Paper II
Question
Dimorphic fungi
Answer
Dimorphic fungi are fungi capable of existing in two distinct morphological forms depending on temperature — growing as a filamentous mould at ambient temperature (25°C) and converting to a yeast (or spherule, one exception) form at body temperature (37°C) in host tissue — a defining feature of the major systemic fungal pathogens, directly relevant to pathogenicity since the yeast form is typically the tissue-invasive phase.
Important examples:
- Histoplasma capsulatum — mould with tuberculate macroconidia at 25°C; small intracellular yeast (within macrophages) at 37°C; causes histoplasmosis, endemic in soil with bird/bat droppings.
- Blastomyces dermatitidis — mould at 25°C; broad-based budding yeast at 37°C; causes blastomycosis.
- Coccidioides immitis — mould with arthroconidia at 25°C; converts to a large, thick-walled spherule (containing endospores) at 37°C rather than a yeast; causes coccidioidomycosis, endemic to arid southwestern USA.
- Sporothrix schenckii — mould at 25°C; cigar-shaped budding yeast at 37°C; causes sporotrichosis, classically via traumatic inoculation from thorns.
- Paracoccidioides brasiliensis — mould at 25°C; yeast with characteristic multiple peripheral budding (“pilot’s wheel” appearance) at 37°C; causes paracoccidioidomycosis, endemic to Latin America.
- Talaromyces (Penicillium) marneffei — mould at 25°C; yeast dividing by fission at 37°C; causes talaromycosis, endemic to Southeast Asia.
Clinical/laboratory significance: demonstrating this thermally-induced dimorphism in culture is a key confirmatory laboratory technique; culture of the mould (highly infectious airborne conidia) phase requires biosafety precautions (often BSL-3) to prevent laboratory-acquired infection.