Evaluation of the Microbial Identification System for identification of clinically isolated yeasts

Abstract

The Microbial Identification System (MIS; Microbial ID, Inc., Newark, Del.) was evaluated for the identification of 550 clinically isolated yeasts. The organisms evaluated were fresh clinical isolates identified by methods routinely used in our laboratory (API 20C and conventional methods) and included Candida albicans (n ‫؍‬ 294), C. glabrata (n ‫؍‬ 145), C. tropicalis (n ‫؍‬ 58), C. parapsilosis (n ‫؍‬ 33), and other yeasts (n ‫؍‬ 20). In preparation for fatty acid analysis, yeasts were inoculated onto Sabouraud dextrose agar and incubated at 28؇C for 24 h. Yeasts were harvested, saponified, derivatized, and extracted, and fatty acid analysis was performed according to the manufacturer's instructions. Fatty acid profiles were analyzed, and computer identifications were made with the Yeast Clinical Library (database version 3.8). Of the 550 isolates tested, 374 (68.0%) were correctly identified to the species level, with 87 (15.8%) being incorrectly identified and 89 (16.2%) giving no identification. Repeat testing of isolates giving no identification resulted in an additional 18 isolates being correctly identified. This gave the MIS an overall identification rate of 71.3%. The most frequently misidentified yeast was C. glabrata, which was identified as Saccharomyces cerevisiae 32.4% of the time. On the basis of these results, the MIS, with its current database, does not appear suitable for the routine identification of clinically important yeasts.

Limitations of the Current Microbial Identification System for Identification of Clinical Yeast Isolates

“…This recommendation is similar to suggestions made previously for other rapid yeast identification systems (6)(7)(8)16). The results of the current study confirm and extend the results of the earlier study of MIS yeast identification (5).…”

“…Only 70 to 71% of the yeast isolates investigated were correctly identified by the MIS during this and a previous study (5). It is of interest that of the isolates of C. glabrata that were misidentified by the GLC system in the current study, all 17 were called S. cerevisiae.…”

“…A previous study reported that the MIS correctly identified only 71% of the clinical isolates of 10 yeast species to the species level (5). In contemplating the use of the MIS (or any other system) for yeast identification, it is important not only to determine the ability of the system to identify multiple isolates of each species accurately but also to document the predictive values for the accuracies of the MIS identifications for each of the species.…”

Variation in Microbial Identification System Accuracy for Yeast Identification Depending on Commercial Source of Sabouraud Dextrose Agar

“…The results of the present study indicate that accurate MIS identification of yeasts to the species level is dependent, at least in part, on the commercial source of prepoured SDA medium that may be used to culture the organisms just prior to chromatographic analysis. The percentage of yeasts correctly identified with BBL SDA in the present study (74%) was similar to both our previous findings with BBL SDA (70% [13]) and the findings of another study (71% [6]) using SDA from an unspecified source, but it was significantly greater than that obtained when yeasts were cultured during the present study on SDA obtained from Remel (49%).…”

“…The MIS has provided an accurate, rapid, and cost-effective alternative for identification of many aerobic gram-positive and gram-negative bacterial isolates (2,12,16). Two recent reports, however, have noted that only 70 to 71% of yeast isolates were correctly identified by the MIS to the species level (6,13). The yeast database for the MIS was derived by the system's manufacturer from yeasts grown on BBL 11584 Sabouraud dextrose agar (SDA), which was purchased in the dehydrated form (Becton-Dickinson Microbiology Systems, Cockeysville, Md.)…”