Histoplasma duboisii
| Histoplasma duboisii | |
|---|---|
| Scientific classification | |
| Kingdom: | Fungi |
| Division: | Ascomycota |
| Subdivision: | Pezizomycotina |
| Class: | Eurotiomycetes |
| Order: | Onygenales |
| Genus: | Histoplasma |
| Species: | H. duboisii |
| Binomial name | |
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Histoplasma duboisii Vanbreuseghem (1952) |
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| Synonyms | |
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Histoplasma duboisii is a saprotrophic fungus responsible for the invasive infection known as African histoplasmosis. This species is a close relative of Histoplasma capsulatum, the agent of classical histoplasmosis, and the two occur in similar habitats.Histoplasma duboisii is restricted to continental Africa and Madagascar, although scattered reports have arisen from other places usually in individuals with an African travel history. Like, H. capsulatum, H. duboisii is dimorphic – growing as a filamentous fungus at ambient temperature and a yeast at body temperature. It differs morphologically from H. capsulatum by the typical production of a large-celled yeast form. Both agents cause similar forms of disease, although H. duboisii predominantly causes cutaneous and subcutaneous disease in humans and non-human primates. The agent responds to many antifungal drug therapies used to treat serious fungal diseases.
Histoplasmosis was first reported from the African continent in 1942. These early reports implicated strains that produced larger yeast cell forms than H. capsulatum, and the Irish mycologist James Thompson Duncan suggested they might represent a distinct taxon. The fungus was described as a new species by Raymond Vanbreuseghem in 1952 based on isolates provided to him by Professor Albert Dubois, director of the Prince Léopold Institute for Tropical Medicine in Antwerp, Belgium, and the species was named in honour of Dubois. Five years after its description, Professor Edouard Drouhet of the Pasteur Institute in Paris reduced the taxon to synonymy with H. capsulatum, designating it as a variant. However, since the 1960s the fungus has been generally accepted as a separate species from H. capsulatum.
Histoplasma duboisii is a dimorphic fungus, growing as either a yeast-like form or a filamentous form depending on the prevailing nutritional and temperature conditions. It is unusual to find both the mycelial and yeast forms co-existing. The mycelial form is characterized by white and cottony colonies that turn brownish with age. The underside of the colony is typically brownish in colour. It is morphologically similar to the closely related species, H. capsulatum, producing warted aleurioconidia though not as prolifically as H. capsulatum. Unlike the small-celled yeast produced by H. capsulatum, H. duboisii initially produces small yeast cells (2–5 μm in diameter) but later develops a mixture of small and large cells after 3–4 weeks in culture culminating in the culture being dominated by large yeast cells (10–15 μm in diameter). The yeast form of H. duboisii are oval in shape with thick cell walls composed of galactomannan intermixed with β-(1,4)-glucan. Cells of H. duboisii have a different fatty acid profile than those of H. capsulatum, and these differences have been suggested to relate to differences in cell sizes between the two species.
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