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School of Biological Sciences
The University of Adelaide
AUSTRALIA 5005

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Dr David Ellis
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Coccidioides immitis/posadasii complex

WARNING: RG-3 organism. Cultures of Coccidioides immitis/posadasii represent a severe biohazard to laboratory personnel and must be handled with extreme caution in Class II Biological Safety Cabinet (BSCII).

Coccidioides immitis has been separated into two distinct species: C. immitis and C. posadasii (Fisher et al. 2002). The two species are morphologically identical and can be distinguished only by genetic analysis and different rates of growth in the presence of high salt concentrations (C. posadasii grows more slowly). C. immitis is geographically limited to California’s San Joaquin Valley region and Mexico, whereas C. posadasii is found in California, Arizona, Texas, Mexico and South America.

Morphological Description: Colonies of C. immitis and C. posadasii grown at 25C are initially moist and glabrous, but rapidly become suede-like to downy, greyish-white with a tan to brown reverse, however considerable variation in growth rate and culture morphology has been noted. Microscopy shows typical single-celled, hyaline, rectangular to barrel-shaped, alternate arthroconidia, 2.5-4 x 3-6 µm in size, separated from each other by a disjunctor cell. This arthroconidial state has been classified in the genus Malbranchea and is similar to that produced by many non-pathogenic soil fungi, e.g. Gymnoascus species.

Comment: Coccidioides immitis and C. posadasii dimorphic fungi, existing in living tissue as spherules and endospores, and in soil or culture in a mycelial form. Culture identification by either exoantigen test or DNA sequencing is preferred to minimise exposure to the infectious propagule.

Key Features: Clinical history, tissue pathology, culture identification by ITS sequence analysis.

Molecular Identification: In endemic areas a DNA probe for recognition of the species is commercially available (Padhye et al. 1994b). ITS sequencing is recommended for differentiation of species (Tintelnot et al. 2007, Binnicker et al. 2011).

References: Ajello (1957), Steele et al. (1977), McGinnis (1980), Chandler et al. (1980), Catanzaro (1986), Rippon (1988), de Hoog et al. (2015), Fisher et al. (2002).


Coccidioides immitis tissue morphology showing typical endosporulating spherules. Young spherules have a clear centre with peripheral cytoplasm and a prominent thick-wall. Endospores (sporangiospores) are later formed within the spherule by repeated cytoplasmic cleavage. Rupture of the spherule releases endospores into the surrounding tissue where they re-initiate the cycle of spherule development.

Culture of Coccidioides immitis
Culture of Coccidioides immitis.


Arthroconidia of C. immitis.

Mycosis: Coccidioidomycosis

Further reading:

Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.