Dimorphic Fungi

Slow growing fungi that grow as molds at 25-30° and yeast at 35-37° (human body temperature) The commonly considered dimorphic fungi include: Of special concern, thermally dimorphic fungi can be dangerous to culture in routine microbiology laboratory settings. Some laboratories recommend transferring young mold colonies from petri dishes to slants as soon as they appear, limiting the chance of Coccidioides arthrospore formation and laboratory worker exposure should C. immitis be cultured (Al-Doory 1980). The slow growth and nonspecific gross appearance of these fungi complicate diagnosis. The following fungi may be confused:
Dimorphic FungiMistaken Fungi
Histoplasma capsulatumSepedonium
Blastomyces dermatitidis
Sporothrix schenckiiCephalosporium (Acremonium)
Blastomyces dermatitidisScedosporium apiospermum

Histoplasma capsulatum: Direct Examination

Histoplasma capsulatum


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


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

Sporothrix schenckii

s. schenckii

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

Penicillium marneffei

p. marneffei
yeast form penicillium marneffei

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.

mold form penicillium marneffei

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Copyright © 2001, William McDonald, M.D.
Revised: 5 September 2002