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 ||Download (.pdf) TABLE 47–1 Features of Systemic Fungal Pathogens
| ||GROWTH || || || || |
|ORGANISM ||CULTURE AT 25°C ||CULTURE AT 37°C ||TISSUE ||SOURCE ||PRIMARY DISEASE ||DISSEMINATED DISEASE |
|Cryptococcus neoformans, C gattii ||Encapsulated yeast ||Encapsulated yeast ||Encapsulated yeast ||Environment, worldwide ||Pneumonia ||Chronic meningitis |
|Histoplasma capsulatum ||Mold, tuberculate macroconidiaa ||Small yeast ||Small intracellular yeastb ||Environment, US Midwestd ||Pneumonia, hilar adenopathy ||RES enlargement |
|Blastomyces dermatitidis ||Molda ||Yeast || ||Environment, US Midwestc ||Pneumonia ||Skin and bone lesions |
|Coccidioides immitis, C posadasii ||Mold, arthroconidia ||(Spherules)e ||Spherules ||Environment, Sonoran desertc,f ||Valley fever ||Pneumonia, meningitis, skin, bone |
|Paracoccidioides brasiliensis ||Mold ||Yeast, multiple blastoconidia || ||Environment, Latin America ||Pneumonia ||Mucocutaneous, RES |
CRYPTOCOCCUS NEOFORMANS AND CRYPTOCOCCUS GATTII
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.
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 ...