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.
Lifecycle 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. This species is an extracellular fungus. 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, appears amoeboid (multilobed), and is 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. Whether meiosis takes place within the cysts, or what the genetic status is of the various cell types, is not known for certain.
Homothallism The lifecycle of P. jirovecii is thought to include both asexual and sexual phases. Asexual multiplication of haploid cells likely occurs by binary fission. The mode of sexual reproduction appears to be primary homothallism, a form of self-fertilization. The sexual phase takes place in the host's lungs. This phase is presumed to involve formation of a diploid zygote, followed by meiosis, and then production of an ascus containing the products of meiosis, eight haploid ascospores. The ascospores may be disseminated by airborne transmission to new hosts.
Medical relevance
Pneumocystis pneumonia is a significant disease affecting immunocompromised individuals—particularly those with HIV infection, but also those with severely suppressed immune systems due to various reasons, such as bone marrow transplants. In individuals with a functioning immune system, P. jirovecii is an exceptionally common but usually asymptomatic infection. Pneumocystis pneumonia can be identified by methenamine silver stain of lung tissue. Type I pneumocytes and type II pneumocytes over-replicate and damage alveolar epithelium, possibly leading to death by asphyxiation. Mucosal fluid leaks into alveoli, producing an exudate of an honeycomb/cotton candy appearance on hematoxylin and eosin-stained slides. Drugs of choice are trimethoprim/sulfamethoxazole, pentamidine, or dapsone. In those with HIV, most cases occur when the CD4 count is below 200 cells per microliter.
Nomenclature At first, the name Pneumocystis carinii was applied to the organisms found in both rats and humans, as the parasite was not yet known to be host-specific. In 1976, the name "Pneumocystis jiroveci" was proposed for the first time, to distinguish the organism found in humans from variants of Pneumocystis in other animals. The organism was named thus in honor of Czech parasitologist Otto Jírovec, who described Pneumocystis pneumonia in humans in 1952. After DNA analysis showed significant differences in the human variant, the proposal was made again in 1999 and has come into common use. The name was spelled according to the International Code of Zoological Nomenclature, since the organism was believed to be a protozoan. After it became clear that it was a fungus, the name was changed to Pneumocystis jirovecii, according to the International Code of Nomenclature for algae, fungi, and plants (ICNafp), which requires such names be spelled with double i (ii). Both spellings are commonly used, but according to the ICNafp, P. jirovecii is correct. A change in the ICNafp now recognizes the validity of the 1976 publication, making the 1999 proposal redundant, and cites Pneumocystis and P. jiroveci as examples of the change in ICN Article 45, Ex 7. The name P. jiroveci is typified (both lectotypified and epitypified) by samples from human autopsies dating from the 1960s. The term PCP, which was widely used by practitioners and patients, has been retained for convenience, with the rationale that it now stands for the more general Pneumocystis pneumonia rather than Pneumocystis carinii pneumonia. The name P. carinii is incorrect for the human variant, but still describes the species found in rats, and that name is typified by an isolate from rats.
Pneumocystis genome Pneumocystis species cannot be grown in axenic culture, so the availability of the human disease-causing agent, P. jirovecii, is limited. Hence, investigation of the whole genome of a Pneumocystis is largely based upon true P. carinii available from experimental rats, which can be maintained with infections. Genetic material of other species, such as P. jirovecii, can be compared to the genome of P. carinii.
The genome of P. jirovecii has been sequenced from a bronchoalveolar lavage sample. The genome is small, low in G+C content, and lacks most amino-acid biosynthesis enzymes.
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