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The page below is a sample from the LabCE course Fungal Infections in Humans. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Dimorphic fungi- Histoplasma spp.

Histoplasma capsulatum yeast cells are usually found within histiocytes or reticuloendothelial cells. They are seen as 2-4 µm ovoid yeasts with narrow-based budding. Histoplasma capsulatum incubated at 25-30ºC is a very slow growing, powdery or cottony white mold that forms septate, hyaline hyphae. Conidiophores produce small (2-5 µm), smooth-walled microconidia or large (7-15 µm) thick-walled, spiny (tuberculate) macroconidia.
Histoplasmosis caused by H. capsulatum is endemic in the eastern regions of the US, especially in the Ohio and Mississippi River valley. In the environment, the fungus seems to favor soil contaminated by droppings from chickens, other birds, and bats because of its high nitrogen content. Histoplasma duboisii is restricted to continental Africa and Madagascar and is morphologically similar to H. capsulatum though H. duboisii produces small yeast cells initially (2-5 µm in diameter), but later develops a mixture of small and large cells after 3-4 weeks in culture which culminates in the culture being dominated by large yeast cells (10-15 µm in diameter).
The top right image shows a lactophenol cotton blue stain preparation of H. capsulatum demonstrating tuberculate (knobby), spheroidal macroconidia, and diaphanous filamentous hyphae. The bottom right image shows a culture plate with H. capsulatum depicting small, round colonies.
Images courtesy of the CDC.