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The fungi discussed in this chapter cause a variety of infections, each ranging in severity from subclinical to progressive, debilitating disease. Some of these species are dimorphic, growing in the infectious mold form in the environment but switching to a round, yeast-like form in infected tissues. They differ from the opportunistic fungi in their ability to cause disease in previously healthy persons. However, the most serious infections still occur in patients with compromised immune systems. With the exception of Cryptococcus neoformans, each of these fungi is predominantly restricted to geographic niches corresponding to the environmental habitats of the mold form of the species. None of these infections is transmitted from human to human. The major features of the systemic pathogens are summarized in Table 47-1.
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CRYPTOCOCCUS NEOFORMANS AND CRYPTOCOCCUS GATTII
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Cryptococcus species were first isolated from environmental sources more than a century ago, and they are now recognized as important human pathogens, especially in the setting of HIV infection. The most important clinical manifestation of cryptococcal disease is a life-threatening meningitis in immunocompromised patients.
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Found throughout the world, Cryptococcus species grow as a budding yeast 4 to 6 μm in diameter. The most characteristic feature of these cells is a large polysaccharide capsule (Figure 47–1), often extending the overall diameter of these cells to 25 μm or more. Cryptococcal species are basidiomycetes, a group of fungi that includes the mushrooms as well as many agricultural pathogens. The Cryptococcus genus contains two pathogenic species complexes, C neoformans and the more recently recognized Cryptococcus gattii. Each has multiple serotypes, subspecies, or varieties.
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