Coccidioides spp. are seen as large (10-100 µm) spherules with thick, hyaline walls that enclose numerous tiny (2-5µm) endospores in tissues sections or primary wet preparations. The endospores can sometimes be seen spilling from the spherules, and individual endospores may be confused with small yeast such as those produced by Histoplasma capsulatum (though the endospores of Coccidioides do not bud compared to the yeast of Histoplasma). There are other fungi that can be confused with Coccidioides spp., inculding Malbranchea spp., Gymnoascus spp., and Uncinocarpus reesii.
Coccidioides spp. grows moderately to rapidly in its mold form whether incubated at 25-30ºC or 37ºC and initially forms glabrous, moist, gray colonies that become white and cottony when mature.
Microscopically, Coccidioides spp. produce thin, hyaline, septate hyphae and arthroconidia (this is seen microscopically when mold-form growth is analyzed). Mature arthroconidia are barrel-shaped and alternate with empty cells.
The top right image shows a slant culture of Coccidiodes. The bottom right picture shows a lactophenol cotton blue preparation demonstrating arthroconidia. The average arthroconidia measures 3.0 x 4.5 µm, and is barrel-shaped.
Images courtesy of the CDC.