Rapid identification of Candida albicans by fluoroplate candida agar

Yeast identification--past, present, and future methods

The focus of this review is the evolution of biochemical phenotypic yeast identification methods with emphasis on conventional approaches, rapid screening tests, chromogenic agars, comprehensive commercial methods, and the eventual migration to genotypic methods. As systemic yeast infections can be devastating and resistance is common in certain species, accurate identification to the species level is paramount for successful therapy and appropriate patient care.

Identification of Candida glabrata by a 30-second trehalase test

Rapid (30-s) trehalase tests done with material from colonies of 482 yeasts suspended in a drop of trehalose solution on a commercially supplied glucose test strip were positive for 225 (99.1%) of 227 Candida glabrata isolates grown on either of two differential media, Candida ID medium or CandiSelect medium. The test was positive for only 3 (1.2%) and 12 (4.7%) of 255 isolates of other medically important yeast species grown on the same two media, respectively. A rapid maltase test done with a subset of 255 yeast isolates was negative for all but 1 of 64 trehalase-positive C. glabrata isolates, raising the specificity of the rapid testing for C. glabrata to 98.4 to 100%, depending on the isolation medium used. Rapid trehalase and maltase tests done independently in two laboratories with 217 yeast isolates showed sensitivities of 96.0 to 98.0% and specificities of 98.2 to 99.4% for identification of C. glabrata from colonies grown on Candida ID medium. The specificity was much lower because of frequent false-positive trehalose test results when the source of colonies was Sabouraud agar formulated with 4% glucose. We conclude that direct recognition of C. albicans as blue colonies on Candida ID isolation medium coupled with the performance of the 30-s trehalase and maltase tests for C. glabrata among the white colonies on this medium will allow the rapid presumptive identification of the two yeast species most commonly encountered in clinical samples.

Yeast identification in the clinical microbiology laboratory: phenotypical methods

Emerging yeast pathogens are favoured by increasing numbers of immunocompromised patients and by certain current medical practices. These yeasts differ in their antifungal drug susceptibilities, and rapid species identification is imperative. A large variety of methods have been developed with the aim of facilitating rapid, accurate yeast identification. Significant recent commercial introductions have included species-specific direct enzymatic colour tests, differential chromogenic isolation plates, direct immunological tests, and enhanced manual and automated biochemical and enzymatic panels. Chromogenic isolation media demonstrate better detection rates of yeasts in mixed cultures than traditional media, and allow the direct identification of Candida albicans by means of colony colour. Comparative evaluation of rapid methods for C. albicans identification, including the germ tube test, shows that chromogenic media may be economically advantageous. Accurate tests for single species include the Bichrolatex Albicans and Krusei Color tests, both immunologically based, as well as the Remel Rapid Trehalose Assimilation Broth for C. glabrata. Among broad-spectrum tests, the RapID Yeast Plus system gives same-day identification of clinical yeasts, but performance depends on inoculum density and geographic isolate source. The API 20 C AUX system is considered a reference method, but newer systems such as Auxacolor and Fungichrom are as accurate and are more convenient. Among automated systems, the ID 32 C strip, the Vitek Yeast Biochemical Card and the Vitek 2 ID-YST system correctly identify >93% of common yeasts, but the ID-YST is the most accurate with uncommon yeasts, including C. dubliniensis. Spectroscopic methods such as Fourier transformed-infrared spectroscopy offer potential advantages for the future. Overall, the advantages of rapid yeast identification methods include relative simplicity and low cost. For all rapid methods, meticulous, standardized multicenter comparisons are needed before tests are fully accepted.

Membrane filtration test for rapid presumptive differentiation of four Candida species

A rapid enzymatic two-step test for the presumptive differentiation of four Candida species commonly occurring in various clinical samples is described. The technique involves membrane filtration of a liquid sample, followed by preincubation of the membrane filter on Sabouraud glucose agar supplemented with ticarcillin-clavulanic acid to yield microcolonies. In a separate assay step, parts of the filter are placed on absorbent pads impregnated with fluorogenic 4-methylumbelliferyl (4-MU) enzyme substrates (4-MU-N-acetyl-beta-D-galactosaminide, 4-MU-phosphate, 4-MU-pyrophosphate, and 4-MU-beta-D-galactoside) in combination with 0.1% digitonin acting as a membrane permeabilizer. The membrane filter in contact with the assay medium is incubated to allow cleavage of the enzyme substrate, resulting in fluorescent microcolonies under long-wavelength UV light. This approach, tested on 301 clinical samples, is able to presumptively differentiate C. albicans, C. glabrata, C. krusei, and C. tropicalis and to distinguish them from other Candida spp. in about 9 to 11 h. Overall agreement with the conventional methods of 94.4% (one Candida species present in the sample) to 83.8% (multiple Candida spp. present) was obtained. The false-negative rates with reference to identification by traditional methods were 1.3% (single species) and 3.8% (multiple species).

