Fungal Morphology

Candida albicans switches between yeast, pseudohyphal and hyphal forms in response to serum, temperature, pH and carbon dioxide. This is polymorphism and not thermal dimorphism. Coccidioides differs from the other thermal dimorphs in forming spherules, not budding yeasts, in tissue.

Yeasts reproduce by budding, in which a daughter cell arises from the mother cell and separates after septum formation, leaving a bud scar. A few reproduce by fission, in which a cell divides by a central septum. Budding pattern is a key identifier in tissue:

• Narrow based budding: Candida species, Cryptococcus, Histoplasma, Emergomyces.
• Broad based budding: Blastomyces. Histoplasma duboisii forms large thick walled yeasts that resemble Blastomyces in size, but the buds arise from a narrower base.
• Multiple budding: Paracoccidioides, with a large mother cell ringed by daughter buds in a pilot wheel pattern.
• Monopolar budding with collarette: Malassezia, a flask shaped cell with a broad bud base and a collar of wall remnant.
• Fission: Talaromyces marneffei divides by a transverse septum, giving elongated sausage shaped cells with a central cross wall.

The germ tube is a hyphal outgrowth with no constriction at its origin. It forms within 2 to 3 hours in serum at 37 degrees Celsius and is produced by Candida albicans and Candida dubliniensis. Both also form thick walled terminal chlamydoconidia on nutrient poor media such as cornmeal agar. Candida glabrata does not filament and is seen only as small budding yeasts, which is why it is rarely recognised as Candida from morphology alone in tissue.

Trichosporon is distinctive among yeast like fungi for forming true hyphae, pseudohyphae, blastoconidia and arthroconidia together.

Mould hyphae are tubular filaments that elongate at the tip and branch behind it. Septate hyphae are divided by regular cross walls with pores. Pauciseptate hyphae have few septa, which are irregular and often missing in tissue, and in older literature are loosely called aseptate or coenocytic.

Hyphae of Aspergillus, Fusarium and Scedosporium are indistinguishable on histology. Culture or molecular identification is needed to separate them. Fusarium and Scedosporium can also form adventitious conidia and yeast like forms within tissue and blood. Aspergillus forms conidial heads within air containing cavities such as aspergillomas and fungal sinus disease. Basidiobolus and Conidiobolus also produce broad pauciseptate hyphae, characteristically surrounded by eosinophilic Splendore Hoeppli material.

"Spore" is the general term for a reproductive propagule. A conidium is an asexual, non motile spore borne externally from a conidiogenous cell. A sporangiospore is an asexual spore formed inside a sporangium. The airborne particles of Aspergillus are conidia, 2 to 3 micrometres in Aspergillus fumigatus, small enough to reach the alveoli.

• Grocott methenamine silver stains the wall of nearly all fungi black and is the most sensitive stain for tissue detection.
• Periodic acid Schiff stains fungal walls magenta.
• Haematoxylin and eosin shows Mucorales hyphae well and Aspergillus faintly. It poorly demonstrates Candida species and Histoplasma, which appear as small dots with a halo.
• Mucicarmine stains the Cryptococcus capsule.
• Fontana Masson stains melanin and identifies capsule deficient Cryptococcus.
• Calcofluor white binds chitin and cellulose and fluoresces under ultraviolet light.
• India ink is a negative stain: the capsule appears as a clear halo.
• Candida species are strongly Gram positive. Cryptococcus stains variably and its capsule is not stained.
• Pneumocystis cyst walls stain with Grocott methenamine silver; trophic forms and intracystic bodies stain with Giemsa and Diff Quik, and cyst walls do not.

Yeast colonies are smooth, moist and cream or white. Mould colonies show aerial mycelium that is cottony, woolly, powdery or velvety, with a reverse side that may be pigmented. Dematiaceous fungi form olive, brown or black colonies. Thermal dimorphs grow as mould at 25 to 30 degrees Celsius and convert to yeast or spherule form only at 37 degrees Celsius on enriched media.