What is the microbiology of the Pneumocystis genus?

Updated: Apr 24, 2019
  • Author: Shelley A Gilroy, MD, FACP, FIDSA; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Pneumocystis is a genus of unicellular fungi found in the respiratory tracts of many mammals and humans. Distinct genomic variability exists between host-specific members of the genus. The organism was first described in 1909 by Chagas and then a few years later by Delanöes, who ultimately named the organism in honor of Dr. Carini after isolating it from infected rats. Years later, Dr. Otto Jirovec and his group isolated the organism from humans, and the organism responsible for PJP was renamed after him. [4, 5]

The taxonomic classification of the Pneumocystis genus was debated for some time. It was initially mistaken for a trypanosome and then later for a protozoan. In the 1980s, biochemical analysis of the nucleic acid composition of Pneumocystis rRNA and mitochondrial DNA identified the organism as a unicellular fungus rather than a protozoan. Subsequent genomic sequence analysis of multiple genes including elongation factor 3, a component of fungi protein synthesis not found in protozoa, further supported this notion.

The organism is found in 3 distinct morphologic stages, as follows:

  • The trophozoite (trophic form), in which it often exists in clusters

  • The sporozoite (precystic form)

  • The cyst, which contains several intracystic bodies (spores)

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