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The fungi discussed in this group cause a variety of infections, each ranging in severity from subclinical to progressive, debilitating disease. Most species are dimorphic, growing in the infectious mold form in the environment but switching to a yeast or other form in tissues to produce infection. They differ from the opportunistic fungi in their ability to cause disease in previously healthy persons, but the most serious disease still occurs in immunocompromised persons. With the exception of Cryptococcus neoformans, each of these species is restricted to a geographic niche corresponding to the environmental habitat of the mold form of the species. None is transmitted from human to human. The major features of the systemic pathogens are summarized in Table 47-1.

TABLE 47–1Features of Systemic Fungal Pathogens



Cryptococcus is a genus of yeast 4 to 6 μm in diameter that produces a characteristic capsule (Figure 47–1), extending the overall diameter to 25 μm or more. It is a basidiomycete, and has two species, C neoformans and the more recently recognized C gattii. Each has multiple serotypes or varieties. Here, unless specified otherwise, the use of Cryptococcus or simply the cryptococcus refers to the classic Cryptococcus neoformans.

FIGURE 47–1.

Cryptococcus neoformans. This India ink preparation was made by mixing cerebrospinal fluid containing cryptococci with India ink. The yeast cells can be seen within the clear space caused by the large polysaccharide capsule excluding the ink particles. Note that the one on the right is budding. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)

Two species and multiple varieties

The capsule is unique ...

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