Pneumocystis carinii (PCP)

General: Debates surrounding its taxonomy notwithstanding, PCP is considered a fungus as of this writing. It consists of small cysts, each producing and containing up to 8 sporozoites; these rupture and the sporozoites mature into trophozoites, which develop into cysts and repeat the cycle. Trophozoites attach to type I alveolar pneumocytes, with subsequent cell death and leakage of amphophilic, foamy, amorphous material composed of the parasites and cell debris into the alveoli.

Clinical: Although ubiquitous, PCP rarely causes infection other than in immunocompromised patients, and is an AIDS defining illness in the HIV+ population. There is no reliable antigen detection method and it cannot be reliably cultured; diagnosis is on morphology, usually cytology of sputum or bronchial washings. Imaging classically shows bilateral, diffuse, often perihilar, fine, reticular interstitial opacification on CXR, which may progress to consolidation, and "ground-glass" appearance on HRCT. Pleural effusion is rare enough to suggest another cause.

Resistance:

Morphology: May form cavitary nodules and may even resemble granulomas. There is fibrosis apparent in septal walls and a "foamy" looking exudate seen in alveoli — empty spaces within the foam are the organisms.

• Staining: Toluidine blue, polychrome. Not visible on routine H and E.

Growth characteristics:

Common/important pathogens: