Pneumocystis jirovecii

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Pneumocystis jirovecii
"P. jirovecii" cysts in tissue
P. jirovecii cysts in tissue
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Fungi
Division: Ascomycota
Class: Pneumocystidomycetes
Order: Pneumocystidales
Family: Pneumocystidaceae
Genus: Pneumocystis
Species:
P. jirovecii
Binomial name
Pneumocystis jirovecii
Synonyms
  • Pneumocystis carinii

Pneumocystis jirovecii (previously P. carinii) is a yeast-like

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

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. Whether meiosis takes place within the cysts, or what the genetic status is of the various cell types, is not known for certain.[2]

Homothallism

The lifecycle of P. jirovecii is thought to include both

ascospores
. The ascospores may be disseminated by airborne transmission to new hosts.

Medical relevance

Further information: Pneumocystis pneumonia

Pneumocystis pneumonia is an important disease of immunocompromised humans, particularly patients with

bone marrow transplant. In humans with a normal immune system, it is an extremely common silent infection.[4]

Identified by methenamine silver stain of lung tissue,

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.

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

Otto Jirovec, 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.[5]

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,[6] according to the International Code of Nomenclature for algae, fungi, and plants (ICNafp), which requires such names be spelled with double i (ii).[7] Both spellings are commonly used, but according to the ICNafp, P. jirovecii is correct.[8] 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. [9]

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.[9]

Pneumocystis genome

Pneumocystis species cannot be grown in 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.[10]

Microscopy image of P. jirovecii

The genome of P. jirovecii has been sequenced from a bronchoalveolar lavage sample.

G+C
content, and lacks most amino-acid biosynthesis enzymes.

History

The earliest report of this genus appears to have been that of Carlos Chagas in 1909,[12] who discovered it in experimental animals, but confused it with part of the lifecycle of Trypanosoma cruzi (causal agent of Chagas disease) and later called both organisms Schizotrypanum cruzi, a form of trypanosome infecting humans.[13] The rediscovery of Pneumocystis cysts was reported by Antonio Carini in 1910, also in Brazil.[14] The genus was again discovered in 1912 by Delanoë and Delanoë, this time at the Pasteur Institute in Paris, who found it in rats and proposed the genus and species name Pneumocystis carinii after Carini.[15]

Pneumocystis was redescribed as a human pathogen in 1942 by two Dutch investigators, van der Meer and Brug, who found it in three new cases: a 3-month-old infant with

co-evolved with each species.[24] Currently, only five species have been formally named: P. jirovecii from humans, P. carinii as originally named from rats, P. murina from mice,[25] P. wakefieldiae[26][27] also from rats, and P. oryctolagi from rabbits.[28]

Historical and even recent reports of P. carinii from humans are based upon older classifications (still used by many, or those still debating the recognition of distinct species in the genus Pneumocystis) which does not mean that the true P. carinii from rats actually infects humans. In an intermediate classification system, the various

taxa in different mammals have been called formae speciales or forms. For example, the human "form" was called Pneumocystis carinii f. [or f. sp.] hominis, while the original rat infecting form was called Pneumocystis carinii f. [or f. sp.] carinii. This terminology is still used by some researchers. The species of Pneumocystis originally seen by Chagas have not yet been named as distinct species.[9] Many other undescribed species presumably exist and those that have been detected in many mammals are only known from molecular sample detection from lung tissue or fluids, rather than by direct physical observation.[29][30]
Currently, they are cryptic taxa.

References

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  2. ^ "see DPDx life-cycle diagram". Dpd.cdc.gov. Archived from the original on 2013-03-18. Retrieved 2013-03-26.
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  7. epithets are formed by adding -i- (stem augmentation) plus the genitive inflection appropriate to the sex and number of the person(s) honoured (e.g. lecardii for Théodore Lecard
    ).
  8. ^ "International Code of Nomenclature for algae, fungi, and plants". www.iapt-taxon.org.
  9. ^
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  10. ^ "Pneumocystis Genome Project". Pgp.cchmc.org. Retrieved 2013-03-26.
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  12. ^ Chagas C (1909). "Neue Trypanosomen". Vorläufige Mitteilung. Arch. Schiff. Tropenhyg. 13: 120–122.
  13. doi:10.1590/S0074-02761909000200008
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  14. ^ Carini A. (1910). "Formas des eschizogonia do Trypanosoma lewisi". Soc Med Cir São Paulo. 38 (8).
  15. ^ Delanoë P, Delanoë M (1912). "Sur les rapports des kystes de Carini du poumon des rats avec le Trypanosoma lewisi". Comptes Rendus de l'Académie des Sciences. 155: 658–61.
  16. ^ van der Meer MG, Brug SL (1942). "Infection à Pneumocystis chez l'homme et chez les animaux". Amer Soc Belge Méd Trop. 22: 301–9.
  17. ^ Vanek J. (1951). "Atypicka (interstitiálni) pneumonie detí vyvolaná Pneumocystis carinii (Atypical interstitial pneumonia of infants produced by Pneumocystis carinii)". Casop Lék Cesk. 90: 1121–4.
  18. ^ Jírovec O. (1952). "Pneumocystis carinii puvodce t. zv intertitialnich plasmocelularnich pneumonii kojencw (Pneumocystis carinii, the cause of interstitial plasmacellular pneumonia in neonates)". CSL. Hyg. Epid. Mikrob. 1: 141.
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  30. PMID 11376046
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External links
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