Rapid identification of Candida dubliniensis with commercial yeast identification systems

Candida dubliniensis is a newly described species that is closely related p hylogenetically to Candida albicans and that is commonly associated with or al candidiasis in human immunodeficiency virus-positive patients. Several r ecent studies have attempted to elucidate phenotypic and genotypic characte ristics of use in separating the two species. However, results obtained wit h simple phenotypic tests were too variable and tests that provided more de finitive data were too complex for routine use in the clinical laboratory s etting. The objective of this study was to determine if reproducible identi fication of C. dubliniensis could be obtained with commercial identificatio n kits. The substrate reactivity profiles of 80 C. dubliniensis isolates we re obtained by using the API 20C AUX, ID 32 C, RapID Yeast Plus, VITEK YBC, and VITEK 2 ID-YST systems. The percentages of C. dubliniensis isolates ca pable of assimilating or hydrolyzing each substrate were compared with the percentages from the C. albicans profiles in each kit's database, and the r esults were expressed as percent C. dubliniensis and percent C. albicans. A ny substrate that showed >50% difference in reactivity was considered usefu l in differentiating the species. In addition, assimilation of methyl-or-D- glucoside (MDG), D-trehalose (TRE), and D-xylose (XYL) by the same isolates was investigated by the traditional procedure of Wickerham and Burton (L. J. Wickerham and K. A. Burton, J. Bacteriol. 56:363-371, 1948). At 48 h (th e time recommended by the manufacturer for its new database), we found that the assimilation of four carbohydrates in the API 20C AUX system could be used to distinguish the species, i.e., glycerol (GLY; 88 and 14%), XYL (0 a nd 88%), MDG (0 and 85%), and TRE (15 and 97%). Similarly, results with the ID 32 C system at 48 h showed that Xn (0 and 98%), MDG (0 and 98%), lactat e (LAT; 0 and 96%), and TRE (30 and 96%) could be used to separate the two species. Phosphatase (PHS; 9 and 76%) and alpha-D-glucosidase (23 and 94%) proved to be the most useful for separation of the species in the RapID Yea st Plus system. While at 24 h the profiles obtained with the VITEK YBC syst em showed that MDG (10 and 95%), XYL (0 and 95%), and GLY (26 and 80%) coul d be used to separate the two species, at 48 h only Xn (6 and 95%) could be used to separate the two species. The most useful substrates in the VITEK 2 ID-YST system were TRE (1 and 89%), MDG (1 and 99%), LAT (4 and 98%), and PHS (83 and 1%). While the latter kit was not yet commercially available a t the time of the study, it would appear to be the most valuable for the id entification of C. dubliniensis. Although assimilation of MDG, TRE, and Xn proved to be the most useful for species differentiation by the majority of commercial systems, the results with these carbohydrates by the Wickerham and Burton procedure were essentially the same for both species, albeit fol lowing protracted incubation. Thus, it is the rapidity of the assimilation achieved with the commercial systems that allows the differentiation of C. dubliniensis from C. albicans.