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

Abstract

The ability of the rapid, computerized Microbial Identification System (MIS; Microbial ID, Inc.) to identify a variety of clinical isolates of yeast species was compared to the abilities of a combination of tests including the Yeast Biochemical Card (bioMerieux Vitek), determination of microscopic morphology on cornmeal agar with Tween 80, and when necessary, conventional biochemical tests and/or the API 20C Aux system (bioMerieux Vitek) to identify the same yeast isolates. The MIS chromatographically analyzes cellular fatty acids and compares the results with the fatty acid profiles in its database. Yeast isolates were subcultured onto Sabouraud dextrose agar and were incubated at 28°C for 24 h. The resulting colonies were saponified, methylated, extracted, and chromatographically analyzed (by version 3.8 of the MIS YSTCLN database) according to the manufacturer’s instructions. Of 477 isolates of 23 species tested, 448 (94%) were given species names by the MIS and 29 (6%) were unidentified (specified as “no match” by the MIS). Of the 448 isolates given names by the MIS, only 335 (75%) of the identifications were correct to the species level. While the MIS correctly identified only 102 (82%) of 124 isolates of Candida glabrata, the predictive value of an MIS identification of unknown isolates asC. glabrata was 100% (102 of 102) because no isolates of other species were misidentified as C. glabrata. In contrast, while the MIS correctly identified 100% (15 of 15) of the isolates of Saccharomyces cerevisiae, the predictive value of an MIS identification of unknown isolates as S. cerevisiae was only 47% (15 of 32), because 17 isolates ofC. glabrata were misidentified as S. cerevisiae. The low predictive values for accuracy associated with MIS identifications for most of the remaining yeast species indicate that the procedure and/or database for the system need to be improved.

The Emergency Medical Service Microbiome

“…Culturing methods also have long turnaround times for results (48 to 96 h), limiting the time frame in which microbial contamination may be detected and corrective action can be taken (e.g., decontamination). Finally, extensive biochemical testing, microscopy, and use of selective media for culturing may still misidentify microbes (42) or provide only coarse-grained (genus-level) identification that may include both pathogenic and innocuous species. The development of alternative pathogen detection methods that provide more rapid (Ͻ8 h) and comprehensive analysis of resident microbes is therefore needed to effectively identify and mitigate EMS workplace risks.…”

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

“…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.)…”

“…Most clinical laboratories, however, buy their culture media prepoured from commercial suppliers. Of the two published studies concerning yeast identification using the MIS, one used yeasts cultured on prepoured BBL SDA (13). The commercial source of SDA for the other study (6) is unknown.…”

“…Isolates of most of the remaining species were identified by using the Yeast Biochemical Card (YBC; bioMerieux Vitek, Hazelwood, Mo.) (7-9, 18), which was inoculated and incubated according to the manufacturer's specifications as previously described (13). Fresh clinical isolates which could not be conclusively identified by using the combination of their microscopic morphology and the YBC were identified by using the API 20C Aux (bioMerieux Inc.) (9,18,20) and/or additional conventional tests as appropriate and as previously described (13).…”

“…Isolated colonies of each yeast to be studied were quadrant streaked onto two commercially obtained SDA plates (BBL and Remel). The cultures were incubated aerobically at 28 Ϯ 1°C for 24 Ϯ 2 h, and extracts were prepared and analyzed chromatographically as specified in the MIS instructions (22) and as previously described (13). Version 3.8 of the YSTCLN database in the MIS computer was used to identify isolates.…”