Harvey's Garden

Morphology of fungi

2 min read

1. Basic Forms

  • Yeast

    • Morphology: Unicellular, round/oval shape.
    • Reproduction: Primarily by budding (asexual).
    • Colonies: Appear smooth, mucoid, or creamy, similar to bacterial colonies.
    • Examples: Candida albicans, Cryptococcus neoformans, Saccharomyces cerevisiae.

  • Mold (Mycelium)

    • Morphology: Multicellular, composed of filamentous structures called hyphae. A tangled mass of hyphae is called a mycelium.
    • Reproduction: Via asexual or sexual spores.
    • Colonies: Appear fuzzy, hairy, or woolly.
    • Examples: Aspergillus, Rhizopus, Penicillium.

  • Pseudohyphae

    • Chains of elongated yeast cells that fail to detach after budding.
    • Distinguished from true hyphae by constrictions at the septal sites.
    • Classic Example: Candida albicans.

2. Hyphae Types (Molds)

  • Septate Hyphae

    • Hyphae are divided by cross-walls called septa.
    • Septa have pores allowing communication between cells.
    • Medically Important Examples: Aspergillus (acute angle branching), Dermatophytes.

  • Aseptate (Coenocytic) Hyphae

    • Lack septa, appearing as continuous, multinucleated tubes. This allows for rapid growth.
    • Irregular, broad, ribbon-like appearance.
    • Medically Important Examples: Mucorales order (e.g., Rhizopus, Mucor), which cause mucormycosis. These hyphae often show wide-angle, irregular branching.

3. Dimorphic Fungi

  • Definition: Fungi that exist as either mold or yeast depending on the temperature. This is a key virulence factor.
  • Pathophysiology: They are typically inhaled as mold spores from the environment and convert to the pathogenic yeast form in the warmer host tissues.
  • Buzzword Mnemonic: “Mold in the Cold, Yeast in the Heat (Beast)”
    • 25°C (environment): Mold form
    • 37°C (in vivo/body temp): Yeast form
  • High-Yield Pathogens:
    • Histoplasma capsulatum: Yeast form is small, often seen within macrophages.
    • Blastomyces dermatitidis: Yeast form is large with a single, broad-based bud.
    • Coccidioides immitis: Forms a spherule (not a true yeast) containing endospores at 37°C.
    • Paracoccidioides brasiliensis: Yeast form looks like a “captain’s wheel” (multiple buds).
    • Sporothrix schenckii: Yeast form is cigar-shaped.

4. Asexual Spores (Conidia)

  • Purpose: Primarily for reproduction and dispersal.
  • Sporangiospores: Spores formed internally within a sac-like structure called a sporangium.
    • Example: Rhizopus and Mucor.
  • Conidia: Spores borne externally on a specialized structure called a conidiophore.
    • Example: Aspergillus (forms a “fruiting body”), Penicillium.

Tip

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

Graph View