Fungi can be broadly classified into two groups; yeasts and molds. Yeasts are single-celled organisms that reproduce by budding but molds grow as filaments-like structures.
Many medically important fungi show unique behavior based on the temperature of growth and habitat, which is known as dimorphism (di: both, morphism: morphological form). These dimorphic fungi exist as molds in the environment at ambient temperature (25°C– 30°C ) and as yeasts (or other structures e.g., spherules in case of Coccidioides immitis) in human tissues at body temperature (35°C -37°C). The mold form can be converted to the yeast form by incubating subcultures on enriched media at 37°C.
Mnemonic: Mold in the Cold, Yeast in the Heat (Beast)
Note: All molds should be handled within appropriate biological safety cabinets to avoid a laboratory-acquired infection.
Most dimorphic fungi cause systemic mycoses and Sporothrix (one of the dimorphic fungi) causes subcutaneous mycoses.
Disease caused by Dimorphic Fungi
Inhalation of spores of these dimorphic fungi causes systemic mycoses;
- Histoplasma capsulatum: H. capsulatum is a dimorphic fungus that exists as a mold in soil and as yeast in tissue. It causes histoplasmosis. (Note: It does not possess a capsule)
- Blastomyces dermatitidis: B. dermatitidis is a dimorphic fungus that exists as a mold in soil and spherule in tissue. The yeast has a characteristic double refractive wall and a single broad-based bud.
- Paracoccidioides brasiliensis: P. brasiliensis is a dimorphic fungus that exists as a mold in soil and yeasts in tissue. The yeast is thick-walled and has multiple buds. It causes Paracoccidioidomycosis, formerly named “South American blastomycosis,”.
- Coccidioides immitis: It is a dimorphic fungus that exists as a mold in soil and as yeast in tissue. It causes coccidioidomycosis.
Among the subcutaneous mycoses causing fungi, Sporothrix schenckii is a notable dimorphic fungus. It causes sporotrichosis.
Talaromyces marneffei, formerly called Penicillium marneffei, is a mold in environmental conditions, and yeast form is seen in tissues. It is an opportunistic pathogen causing systemic penicilliosis in AIDS patients in Thailand, China, Hong Kong, Vietnam, and other parts of Southeast Asia.
Mnemonics to Remember Dimorphic Fungi
- Mold in the Cold, Yeast in the Heat (Beast)
- Body Heat Probably Changes ShaPe of the dimorphic fungi
- Penicillium marneffei (Talaromyces marneffei)
Note: Candida and Cryptococcus are not dimorphic fungi they are true yeast.
Cryptococcus neoformans is an encapsulated, unicellular (yeast). It can cause pneumonia and cryptococcal meningitis in immunocompromised patients.
Candida is a yeast; it can cause candidiasis “thrush“. In immunocompromised individuals, invasive candidiasis may develop.
Colony Morphology of Dimorphic Fungi
- If the organism is dimorphic fungi, you may observe mixtures of yeast and mold forms. In slow-growing dimorphic fungi, mold appears hair-like or cobweb-like consistency.
- During a mold-to-yeast conversion of Blastomyces dermatitidis, an intermediate form known as the “prickly stage” can be seen. A microscopic preparation of such stage may show a mixture of hyphae, budding yeasts, and forms intermediate between the two.
- Yeast forms of dimorphic fungi are similar to other true yeasts. In contrast to yeasts, they are generally slow-growing and the colonies remain quite small.
Acharya TankeshwarHello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.
Blastomyces dermatitidis: Pathogenesis, Lab Diagnosis
Blastomyces dermatitidis is a thermally dimorphic fungus and causes blastomycosis. Single broad-based bud in a clinical sample is diagnostic.