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School of Molecular & Biomedical Science |
Yeasts
Identification.Yeast-like fungi may be basidiomycetes, such as Cryptococcus neoformans or ascomycetes such as Candida albicans. 1. Ensure that you start with a pure culture; streak for single colony isolation if necessary. 2. Germ Tube Test: lightly inoculated 5 ml of serum, containing 0.5% glucose and incubated at 35oC for 2-3 hours.
3. For the identification of germ tube negative yeasts, morphological (Dalmau plate culture), physiological and biochemical tests are essential. Dalmau Plate Culture: To set up a yeast morphology plate, dip a flamed sterilized straight wire into a light inoculum (sterile distilled water suspension) and then lightly scratch the wire into the surface of a cornmeal/tween 80, rice/tween 80 or yeast morphology agar plate, then place a flamed coverslip onto the agar surface covering the scratches. Dalmau morphology plates are examined in situ using the lower power of a microscope for the presence of pseudohyphae which may take up to 4-5 days at 26oC to develop. C. albicans also produces characteristic large, round, terminal, thick-walled vesicles (often called chlamydoconidia). The key features to remember are to use a light inoculum and to scratch the surface of the agar with the wire when inoculating. Physiological and biochemical tests: including fermentation and assimilation studies should be performed based on those used at the Centraalbureau voor Schimmelcultures, Delft, The Netherlands (Kreger-Van Rij, 1984). Reliable commercially available yeast identification kits are the API 20C AUX, ATB 32C, MicroScan and Vitek systems.
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