Dimorphic Fungi

• Sporothrix schenckii
• Histoplasma capsulatum
• Blastomyces dermatitidis
• Paracoccidioides brasiliensis
• Coccidioides immitis
• Penicillium marneffei

Histoplasma capsulatum: Direct Examination

• Small budding yeast
• Relatively uniform in size, ranging from 3-5 μ
• Single bud attached at narrow base
• Often present extracellularly or within macrophages

Histoplasma capsulatum

• Hyphal colonies (25-30°) appear nondescript, white or slightly pink or brown on Sabouraud. dextrose agar, and cobweb-like. Mold colonies may be moist and white on brain heart infusion agar; yeast colonies may be small and yellow on 5% sheep blood agar
• Colonies grow slowly
• Several generations of culture may be necessary to convert hyphal form to yeast form
• Cyclohexamide inhibits the yeast form
• Tuberculate macroconidia and microconidia observed
• Sepedonium species must be considered

FIG. 1. Histoplasma in slide culture. Arrows indicate good examples of tuberculate macroconidia.

FIG. 2. Histoplasma in slide culture. Macroconidia and microconidia are present here.

Coccidioides immitis

• Colonies grow moderately rapidly
• Mold colonies on 5% sheep blood agar are typically gray-white, and have a delicate, hair-like texture; on Sabouraud. dextrose agar colonies evolve from moist, membranous gray appearance to white and cottony with the appearance of aerial mycelium. Colonies often darken with age. The reverse is gray or white
• Yeast colonies (35-37°) may take several generations to obtain and appear moist and white on brain heart infusion agar
• Cyclohexamide inhibits yeast forms
• In tissue, large, irregularly sized thick walled spherules ranging in size from 10-50 μ to as much as 10-200 μ are characteristic
• Spherules contain small, round endospores (2-4 μ) which may be confused with Histoplasma

FIG. 1. High power photomicrograph of Coccidioides immitis in lung tissue, H&E.

Sporothrix schenckii

• Subcutaneous mycosis; found world-wide
• Rose gardener's disease
• Mold colonies mature rapidly and are white to yellow, wrinkling and folding with maturation
• Microscopically hyphae are delicate with small, unicellular, oval to round microconidia (2-6μ) developing laterally along hyphae in a sleeve-like pattern
• Yeast forms vary in size and shape but may be round, oval, and fusiform; budding forms are seen

FIG. 1. Sporothrix in slide culture. Note the sleeve-like way that microconidia cling to the hyphae.

Penicillium marneffei

• Endemic in southeast Asia, P. marneffei is known to cause a wide variety of disease among immunocompromised and, less frequently, immunocompetent patients.
• Hyphal colonies appear rapidly and white-tan, velvety, and flat, although distinct spicules of growth may protrude from the surface of the culture. Red soluble pigment is noted in maturing colonies, although this finding is not specific for P. marneffei.
• The mold form (grown at 25-30°) resembles an otherwise typical Penicillium, with septate hyphae and smooth generally smooth conidia aloft phialides which in turn are borne by metulae. The conidiophore typically carries 4-5 metulae, each of which bears 4-6 phialides.
• The yeast form (grown at 35-37°) are round to oval and are 3-7μ in diameter; yeast reproduce by fission rather than budding.

FIG. 1. Penicillium marneffei yeast form. An arrow indicates a yeast cell reproducing by fission.

FIG. 2. Penicillium marneffei in slide culture. The mold form resembles other Penicillium species.