DEFINITION AND DESCRIPTION
Pneumocystis is an opportunistic fungal pulmonary pathogen that is an important cause of pneumonia in the immunocompromised host. Although organisms within the Pneumocystis genus are morphologically very similar, they are genetically diverse and host-specific. P. jirovecii infects humans, whereas P. carinii—the original species described in 1909—infects rats. For clarity, only the genus designation Pneumocystis will be used in this chapter.
Developmental stages of the organism include the trophic form, the cyst, and the precyst (an intermediate stage). The life cycle of Pneumocystis probably involves sexual and asexual reproduction, although definitive proof awaits the development of a reliable culture system. Pneumocystis contains several different antigen groups, the most prominent of which is the 95- to 140-kDa major surface glycoprotein (MSG). MSG plays a central role in the interaction of Pneumocystis with its host.
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