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Journal of Clinical Microbiology, January 2006, p. 227-228, Vol. 44, No. 1
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.1.227-228.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Comparative Study of the New Colorimetric VITEK 2 Yeast Identification Card versus the Older Fluorometric Card and of CHROMagar Candida as a Source Medium with the New Card

C. L. Aubertine, M. Rivera, S. M. Rohan, and D. H. Larone^*

Weill Cornell Medical Center/New York Presbyterian Hospital, Department of Pathology and Laboratory Medicine, 525 East 68th Street, New York, New York 10021

Received 22 June 2005/ Returned for modification 29 July 2005/ Accepted 12 October 2005

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The new VITEK 2 colorimetric card was compared to the previous fluorometric card for identification of yeast. API 20C was considered the "gold standard." The new card consistently performed better than the older card. Isolates from CHROMagar Candida plates were identified equally as well as those from Sabouraud dextrose agar.

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The identification of clinically encountered yeasts by using conventional methods is time-consuming, labor-intensive, and expensive (4). Commercial automated identification systems have been developed, are currently on the market, and are in use in clinical microbiology laboratories. These systems allow for accurate and more rapid identification of medically relevant bacteria and yeasts, improving the quality and cost effectiveness of patient care (1, 4, 6). The VITEK 2 system has been shown to be a reliable fully automated instrument for the identification of microorganisms, including yeasts using the ID-YST cards (1). bioMerieux (Marcy l'Etoile, France) has recently developed a new VITEK 2 YST card (NC) using a colorimetric system to replace the older fluorometric VITEK 2 ID-YST card (OC) in order to broaden the VITEK 2 database and improve the quality of the system in the identification of medically relevant yeasts. CHROMagar Candida (CAC; Becton Dickinson, Sparks, MD) is increasingly being used in clinical laboratories as a primary yeast isolation medium, but it is not included by the VITEK 2 manufacturer as a source medium for yeast to be tested on the NC (5). The aim of our study was twofold: (i) to assess the ability of the NC, compared to the OC, to identify a variety of yeast isolates and (ii) to evaluate CAC as a source medium from which isolates may be taken for testing on the NC. This study was not meant to be a definitive inspection of the entire range of the NC (as was performed for the FDA-approval study [unpublished]) but a real-time determination of whether or not users (like us) of the OC could expect to see improved results with the NC.

The OC consists of 64 wells with 46 fluorescent biochemical tests with the final results generated in approximately 15 h. Its database contains 46 species across 11 genera. The NC also consists of 64 wells, but the 46 biochemical tests are colorimetric, with results available in approximately 18 h. It has an expanded database of 53 species and 14 genera.

A total of 97 yeast isolates, including 24 species of six genera from clinical specimens and stock cultures, were subcultured to Sabouraud dextrose agar (SDA; Becton Dickinson) and incubated at 30°C for 48 h. Each isolate was subsequently tested for identification to genus and species levels with both the OC and NC in accordance with manufacturer's instructions, and the results were compared. Additionally, 46 of these isolates, including 23 species of six genera were grown on CAC under the same conditions and tested on the NC. The large majority of these were species that could not be identified by color or texture on CAC. The API 20C AUX test (bioMerieux) was performed on all 97 isolates and used as a "gold standard" for accuracy of identification. The results from the two systems were obtained and assigned to one of four categories: (i) correctly identified, (ii) identified with low discrimination, (iii) not identified, and (iv) misidentified.

As seen in Table 1, by using isolates from SDA with the OC, three isolates of Candida were misidentified and five were unidentified; one Cryptococcus isolate was misidentified and one was unidentified; and one Trichosporon isolate was unidentified. All of the Geotrichum, Rhodotorula, and Saccharomyces species were correctly identified. By use of the NC with isolates from SDA, only one species of Candida was misidentified. Candida glabrata was identified as Candida sphaerica, an extremely rare organism whose identification would require confirmation; it was unidentified on the OC. All of the remaining isolates of the various genera were correctly identified, albeit some with low discrimination. Overall, the OC correctly identified 88.7% of the isolates, including 5.2% yielding a correct result with low discrimination. In contrast, the NC correctly identified 99.0% of the isolates, including 4.1% yielding the correct result with low discrimination, requiring supplemental testing for a definitive identification, including testing growth at 42 to 45°C, the presence or lack of well-formed pseudohyphae on cornmeal-Tween 80 agar, and KNO₃ assimilation (2, 3).

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TABLE 1. Comparison of isolates using the older fluorometric VITEK 2 YST card versus the new colorimetric card

The identification of all Cryptococcus, Geotrichum, Saccharomyces, and Trichosporon spp. grown on CAC using the NC were categorized as correctly identified. Five isolates from CAC identified as Candida guilliermondii (two isolates), Candida dubliniensis (two isolates), and Rhodotorula glutinis (one isolate) by API 20C showed low discrimination on the NC, and easy-to-perform supplementary tests such as those mentioned above were required for definitive identification. The NC results from CAC therefore yielded 100% overall correct identification (encompassing 10.8% with low discrimination); no isolates were misidentified or unidentified.

The results indicate that the new colorimetric methodology consistently performed better than the older fluorometric card system in the identification of yeast cultured on SDA. Organisms cultured on CAC were identified equally as well on the new colorimetric card as isolates from SDA. CAC should therefore be considered an acceptable source medium for isolates that are to be tested for identification on the new VITEK 2 colorimetric yeast card.

 
* Corresponding author. Mailing address: Clinical Microbiology—Starr 737, Weill Cornell Medical Center, 525 East 68th Street, New York, NY 10021. Phone: (212) 746-2405. Fax: (212) 746-8945. E-mail: dhlarone{at}med.cornell.edu.

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2. Kurtzman, C. P., and J. W. Fell. 2000. The yeast. A taxonomic study, 4th ed., p. 161-162, 308-309. Elsevier Science BV, Amsterdam, The Netherlands.
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3. Larone, D. H. 2002. Medically important fungi: a guide to identification, 4th ed. American Society for Microbiology, Washington, D.C.
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4. Lin, C. C. S., and D. Y. C. Fung. 1987. Conventional and rapid methods for yeast identification. Crit. Rev. Microbiol. 14:273-288.[Medline]
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5. Murray, M. P., R. Zinchuk, and D. H. Larone. 2005. CHROMagar Canidida as the sole primary medium for isolation of yeasts and as a source medium for the rapid-assimilation-of-trehalose test. J. Clin. Microbiol. 43:1210-1212.[Abstract/Free Full Text]
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6. Quadri, S. M., D. J. Flournoy, S. G. Quadri, and E. G. Ramirez. 1986. Rapid identification of yeasts by semi-automated and conventional methods. Med. Microbiol. Immunol. (Berlin) 175:307-316.[Medline]

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Journal of Clinical Microbiology, January 2006, p. 227-228, Vol. 44, No. 1
0095-1137/06/$08.00+0     doi:10.1128/JCM.44.1.227-228.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Aubertine, C. L. Articles by Larone, D. H. Search for Related Content PubMed PubMed Citation Articles by Aubertine, C. L. Articles by Larone, D. H.