Thermally dimorphic fungus found in
Common natural habitat: soil
contaminated with bird droppings or excrements of bats;
Endemic in the
Tennessee-Ohio-Mississippi river basins.
mycosis/histoplasmosis: varying from an acute benign pulmonary
infection to a chronic pulmonary or fatal disseminated disease.
In cases of dissemination of Histoplasma capsulatum var.
capsulatum infection, reticuloendothelial system (RES) is
most frequently involved. The fungus resides intracellularly in
Dissemination and fatal course are
more common in the immunocompromised and elderly. Chronic
cavitary histoplasmosis is most commonly observed in individuals
with underlying pulmonary disease.
Being a thermally dimorphic fungus,
Histoplasma capsulatum grows in mould form at 25°C, and
in yeast form at 37°C. Below are the macroscopic characteristics
at varying temperatures and for both varieties.
At 25°C: Colonies are slow
growing and granular to cottony in appearance. The color is
white initially and usually becomes buff brown with age. The
colonies are not sensitive to cycloheximide in the culture
media. From the reverse, a yellow or yellowish orange color may
be observed. While these features are best observed on Sabouraud
dextrose agar (SDA), brain heart infusion agar (BHIA) enhances
growth more efficiently.
At 37°C: Creamy,
slowly growing, moist and yeast-like colonies are formed. This
phase is observed in infected tissues and in vitro on enriched
media, such as BHIA containing 5-10% blood.
At 25°C: hyphae-like
conidiophores which arise at right angles to the parent hyphae.
It has both macro- and microconidia. Macroconidia are
tuberculate, thick-walled, round, unicellular, hyaline, large
and often have fingerlike projections on the surface. These
macroconidia are also referred to as tuberculochlamydospores or
macroaleurioconidia. Microconidia (microaleurioconidia) are
unicellular, hyaline and round, with a smooth or rough wall.
At 37°C: narrow-based,
ovoid, budding yeast cells are formed. Yeasts of var.
capsulatum are smaller than (2-4 µm) those of var.
duboisii (12-15 µm).