Evaluation of CHROMagar Candida for rapid screening of clinical specimens for Candida species

CHROMagar Candida is a new differential culture medium that allows selective isolation of yeasts and simultaneously identifies colonies of Candida albicans, Candida glabrata, Candida tropicalis and Candida krusei. We evaluated this medium and compared it with a reference medium, Sabouraud glucose agar, for the presumptive identification of yeast species isolated directly on the medium from 1150 clinical specimens. A total of 731 specimens showed no growth, 299 isolates (70.2%) showed growth to the same extent on both media. Forty mixed cultures were detected on both media. More than one isolate was detected in 30 of the tested specimens on either CHROMagar (26 specimens) or Sabouraud glucose agar (four specimens). We found a sensitivity of 98.8% and a specificity of 100% for C. albicans, 66.7% and 99.8% for C. tropicalis, 100% and 100% for C. krusei, and 98% and 95.7% for C. glabrata. Regarding these results, CHROMagar Candida is recommended as a useful isolation medium capable of the presumptive identification of yeasts and better detection of mixed cultures in clinical specimens.

Comparison of three commercial media for direct identification and discrimination of Candida species in clinical specimens

One hundred and ninety-two clinical specimens were used to compare the three chromogenic media Albicans ID, Candiselect, and CHROMagar Candida to a standard method using a Sabouraud-chloramphenicol agar medium and standard methods for identification of yeasts. The detection rates were 83.79, 83.24, 86.59 and 84.91% respectively. For the chromogenic media, the rates of direct identification (growth plus specific pigmentation) for Candida albicans were 56.50, 37.68 and 11.59% after 24 hours' incubation and 92.75, 91.30 and 88.57% after 72 hours' incubation respectively, with 100% specificity. Furthermore, CHROMagar Candida identified the five Candida tropicalis and the two Candida krusei strains detected after 48 hours' incubation.

Direct identification and recognition of yeast species from clinical material by using albicans ID and CHROMagar Candida plates

Two chromogenic media, Albicans ID and CHROMagar Candida agar plates, were compared with a reference medium, Sabouraud-chloramphenicol agar, and standard methods for the identification of yeast species. This study involved 951 clinical specimens. The detection rates for the two chromogenic media for polymicrobial specimens were 20% higher than that for the Sabouraud-chloramphenicol agar plates. The rates of identification of Candida albicans for Albicans ID and CHROMagar Candida agar plates were, respectively, 37.0 and 6.0% after 24 h of incubation and 93.6 and 92.2% after 72 h of incubation, with specificities of 99.8 and 100%. Furthermore, CHROMagar Candida plates identified 13 of 14 Candida tropicalis and 9 of 12 Candida krusei strains after 48 h of incubation.

Routine use of CHROMagar Candida medium for presumptive identification of Candida yeast species and detection of mixed fungal populations

OBJECTIVE: To assess the value of the new differential culture medium CHROMagar Candida for routine investigation of clinical specimens. METHODS: During a whole year, 6150 clinical samples were plated on CHROMagar Candida medium. After incubation, the green colonies were considered to be Candida albicans. The colonies of other colors were identified using Bichrolatex-krusei, or by their assimilation pattern on ID 32C test strips and their morphology on rice cream-agar-Tween. RESULTS: Among the 6150 clinical samples, 1643 were positive for fungi. Aspergillus fumigatus and Geotrichum sp. were the predominant filamentous fungi isolated. Candida albicans was the most common species isolated (1274 of the positive samples; 77.5%), and Candida glabrata was the second most common yeast isolated (174 positive samples; 10.6%). Other yeast species were detected at lower frequencies, mainly Candida tropicalis (3.8%), Candida krusei (2.7%), Saccharomyces cerevisiae (2.7%) and Candida kefyr (2.3%), and 16 samples revealed a lipophilic species, Malassezia furfur. Mixed fungal populations accounted for 14.7% of the positive samples. Two or more yeast species were detected in 206 of the 242 specimens containing mixed fungal populations, and five yeast species were detected in one sample. Additionally, we did not observe significant differences in the isolation of yeasts or filamentous fungi from the 366 samples simultaneously plated on CHROMagar Candida and Sabouraud dextrose agar. Close agreement between the two culture media was observed for 89.9% of these samples. CONCLUSIONS: CHROMagar Candida medium was shown to be extremely helpful in a routine clinical mycology service, facilitating the detection of mixed cultures of yeasts and allowing direct identification of C. albicans, as well as rapid presumptive identification of the other yeasts: C. glabrata, C. tropicalis, C. krusei and S. cerevisiae. This chromogenic medium thus appears to be suitable as a primary culture medium, particularly for the mycologic surveillance of immunocompromised patients.

