Blastomycosis is a fungal infection caused by Blastomyces dermatitidis, a dimorphic fungus. This means the fungus can exist in two different forms, depending on its environment. Understanding what this fungus looks like involves examining its appearance both in its natural habitat and once it has entered the body, as well as the visible signs it can cause on the skin.
The Fungus in its Natural Habitat
In its natural environment, Blastomyces dermatitidis exists as a mold at temperatures around 25°C (77°F). When grown in a laboratory culture at room temperature, it appears as white to off-white colonies. As the culture matures, these colonies can develop aerial hyphae, leading to a gray to brown, cottony, or fluffy appearance.
Under a microscope, the mold form consists of thin, thread-like, septate hyphae. These hyphae bear spherical to pear-shaped microconidia, measuring approximately 3 to 5 micrometers in diameter. These conidia are the infectious particles that can become airborne when the fungal colony is disturbed. The fungus thrives in moist soil enriched with decaying organic matter, such as rotting wood and animal droppings, often near waterways.
The Fungus Inside the Body
Once inhaled into the lungs, the Blastomyces dermatitidis fungus undergoes a transformation from its mold form to a yeast form, which occurs at body temperature, around 37°C. This yeast form is the pathogenic form observed within infected human and animal tissues. Under a microscope, these yeast cells are large, measuring 8 to 15 micrometers in diameter.
A distinguishing feature of the Blastomyces dermatitidis yeast is its thick, double-contoured cell wall. These yeast cells reproduce by budding, where a new cell grows directly from the parent cell. The bud is attached to the parent cell by a broad base, a unique feature among dimorphic fungi that aids in its identification. This thick cell wall and broad-based budding make the yeast forms highly recognizable during microscopic examination of tissue samples or clinical specimens.
Visible Signs on the Skin
When Blastomyces dermatitidis disseminates from the lungs, the skin is a common site for visible signs. Skin lesions begin as small, raised bumps (papules), pus-filled bumps (pustules), or nodules under the skin. These initial lesions evolve into crusted, wart-like (verrucous) plaques.
These lesions present with raised, irregular borders that may be purplish-red and studded with small abscesses. As the lesions enlarge, their centers may begin to heal, sometimes forming atrophic scars. Ulceration can also occur, with pale, irregular borders. These skin lesions are observed on exposed body surfaces like the face, neck, and extremities.