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Journal of Clinical Microbiology, November 2008, p. 3784-3787, Vol. 46, No. 11
0095-1137/08/$08.00+0     doi:10.1128/JCM.01318-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Evaluation of New Colorimetric Vitek 2 Yeast Identification Card by Use of Different Source Media

Giuseppe Valenza,^* Jörn Strasen, Frauke Schäfer, Matthias Frosch, Oliver Kurzai, and Marianne Abele-Horn

University of Würzburg, Institute of Hygiene and Microbiology, Würzburg, Germany

Received 11 July 2008/ Returned for modification 25 August 2008/ Accepted 10 September 2008

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The new colorimetric Vitek 2 YST card was evaluated for identification of yeasts (136 strains) with respect to the influence of different source media. The Vitek 2 YST card achieved satisfactory results for all yeast species tested, with the exception of Candida guilliermondii, Candida norvegensis, Candida parapsilosis, Candida rugosa, and Candida tropicalis. After simple additional tests, 93.7% of all the strains tested were correctly identified. A significant influence of the isolation medium on the identification rate could not be observed.

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Rapid identification of yeasts provides timely information to physicians for patient management. This is particularly true due to the fact that various yeast species such as Candida glabrata, Candida krusei, Candida lusitaniae, and Candida parapsilosis are recognized to be inherently or potentially resistant to azoles, amphotericin B, or echinocandins (6). Identification of yeasts is mainly based on biochemical reaction patterns. The Vitek 2 system (bioMérieux, Marcy l'Etoile, France) first introduced a fluorometric and then a colorimetric card for the rapid identification of yeast species. The performance of both cards has been evaluated in several studies (1, 2, 5, 7, 8, 10) and was shown to be useful for species identification. However, all evaluations performed so far have been performed with yeasts grown on Sabouraud dextrose agar. In contrast, during routine diagnostics, yeasts are also commonly isolated on other media, including CHROMagar Candida (9). The Vitek 2 manufacturer recommends different source media, such as Sabouraud dextrose agar, Columbia agar with sheep blood, Columbia agar with horse blood, Trypticase soy agar, Trypticase soy agar with sheep blood, and yeast selective agar. Currently, the most common agar media used for routine diagnostics, chromogenic media and Sabouraud agar with antibiotics, are not included among the recommended media. Only one study has comparatively evaluated the performance of the new colorimetric Vitek 2 YST card using two different source media for a limited number of isolates (1). In this study, we evaluated the new colorimetric Vitek 2 YST card and systematically tested the influence of source media on the identification rate.

A total of 136 strains representing 16 yeast species were included in this study. All strains were clinical isolates from unrelated patients belonging to the clinical strain collection of the Institute of Hygiene and Microbiology of the University of Würzburg. The sources of the isolates included blood, stool, urine, and respiratory tract samples. The identification of the strains was performed using the API ID 32C kit (bioMérieux) or internal transcribed spacer (ITS) sequencing (3, 4) as a reference method. For the study, the strains were transferred from storage at –70°C onto Sabouraud agar (produced in-house) and subcultured after 24 h on the following eight different media: Sabouraud agar (produced in-house), Sabouraud agar with penicillin and streptomycin (produced in-house), Sabouraud agar with gentamicin and chloramphenicol (bioMérieux), Columbia agar with sheep blood from three manufacturers (bioMérieux; Becton Dickinson, Heidelberg, Germany; and Oxoid, Wesel, Germany), CAN2 chromogenic agar (bioMérieux), and CHROMagar Candida (Becton Dickinson). Once isolates were streaked onto the different test media, they were incubated for 24 h at 37°C. Subsequently, a yeast suspension made in 0.45% aqueous NaCl was adjusted to a McFarland standard of 2 with a Vitek 2 DensiCheck instrument. Each isolate, obtained from the eight different source media, was tested with the Vitek 2 system, using the new YST card, according to the instructions of the manufacturer. Data were analyzed using software provided by the manufacturer. The four categories for the results were as follows: (i) correct identification (unambiguous correct identification to the species level), (ii) low level of discrimination (either identification to the genus level or a low level of discrimination between two or more species, including the correct species), (iii) misidentification (the species identified with the YST card was different from that identified by the reference method), and (iv) no identification (strains not producing any results). The identification rates of the Vitek 2 YST card obtained using diverse source media were analyzed by Fisher's exact test as provided at http://www.langsrud.com/fisher.htm/.

