COCCI dioides immitis
Coccidioides immitis is also a dimorphic fungus, but instead of a yeast phase, a large (12-to 100-μm), distinctive, round-walledspherule (Fig 49–4A and C) is produced in the in-vasive tissue form. This structure is unique among the pathogenic fungi. Its formation takes place in a process illustrated in Figure 49–5. Spherule development requires simul-taneous invagination of the fungal membrane (plasmalemma) and production of new cell wall to form the large multicompartmental structure. The compartments differentiate into uninucleate structures called endospores, each with a thin wall layer. Multiple endospores develop within each spherule and the entire structure is surrounded by an extracellular matrix. The spherule eventually ruptures, releasing 200 to 300 endospores (Fig 49–6) each of which can differentiate into another spherule.
In alkaline soils and in culture, C. immitis grows only as a mold regardless of tem-perature. Growth becomes visible in 2 to 5 days. The hyphae are septate and produce thick-walled, barrel-shaped arthroconidia (see Fig 49–4B), which are the infectious unit in nature and highly infectious when they develop in the laboratory. Spherules have been produced from arthroconidia in vitro under specialized conditions.
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