Mycology
This domain tests media selection, rapid phenotypic tests, and the temperature-dependent conversion of dimorphic fungi.
Media and rapid tests
- Sabouraud dextrose agar with chloramphenicol is the general medium; add cycloheximide (Mycosel) to select dermatophytes - but never for CSF or sterile sites, because it inhibits Cryptococcus.
- Germ tube test in serum at 35–37 °C for 2–3 hours identifies Candida albicans and C. dubliniensis.
- Cornmeal with Tween 80 induces chlamydospores; CHROMagar gives presumptive colony-color identification.
- Cryptococcus: urease positive, encapsulated on India ink, antigen positive, brown colonies on birdseed (niger seed) agar.
- Dimorphic pathogens grow as molds at 25–30 °C and convert to yeast at 37 °C: Histoplasma, Blastomyces, Coccidioides, Sporothrix. Malassezia requires lipid supplementation.
Worked example
CSF from a patient with AIDS shows an encapsulated yeast on India ink. The germ tube is negative, urease is positive, and colonies turn brown on birdseed agar. Reasoning: germ tube negativity excludes C. albicans; the capsule, urease, and melanin production identify Cryptococcus neoformans. Confirm with cryptococcal antigen and report.
Traps
Using cycloheximide-containing medium for CSF and missing Cryptococcus; reading a germ tube at 30 minutes instead of 2–3 hours; expecting C. glabrata to form germ tubes (it does not); and assuming dimorphism without a temperature shift, since the conversion requires 37 °C, not a special agar.