Fungal Forms and Dimorphism
Fungi grow in more than one body plan. The two classic forms are yeasts and moulds. Some species can switch between them. That switch is called dimorphism. It helps the fungus survive a change in temperature, nutrients, or host tissue.
Yeasts are single cells. They usually reproduce by budding. Colonies look smooth and pasty on agar. Candida and Cryptococcus are familiar examples. Yeasts spread easily in liquid and on moist surfaces.
Moulds grow as filaments called hyphae. Hyphae branch and form a mycelium. Colonies look fuzzy. Spores form on specialised stalks and travel in air. Aspergillus and many dermatophytes follow this pattern. A mould can colonise soil, food, or a damp wall.
Dimorphic fungi use both plans. In the environment they often grow as moulds at cooler temperatures. In tissue, near body temperature, they convert to yeasts or another tissue form. Histoplasma, Blastomyces, and Coccidioides are classic examples. The change is not cosmetic. The tissue form resists immune attack better than the environmental mould.
Temperature is the main cue. Nutrients, carbon dioxide, and pH also matter. The fungus rebuilds its wall and shifts its metabolism. Laboratories therefore incubate suspect cultures at both 25°C and 37°C. Growth at both temperatures supports a dimorphic identification.
Not every pathogen is dimorphic in that strict sense. Candida is usually a yeast but can form hyphae and pseudohyphae in tissue. That plasticity still aids invasion. Cryptococcus stays yeast-like and relies on a capsule instead.
Form guides diagnosis. A mould colony plus a yeast phase in the host points to a dimorphic pathogen. Direct stains, culture, and antigen tests then confirm the species. Treating the infection also depends on the site and the host, not on the shape alone.
Fungal form is therefore a survival strategy. Yeasts suit moist, cellular life. Moulds suit air and surfaces. Dimorphism lets one species use both. This overview is general information. It is not medical advice.