Mycology
Core concepts
Separate yeasts from molds and know the media and temperatures that each requires. Sabouraud dextrose agar with antibiotics is standard; chromogenic agar color-codes common Candida species; cornmeal with Tween 80 induces chlamydospores and true hyphae; bird seed agar detects the melanin produced by Cryptococcus neoformans. The germ tube test identifies Candida albicans within two to three hours, and chlamydospore formation confirms it.
For molds, examine lactophenol cotton blue preparations for hyphal width, septation, and branching angle. Aspergillus shows narrow, septate hyphae with acute-angle branching; Mucorales show broad, irregular, largely nonseptate hyphae with right-angle branching. Dimorphic fungi grow as molds at 25 to 30 degrees Celsius and as yeasts at 37 degrees: Histoplasma (intracellular yeast, tuberculate macroconidia), Blastomyces (broad-based bud), Coccidioides (spherules and endospores), Sporothrix (cigar-shaped yeast, rosette mold). Dermatophytes are identified by macroconidia and conidia arrangement.
Traps
- Confusing pseudogerm tubes of Candida dubliniensis with true germ tubes.
- Assuming every Candida glabrata isolate forms pseudohyphae; it typically does not.
- Exposing plates of Histoplasma or Coccidioides in the open laboratory; handle suspicious molds in a biosafety cabinet.
- Calling Cryptococcus on India ink alone; urease, melanin production, and antigen testing confirm it.
- Overlooking Malassezia because it requires lipid supplementation.
Rule of thumb
Germ tube positive plus chlamydospores on cornmeal equals Candida albicans. A germ tube negative yeast from blood that is urease positive and produces brown colonies on bird seed agar is Cryptococcus neoformans, confirmed by capsular antigen.