Yeasts

• The term "yeast" is not always strictly defined; generally single-cell fungi. Other, more rigorous definitions may be applied.
• Yeasts are normal components of gastrointestinal and genitourinary tracts
• Most common fungal isolate from patients
• Unicellular, eukaryotic, generally round to oval
• Multiply principally by blastoconidia (buds)
• While pseudohyphae are the rule, true septate hyphae may be formed by some yeasts under certain conditions
• While morphology may contribute to identification, biochemical tests are usually necessary for definitive diagnosis
• Growth at 37°, germ tube test, carbohydrate assimilation, fermentation, urease test, and phenol oxidase test may be employed to speciate

Candida Species: Direct Examination

• Hyphae show distinct points of constriction resembling sausage links
• Budding yeast forms (blastospores) often seen

Candida Species

• Generally grow at 37°, ferment glucose and may ferment other carbohydrates, and form pseudo- or true hyphae
• Harbored by the gastrointestinal tract

Candida albicans

• Germ tube positive
• Creamy colonies, as other yeasts
• May display pseudohyphae and true hyphae
• Most commonly isolated candidiasis
• Virulence factors include rapid germination within tissue, protease production, surface integrin-like molecules for binding extracellular matrix, complement protein binding receptor, phenotypic switching, and surface variation and hydrophobicity

FIG. 1. Candida albicans in the cerebrospinal fluid of a patient with a malfunctioning ventriculoperitoneal shunt.

FIG. 2. Candida albicans incubated in rabbit serum at 37° (germ tube test). Germ tubes are indicated by arrows and are the beginnings of true hyphae: no constriction is at the origin of the germ tube and the parent cell.

FIG. 3. Candida albicans on blood agar; moist, opaque colonies are characteristic of yeast.

FIG. 4. Candida albicans on a Dalmau plate. Clusters of round blastoconidia are present at some septae; thick-walled terminal chlamydospores (characteristic of C. albicans) are not seen in this image.

Candida tropicalis

• May cause infection, especially of the immunocompromised; may also be isolated without evidence of disease
• Creamy colonies with mycelial fringe
• Blastoconidia arise singly or in small groups anywhere along long pseudohyphae

FIG. 1. Candida tropicalis on a Dalmau plate.

Candida parapsilosis

• Relatively frequent cause of candidal endocarditis
• Creamy colonies; sometime with lacy appearance
• Blastoconidia arise singly or in small groups anywhere along pseudohyphae; at low power has characteristic shaggy star appearance
• At high power, crooked or curved, short pseudohyphae and occasional large hyphal elements (termed "giant cells") can be seen

FIG. 1. Low power appearance of Candida parapsilosis on Dalmau plate. Note the characteristic "shaggy star" morphology.

Cryptococcus Species

• Do not form pseudo- or true hyphae, except for Cryptococcus neoformans, which rarely forms them
• Urease positive
• Cryptococcus neoformans is the only common, medically important yeast to produce phenol oxidase, colonies grown on birdseed agar (which contains niger or thistle seeds) turn dark brown in 2-5d of incubation

Cryptococcus Species: Direct Examination

• Cells spherical and irregular in size, ranging from 5-20μ in diameter
• Classically have thick polysaccharide capsule, however, poorly encapsulated strains are becoming more common
• One or more budding forms are attached by a narrow constriction

FIG. 1. Cryptococcus neoformans in a bronchoalveolar lavage specimen; wright stain.

FIG. 2. Cryptococcus neoformans in blood culture; gram stain.

FIG. 3. Cryptococcus neoformans in sputum; wright stain.

FIG. 4. Cryptococcus neoformans in lung; H&E stain, low-medium power.

FIG. 5. Cryptococcus neoformans in lung; mucicarmine stain helps identify organisms.

FIG. 6. Cryptococcus neoformans in lung; GMS stain is a good, general-purpose fungal stain on tissue sections. The tiny black dots seen in this image are yeast within alveoli, seen at low power.

Cryptococcus neoformans

• Common infection in immunocompromised patients
• CNS infection most common, but may cause other disease, especially of lungs
• Capsule is seen in classic India ink preparations; but given the appearance of poorly encapsulated strains, this technique is becoming less useful
• Mucicarmine stains and Gomori methenamine silver (GMS) stain are useful on histologic sections, see figures under Direct Examination, above

Cryptococcus laurentii

• Colonies are white or greenish on birdseed agar; cream to yellow, pink, orange, or tan on routine growth media
• Like C. neoformans, may grow at 37° C on Sabouraud. dextrose agar
• Like other Cryptococcus species, urease positive
• Used within the laboratory as a control organism to test purity of sugars within certain media
• Yeast ovoid to elongate, as single cells or in pairs or occasionally in chains of 3 or more cells

FIG. 1. Cryptococcus laurentii on Dalmau plate. Note the absence of hyphae.

Geotrichum species: yeast-like organisms

• Usually regarded a contaminant, except in extremely immunocompromised patients
• Colonies are white to cream, mealy, and subsurface, resembling yeast; mature colonies may appear more mold-like
• Septate, hyaline, true hyphae are seen microscopically; these break into arthroconidia with maturation
• Geotrichum species are often confused with yeast grossly, but show hyphae on slide culture
• Given presence of arthroconidia, may resemble Coccidioides
• Differentiated from Trichosporon species based on carbohydrate assimilation tests; also, does not produce urease or blastoconidia

FIG. 1. Geotrichum in slide culture. Arthroconidia and hypha are seen here.