Last updated on May 30th, 2021
If you observe a probable fungus in the culture plate, colony characteristics are first assessed to determine the broad group of the isolate. Once the initial observations are made, the following microscopic criteria are used to make a genus/species identification of the fungal isolate.
- If hyphae are observed, determine the structure of the hyphae. Are they:
- Septate or aseptate
- Branching (if so, at what angles) or not branching?
- Pigmented or not pigmented
- Even or uneven in width
- Composed of arthroconidia or pseudohyphae
- Determine the structure and derivation of fruiting bodies
- Visualize the type of conidiation:
- The size and shape of the spores or conidia
- The size, shape, and arrangement of the spores
- Look for the presence of special diagnostic structures: pycnidia, cleistothecia, Hulle cells
- If only yeast cells are observed:
- Note their size, shape, and arrangement.
- Look for the presence of or absence of capsules.
- Determine the type of bastoconidiation- are daughter cells single or multiple?
Presumptive identification of fungi based on Direct Microscopic Examination of material from Clinical Specimen.
- Hyphae relatively small (3-6 micrometer) and regular in size, dichotomously branching at 45-degree angles with distinct cross-septa: Aspergillus spp
- Hyphae irregular in size (6-50 micrometer), ribbonlike, and devoid of septa: Zygomycetes (Phycomycetes); Rhizopus-Mucor-Absidia
- Hyphae small (2-3 micrometer) and regular, some branching, with rectangular arthrospores sometimes seen: found only in skin, nail scrapings, and hair: Dermatophyte group (Microsporum spp, Trichophyton spp, Epidermophyton spp)
- Hyphae regular in diameter (3-6 micrometer), parallel walls, irregular branching, septate, dark yellow, brown or hyaline: Phaeohyphomyces spp, Hyalohyphomyces spp.
- Hyphae, distinct points of constriction simulating link sausages (pseudohyphae), with budding yeast forms (blastospores) often seen: Candida spp
- Yeast forms, cells spherical and irregular in size (5-20 micrometer), classically with a thick polysaccharide capsule (not all cells are encapsulated), with one or more buds attached by a narrow constriction: Cryptococcus neoformans, Cryptococcus spp., nonencapsulated.
- Small budding yeast, relatively uniform in size (3-5 micrometer), with a single bud attached by a narrow base, extracellular or within macrophages: Histoplasma capsulatum
- Yeast forms, large (8-20 micrometer), with cells appearing to have a thick, double-contoured wall, with a single bud attached by a broad base: Blastomyces dermatitidis
- Large, irregularly sized (10-50 micrometer), thick-walled spherules, many of which contain small (2-4 micrometer), round endospore: Coccidioides immitis