The YST card performed a correct identification, independent of the isolation medium, in 909 cases (83.5%) without the need for extra assays; 115 tests (10.6%) resulted in low discrimination, 51 tests (4.7%) resulted in misidentification, and 13 tests (1.2%) gave no valid result (Table 1). The identification results were improved to 1,019 tests (93.7%) when simple additional assays were applied to clarify the results for strains identified with low discrimination (Table 2). With the exception of Candida guilliermondii (32.4%), C. parapsilosis (71.7%), and Candida tropicalis (79%), the YST card achieved satisfactory results for all common clinical yeasts, e.g., Candida albicans (97.9%), C. glabrata (98.8%), Candida dubliniensis (96.1%), C. krusei (92.2%), C. lusitaniae (89.6%), and Cryptococcus neoformans (95.3%). Through additional tests, identification rates could be increased, especially for C. guilliermondii (85.9%), C. parapsilosis (93.3%), and C. tropicalis (98.7%). Despite supplemental tests, less convincing results were achieved when rarer species, e.g., Candida pelliculosa (62.5%) and Candida rugosa (60%), were investigated.

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TABLE 1. Results obtained for the identification of yeast isolates by the Vitek 2 YST card independently of the source media

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TABLE 2. Additional tests for definitive identification of strains with low discrimination by the Vitek 2 YST carda

The highest identification rate could be achieved using the Columbia agar with sheep blood (Becton Dickinson and bioMérieux) (86.8%) and the lowest identification rate was obtained using CHROMagar (Becton Dickinson) (78.7%) as the source medium (Table 3). However, a significant influence of the source medium on the performance of the Vitek 2 YST card could not be shown for either the overall identification rate or the identification rate of a single species (Fisher's exact test, P > 0.1) (Table 3). Thus, the Vitek 2 YST card can be used for identification of yeasts grown on a wide range of source media without previous subcultivation. This novel finding will, in many cases, speed up the identification of clinically relevant yeasts.

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TABLE 3. Results obtained for the identification of yeast isolates by Vitek 2 YST carda

The results indicate that the Vitek 2 YST card is able to correctly identify the majority of the common clinical yeasts, e.g., C. albicans, C. glabrata, and C. krusei, without the use of supplemental tests but is weaker at identifying C. guilliermondii, Candida norvegensis, C. parapsilosis, C. rugosa, and C. tropicalis. The misidentification or delayed identification of the aforementioned species could have a significant clinical impact, since C. guilliermondii exhibits resistance to fluconazole, amphotericin B, and echinocandins, and C. parapsilosis has been suggested to be less responsive to echinocandins. The apparent lack of accuracy of Vitek 2 for rare yeast species has also been confirmed in a recent case report (11). Among the species most frequently isolated from blood cultures, a high level of accuracy (97.4% mean value) for the identification of C. albicans, C. glabrata, and C. krusei was achieved. Less satisfactory results for C. tropicalis (79%) and C. parapsilosis (71.7%) were obtained; through the use of supplemental tests, the identification rate could be increased to 98.7% and 93.3%, respectively. We would like to emphasize that in this study the new Vitek 2 YST card showed a definitive improvement in the identification of C. dubliniensis (96.1%). Furthermore, in contrast to previous studies, the YST card was able to reliably differentiate C. dubliniensis from the closely related C. albicans (2). As in other studies, identification of C. guilliermondii was problematic (32.4%). Thirty-three of 64 strains were identified with low discrimination as C. guilliermondii or Candida famata; 9 strains were misidentified as C. famata. These disappointing results are due to the fact that C. guilliermondii and C. famata show similar biopatterns, which could not be sufficiently discriminated by the YST card. In this study, unequivocal results could be obtained by using supplemental tests. They are based on simple techniques like microscopic morphology or pigment production. In a few cases, especially for identification of rarely occurring yeasts, complete resolution is only possible by the use of molecular methods.

Moreover, the study showed that the performance of the Vitek 2 YST card was not significantly influenced by the source media, independent of the type and manufacturer. Therefore, the performance of identification with the YST card is possible with a wide range of culture media, including chromogenic media.

In conclusion, the Vitek 2 YST card, supported by simple additional tests, is a valuable method for the identification of commonly encountered, medically important yeast species and can be used on isolates grown on a wide range of source media. However, identification results for rare species like C. pelliculosa and C. rugosa have to be viewed with caution.

 
We are grateful to Birgit Hofmann and Monika Panke for their diagnostic mycological expertise and to Oliver Morton for his assistance in editing the manuscript. We also thank bioMérieux for its support to this study by providing the Vitek 2 YST cards, the Sabouraud agar plates with gentamicin and chloramphenicol, the Columbia agar plates with sheep blood, and the CAN2 plates.

 
* Corresponding author. Mailing address: Institute of Hygiene and Microbiology, University of Würzburg, Josef-Schneider-Str. 2/E1, 97080 Würzburg, Germany. Phone: 49-(0)931-201 46901. Fax: 49-(0)931-201 46445. E-mail: gvalenza{at}hygiene.uni-wuerzburg.de

Published ahead of print on 17 September 2008.

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Journal of Clinical Microbiology, November 2008, p. 3784-3787, Vol. 46, No. 11
0095-1137/08/$08.00+0     doi:10.1128/JCM.01318-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

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 Google Scholar Google Scholar Articles by Valenza, G. Articles by Abele-Horn, M. Search for Related Content PubMed PubMed Citation Articles by Valenza, G. Articles by Abele-Horn, M.