Pneumocysitis jirovecii's evolving taxonomy and role in disease

P. jirovecii is a unicellular eukaryote which shares characteristics with both protozoa and fungi leading to years of debate on the proper taxonomy of this microorganism. P. jirovecii was initially reported by Chagas in 1909 as a morphologic form of Trypanosoma cruzi, but later proved to be a separate genus and was named Pneumocystis carinii in 1912. P. carinii was classified as a protozoan until 1988, when P. carinii was placed in the fungal kingdom following ribosomal RNA (rRNA) analysis. The molecular data demonstrated that despite the fact that many of P. jirovecii’s morphological features are similar to those of protozoa, the rRNA and mitochondrial sequence of Pneumocystis are homologous to those of fungi. For more on the morphological features of P. jirovecii check out the life cycle post below

Further work revealed that the Pneumocystis from humans and other animals are different, indicating that there are multiple species in this genus. In 1994 an interim trinomial name change was adopted, the Pneumocystis that infects humans was called P. carinii f.sp hominis, and P. carinii f.sp carinii was used for one of the two species infecting rats.  In 1976 Frenkel suggested the name P. jiroveci (pronounced as "yee row vet zee"), for Pneumocystis isolated from humans, but this was not accepted until 2002. According to the International Code of Botanical Nomenclature (ICBN), P. jiroveci was formed incorrectly and was changed to P. jirovecii in 2006. Currently there are five species identified under the genus Pneumocystis. P. carinii and P. wakefieldiae infect rats, P. murina infect mice, P. oryctolagi infect rabbits, and P. jirocecii infects humans. (Aliouat-Denis et al., 2008) Further genetic research is underway to identify new species of Pneumocystis, as well as new strains in known species.

Pneumonia due to Pneumocystis (PCP) rarely occurs in people with normal immune systems, but is common among people with weakened immune systems, such as infants, patients with AIDS, and patients that are taking immunosuppressive medications or recently underwent an organ or bone marrow transplant. The risk of pneumonia due to P. jirovecii increases when CD4 positive cell levels are less than 200 cells per microliter. For this reason PCP was once recognized as the most common AIDS defining disease; however, the incidence of PCP in patients with AIDS has declined due to the introduction of HAART (highly active antiretroviral therapy) in 1996. P. jirovecii remains one of the major causes of opportunistic mycoses in immunocompromised patients.Check out the videos below for more information on PCP.

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References:
Tortora, G. J., Funke, B. R., Case, C. L. (2010). Microbiology: An introduction (10^th ed) (pp. 697). San Francisco, CA: Pearson Benjamin Cummings.

DoctorFungus Corporation. (2007). Pneumocystis spp., Retrieved from http://www.doctorfungus.org/thefungi/pneumocystis.php

CDC Division of Parasitic Diseases and Malaria. (July 2009). Pneumocystis jirovecii Infection, Retrieved from http://www.dpd.cdc.gov/dpdx/html/pneumocystis.htm

Hughes, W. T. (February 2003). Pneumocystis carinii versus Pneumocystis jiroveci: Another Misnomer. Emerging Infectious Diseases, Volume 9 (2). doi: 103201/0902020602

Tietjen, P. A. (May 2010). Clinical presentation and diagnosis of Pneumocystis carinii (P. jirovecii) infection in HIV-infected patients, Retrieved from http://www.uptodate.com/patients/content/topic.do?topicKey=~Vgggn9DNrPyrIl

Larsen, J. H. (May 2004). Pneumocystis jirovecii, Retrieved from http://www.danmedbul.dk/DMB_2004/0304/0304-disputatser/DMB3677.pdf

Aliouat-Denis, C., Chabe, M., Demanche, C., Aliouat, E. M., Viscogliosi, E., Guillot, J., Delhases, L., Dei-Cas, E. (September 2008). Pneumocystis species, co-evolution and pathogenic power. Infection, Genetics, and Evolution, Volume 8 (5). doi: 101016/200805001

Life cycle:

Asexual phase: trophic forms replicate by mitosis to .  Sexual phase: haploid trophic forms conjugate and produce a zygote or sporocyte (early cyst) .  The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst) .  Spores exhibit different shapes (such as, spherical and elongated forms).   It is postulated that elongation of the spores precedes release from the spore case.  It is believed that the release occurs through a rent in the cell wall.  After release, the empty spore case usually collapses, but retains some residual cytoplasm .  A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist.  The organism causes disease in immunosuppressed individuals.

Image & caption found on the following website:

 http://www.dpd.cdc.gov/dpdx/html/pneumocystis.htm

Histopathology Lung--Pneumocystis pneumonia

Please note that the speaker is referring to Pneumocystis jirovecii by the term Pneumocystis carinii. This was the correct name for this species at the time that this video was made; however, Pneumocystis jirovecii is the most current and correct term.

What is Pneumonia ? animation