DESCRIPTIONS OF MEDICAL FUNGI

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Descriptions of Medical Fungi
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 25 O C
are initially moist and glabrous, but rapidly become suede-like to downy, greyishwhite
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.
20 μm
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.
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).