Utility of Albicans ID plate for rapid identification of Candida albicans in clinical samples. Rapid identification of Candida albicans

Albicans ID (bioMérieux, Marcy l'Etoile, France) is a ready-to-use medium that contains a chromogenic substrate that allows rapid detection and specific identification of Candida albicans. We have evaluated its clinical performance by culturing 846 clinical specimens from pregnant women and neonates. A 99.2% sensitivity and a 100% specificity were observed in the identification of C. albicans isolates from primary culture.

Rapid identification of Candida albicans by using Albicans ID and fluoroplate agar plates

Two commercially available agar media, Albicans ID and Fluoroplate, that use a chromogenic or a fluorogenic substrate for the detection and identification of Candida albicans were evaluated. From 1,006 clinical samples containing 723 yeast strains, 352 C. albicans strains were detected with either of the two media. The sensitivity of each of the two media was 93.8% and the specificity was 98.6%, with five false-positive reactions for Candida tropicalis and no false-negative reactions.

Evaluation of the albicans IDR plate method for the rapid identification of Candida albicans

Albicans IDR is a ready-to-use medium containing a hexosaminic substrate that allows rapid and specific identification of Candida albicans. In this study, we have evaluated its performance using 330 pure cultures of yeast strains previously identified by conventional mycological methods. The test correctly identified all the Candida albicans strains tested (186 isolates), but there were 28 false-positive reactions after 48 h of culture incubation. The false-positive isolates corresponded to the species Candida tropicalis (13 strains), Trichosporon beigelii (12 strains), Cryptococcus neoformans (one strain), Candida lusitaniae (one strain) and Candida membranaefaciens (one strain). The sensitivity of the method was 100% and the specificity was 90.3% at 24 h and 86.6% at 48 h of incubation. Albicans IDR plate is a new, easy-to-perform and inexpensive method for C. albicans identification, and it could be used for direct isolation and identification of this species from clinical samples.

Evaluation of two commercialized systems for the rapid identification of medically important yeasts

A total of 77 recent clinical isolates of Candida albicans and other medically important yeasts were identified by two different commercial tests, Rapidec albicans (API-bioMérieux) and Fongiscreen 4H (Sanofi Diagnostics Pasteur), and conventional mycological methods. The strains were from 13 different species of yeasts and consisted of strains of 36 C. albicans, three of Candida famata, nine of Candida (Torulopsis) glabrata, five of Candida guilliermondii, two of Candida kefyr, three of Candida krusei, one of Candida lusitaniae, four of Cryptococcus neoformans, five of Candida parapsilosis, six of Candida tropicalis, one of Candida viswanathii, one of Rhodotorula rubra and one of Saccharomyces cerevisiae. According to the reactivity profiles of the isolates, identification was always correct with Fongiscreen 4H and was correct in 97.3% of the strains with Rapidec albicans. The latter test did not identify two C. albicans isolates that were correctly identified by Fongiscreen 4H. Both methods (97.3% correlation) were very useful for identification of C. albicans achieving the aim of their manufacturers. Additionally, Fongiscreen 4H was very useful for the identification of three other species of yeasts: C. glabrata, C. tropicalis and Cr. neoformans. The results of our study indicate that the accuracy of Rapidec albicans and Fongiscreen 4H is similar to that of the conventional methods used in this study for the identification of C. albicans. The same is true of Fongiscreen 4H in the identification of C. glabrata, C. tropicalis and Cr. neoformans. Both tests could be rapid and easy-to-perform tools in the clinical microbiology laboratory, but differences in cost must be taken into account.