Question
Dimorphic fungi
Answer
Dimorphic fungi are fungi capable of existing in two distinct morphological forms depending on temperature/environmental conditions — growing as a filamentous mould (mycelial form) at ambient/environmental temperature (typically 25°C) and converting to a yeast (or spherule, in one notable exception) form at body temperature (37°C) in host tissue — this thermal dimorphism is a defining biological feature of the major systemic/deep fungal pathogens and is directly relevant to their pathogenicity, since the yeast form is typically the tissue-invasive, disease-causing phase.
Important medically significant dimorphic fungi:
- 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 — an important exception to the yeast pattern: mould with arthroconidia at 25°C, but converts to a large, thick-walled spherule (containing numerous endospores) at 37°C, rather than a budding yeast; causes coccidioidomycosis (“Valley fever”), endemic to arid regions of the southwestern USA.
- Sporothrix schenckii — mould at 25°C; cigar-shaped budding yeast at 37°C; causes sporotrichosis, classically via traumatic inoculation from thorns/plant material.
- Paracoccidioides brasiliensis — mould at 25°C; yeast with characteristic multiple peripheral budding (“pilot’s/ship’s wheel” appearance) at 37°C; causes paracoccidioidomycosis, endemic to Latin America.
- Talaromyces (Penicillium) marneffei — mould at 25°C; yeast dividing by fission (not budding) at 37°C; causes talaromycosis (penicilliosis), endemic to Southeast Asia, an important AIDS-defining opportunistic infection in that region.
Clinical/laboratory significance: demonstrating this characteristic thermally-induced dimorphism in culture (mould at room temperature, converting to the corresponding tissue form at 37°C) is a key confirmatory laboratory technique for identifying these organisms; because the mould (mycelial) phase of most of these organisms produces highly infectious airborne conidia, culture of suspected dimorphic fungi requires careful biosafety precautions (often BSL-3) to prevent laboratory-acquired infection via inhalation.