Blastomyces spp. produce large (size), broad-based budding yeast in tissue. The yeast cell walls are thick and double contoured.
When incubated at 25 - 30°C, 'Blastomyces" spp. are slow growing and produce a white, cottony mycelium that darkens to tan with age. Microscopically, the mold demonstrates septate, hyaline hyphae with short unbranched conidiophores, producing single pyriform to round, smooth conidia that measure 2-10 µm. Definitive identification of Blastomyces spp. requires either conversion to its yeast form at 37ºC, or exoantigen or molecular confirmatory tests.
Blastomyces spp. are found in the Mississippi and Ohio River valleys. They are also endemic to the southeastern US and in areas bordering the Great Lakes. It is thought to thrive in the soil of wooded areas.
The top right image shows a culture of a Blastomyces sp. The bottom right image shows a lactophenol cotton blue preparation of Blastomyces.
Images courtesy of the CDC.