Evaluation of Media for Direct Isolation and Identification of Yeasts

Objectives: To evaluate four chromogenic yeast media both qualitatively and quantitatively. Methods: Media evaluated included Sabourauds dextrose agar (SDA) (LabM), BiGGY agar (Oxoid), Candida ID (bioMérieux), CHROMagar Candida (CHROMagar Company Ltd) and Pagano-Levin (SDA containing 0.1 g/litre 2,3,5-Triphenyl tetrazolium chloride (Sigma). Qualitative analysis was performed using seventeen yeast isolates including Candida albicans, C. pseudotropicalis, C. glabrata, C. tropicalis, C. guilliermondii, C. parapsilosis, C. rugosa and S. cerevisiae. Isolates were streaked onto a whole plate of each media and incubated at 37°C. Appearance was observed at 24 and 48 hours. To determine the quantification of yeasts a range of 10⁶ to 10² cfu/ml of C. albicans were prepared. These were inoculated onto SDA by i) streaking with a 10 ml standard loop and ii) spreading 100 ml. Plates were incubated at 37°C for 24 hours. Results: BiGGY and Pagano-Levin agars did not adequately support growth or discriminate yeasts. Candida ID and CHROMagar Candida could distinguish C. albicans at 24 hours. One potential problem with CHROMagar Candida was that S. cerevisiae could be mistaken for C. tropicalis. Candida ID did not produce the same diversity of colours as CHROMagar Candida, but did accurately identify C. tropicalis, and C. guilliermondii. Our one strain of C. pseudotropicalis also fell into this group. The percentage recovery of C. albicans from 100 ml of 10², 10³ and 10⁴ cfu/ml was 100%, 75.8% and 67.8% respectively. ³10⁵ cfu/ml was too heavy to count. Plates inoculated with 10 ml of 10⁵ and 10⁶ cfu/ml suspensions were scored as + and ++ respectively. Conclusions: C. albicans was distinguishable from other yeasts by Candida ID and CHROMagar Candida after 24 hours. Accurate quantification can be obtained for counts of <10⁴ cfu/ml from a 100 ml inoculum. For >10⁴ cfu/ml a 10 ml loop with estimation of growth is sufficient.