. glabrata, C. krusei, C. parapsilosis, C. tropicalis, and C. kefyr, keeping in mind that numerous other species, uncommonly recovered in the clinical laboratory, might resemble microscopically any of the previously mentioned species. In general, this method performs well, because the previously mentioned genera and species are more commonly seen in clinical laboratories.
Cornmeal agar morphology has less value for uncommonly encountered isolates. It should be used as an adjunct test with most commercially available yeast identification systems (e.g., to differentiate C. albicans from C. dubliniensis). It aids the differentiation of yeasts that yield similar biochemical profiles and helps prevent mis identifications, particularly of less commonly encountered species that may not be well represented in the commercial database.
CARBOHYDRATE UTILIZATION
Carbohydrate utilization patterns are the most commonly used conventional methods for definitive identification of yeasts recovered in a clinical laboratory. Various methods have been advocated for use in determining carbohydrate utilization patterns by clinically important yeasts, and all work equally well. Procedure 1, which can be found on the Evolve site, outlines the method previously found to be most useful by the Mayo Clinic Mycology Laboratory; however, this method is not commonly used in clinical laboratories. Most use commercially available methods.
PROCEDURE 1
Once the carbohydrate utilization profile has been obtained, reactions may be compared with those listed in tables in most mycology laboratory manuals. In most instances, carbohydrate utilization tests provide definitive identification of an organism, and additional tests are unnecessary. Some laboratories prefer carbohydrate fermentation tests, which are simply performed using purple broth containing different carbohydrate substrates. In general, carbohydrate fermentation tests are unnecessary and are not recommended for routine use.
PHENOLOXIDASE DETECTION USING NIGER SEED AGAR
Use of a simplified Guizotia abyssinica (niger seed) medium is a definitive method for detecting phenoloxidase production by yeasts. Most isolates of C. neoformans readily produce phenoloxidase; however, some do not. In addition, in some instances, cultures of C. neoformans have been shown to contain both phenoloxidase-producing and phenoloxidase-deficient colonies in the same culture.
If conventional methods are used, all criteria, including urease production, carbohydrate utilization, and the phenoloxidase test, must be met before a final identification of C. neoformans is made.
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