• Monomorphic fungi: fungi that exist exclusively as either yeast or mold
  • Yeasts: unicellular fungi, occasionally organize into hyphae and pseudohyphae
    • Pseudohyphae have:
      • Variable width
      • Clear constrictions at cell junctions
      • Irregular septation
      • Irregular branching
  • Molds: multicellular fungi, organized into hyphae
    • True hyphae have:
      • Parallel-sided walls (uniform diameter)
      • No constrictions at septa (cross walls)
      • Regular, evenly spaced septation
      • Regular branching
• Dimorphic fungi: fungi that can exist as both mold or yeast, depending on temperature
  • Molds typically grow at approx. 20°C and yeasts grow at 37 °C.
  • Examples: Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides brasiliensis, and Sporothrix schenckii
  • Most commonly cause subacute pulmonary infections

Mnemonic

Mold in the cold, yeast in the heat!

Tip

• Canonical dimorphic fungi: yeasts (25 ℃) → mold (37 °C, = true hyphae)
• Candidiasis: pseudohaphae (25 ℃, formed by yeasts) → germ tubes (37 °C, = true hyphae)