Pneumocystis carinii
| Pneumocystis | |
|---|---|
 |
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| P. jirovecii cysts in tissue | |
| Scientific classification | |
| Kingdom: | Fungi |
| Phylum: | Ascomycota |
| Class: | Pneumocystidomycetes |
| Order: | Pneumocystidales |
| Family: | Pneumocystidaceae |
| Genus: | Pneumocystis |
| Species: | P. jiroveci |
| Binomial name | |
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Pneumocystis jiroveci J.K.Frenkel |
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Pneumocystis jirovecii (previously P. carinii) is a yeast-like fungus of the genus Pneumocystis. The causative organism of Pneumocystis pneumonia, it is an important human pathogen, particularly among immunocompromised hosts. Prior to its discovery as a human-specific pathogen, P. jirovecii was known as P. carinii.
The complete lifecycles of any of the species of Pneumocystis are not known, but presumably all resemble the others in the genus. The terminology follows zoological terms, rather than mycological terms, reflecting the initial misdetermination as a protozoan parasite. It is an extracellular parasite. All stages are found in lungs and because they cannot be cultured ex vivo, direct observation of living Pneumocystis is difficult. The trophozoite stage is thought to be equivalent to the so-called vegetative state of other species (such as Schizosaccharomyces pombe) which, like Pneumocystis, belong to the Taphrinomycotina branch of the fungal kingdom. The trophozoite stage is single-celled and appears amoeboid (multilobed) and closely associated with host cells. Globular cysts eventually form that have a thicker wall. Within these ascus-like cysts, eight spores form which are released through rupture of the cyst wall. The cysts often collapse forming crescent-shaped bodies visible in stained tissue. If meiosis takes place within the cysts, or what the genetic status is of the various cell types is not known for certain.
Pneumocystis pneumonia is an important disease of immunocompromised humans, particularly patients with HIV, but also patients with an immune system that is severely suppressed for other reasons, for example, following a bone marrow transplant. In humans with a normal immune system, it is an extremely common silent infection.
Identified by methenamine silver stain of lung tissue, type I pneumocytes, and type II pneumocytes over-replicate and damage alveolar epithelium, causing death by asphyxiation. Fluid leaks into alveoli, producing an exudate seen as honeycomb/cotton candy appearance on hematoxylin and eosin-stained slides. Drug of choice is Trimethoprim/sulfamethoxazole, pentamidine, or dapsone. In HIV patients, most cases occur when the CD4 count is below 200 cells per microliter.
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