Commercially available yeast identification systems
المؤلف:
Patricia M. Tille, PhD, MLS(ASCP)
المصدر:
Bailey & Scotts Diagnostic Microbiology
الجزء والصفحة:
13th Edition , p779-780
2025-12-11
66
Commercially available yeast identification systems have provided laboratories of all sizes with standardized identification methods. For the most part, the methods are rapid, providing results within 24 to 72 hours. The major advantage is that the systems provide an identification based on a database of thousands of yeast biotypes that considers a number of variations and substrate utilization patterns. Another advantage is that manufacturers of these products provide computer consultation services to help the laboratorian identify isolates that produce an atypical result. Although these systems are powerful tools, they should not be used as the sole method of identification; traditionally, they are most effectively used in conjunction with yeast morphology on blood, chocolate, or cornmeal agar.
API-20C AUX YEAST SYSTEM
The API-20C AUX yeast identification system has perhaps the most extensive computer-based data set of all commercial systems available. The system consists of a strip that contains 20 microcupules, 19 of which contain dehydrated substrates for determining the utilization profiles of yeasts. Reactions are compared with growth (turbidity) in the first cupule, which lacks a carbohydrate substrate. Reactions are read and results are converted to a seven-digit biotype profile number. Most of the yeasts are identified within 48 hours; however, some Cryptococcus and Trichosporon spp. may require up to 72 hours. The API-20C AUX yeast identification system, as well as all other com mercially available products, requires that the microscopic morphologic features of yeast grown on cornmeal agar containing 1% Tween 80 and trypan blue be used in conjunction with the substrate utilization patterns. This is particularly helpful when more than one possibility for an identification is provided; the microscopic morphologic features can be used to distinguish between the possibilities given by the profile register.
Several evaluations of the API-20C AUX yeast identification system have been performed, and the results have all been favorable. This system is limited in that it cannot identify unusual species; however, most of those seen in the clinical laboratory are accurately identified to the species level, especially the non-germ tube forming Candida spp. When this system is used to differentiate C. albicans from C. dubliniensis, assimilation results for xylose and alpha-methyl-d-glucoside may help distinguish between the two species. The results are negative for C. dubliniensis in 100% and 95% of the strains and positive for C. albicans in 100% and 95% of the strains.
MICROSCAN YEAST IDENTIFICATION PANEL
The MicroScan Yeast Identification Panel (Siemens, Deer field, Illinois) is a 96-well, microtiter plate containing 27 dehydrated substrates. It was introduced as an alternative to the API-20C AUX yeast identification system. It uses chromogenic substrates to assess specific enzyme activity, which can be detected within 4 hours. Specific enzyme profiles have been generated for many of the yeasts commonly encountered in the clinical microbiology laboratory. The most recent evaluation of the method showed that it was moderately accurate within 4 hours using no supplementary tests. When supplementary tests were used, the sensitivity was excellent compared with that of the API-20C AUX yeast identification system. Accuracy for identification of common yeasts was high, and uncommon yeasts were identified in most instances.
VITEK BIOCHEMICAL CARDS
The Yeast Biochemical Card (bioMérieux, Durham, North Carolina) is a 30-well, disposable plastic card that contains conventional biochemical tests and negative controls. It is used with the automated Vitek II system (BioMérieux, Durham, North Carolina), which is used for bacterial identification and susceptibility testing in many laboratories. The most recent evaluation of this system showed an overall accuracy of identification near 100% compared with API-20C AUX. Fewer than one fourth of the yeasts required supplemental biochemical or morphologic features to confirm their identification. Of all correctly identified yeasts, more than half were reported after 24 hours of incubation. The accuracy of identification of common and uncommon species was satisfactory. Identifying germ tube–positive yeasts is not necessary with this system. For laboratories already using this system, accurate and reliable identification of most commonly encountered yeasts can be accomplished.
Interest in commercially available yeast identification systems has taken precedence over the more cumber some, labor-intensive conventional yeast identification methods. Currently the rapid identification methods are financially feasible and provide laboratories of all sizes with the capability to identify yeasts. Commercially available systems, which provide accurate and rapid identification of yeasts and yeastlike organisms, are recommended for all laboratories. In general, they are easy to use, easy to interpret, and relatively inexpensive compared with conventional methods. In most cases they are faster than conventional systems, provide more standardized results, and require less technical skill to perform. As with any system, uncommon identifications should be scrutinized to prevent misidentifications.
CHROMAGAR CANDIDA
CHROMagar is a differential medium useful for the recovery of Candida organisms in clinical specimens, differentiation of Candida spp., and isolation of colonies. Distinct enzymes of different Candida spp. react with chromogenic substrates to yield a characteristic colony color. When used with colonial morphologic features, this system can provide a presumptive identification. Sand-Millan et al. reported an evaluation of 1537 isolates of yeast, which after 48 hours of incubation at 37°C showed that CHROMagar had a sensitivity and specificity near 100% for C. albicans, C. tropicalis, and C. krusei. Another evaluation by Pfaller et al. showed that more than 95% of stock and clinical isolates of C. albicans, C. tropicalis, and C. krusei were correctly identified. A similar sensitivity was observed for C. glabrata. CHROMagar also was evaluated as a recovery medium and was found to detect mixed cultures of Candida spp. Considering that the previously mentioned species account for approximately 90% of the yeast recovered in the clinical laboratory, CHROMagar appears to be a suitable alternative to other yeast identification systems.
MATRIX-ASSISTED LASER DESORPTION IONIZATION TIME-OF-FLIGHT (MALDI-TOF)
Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) is emerging as a potential rapid technique for the identification of yeast and fungal isolates within the microbiology laboratory. MALDI-TOF requires that isolates be cultured overnight and then processed using a standardized extraction procedure. Several studies report the correct identification of yeast to the species level up to approximately 98% in comparison to traditional culture methods. In addition, some laboratories have required the direct isolation of organisms from blood culture systems without subculturing to another medium to facilitate identification. Evidence suggests that MALDI-TOF is able to resolve species discrepancies more accurately than traditional culture methods and will reduce the time needed to identify an organism significantly.
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