Opportunistic Atypical Fungus: Pneumocystis jiroveci

Published on 08/02/2015 by admin

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Last modified 08/02/2015

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Opportunistic Atypical Fungus

Pneumocystis jiroveci

Genus and Species to be Considered

Current Name Previous Name
Pneumocystis jiroveci Pneumocystis carinii

General Characteristics

In 1999 the name of the organism that causes a pneumonia in immunocompromised humans, commonly called pneumocystis pneumonia (PCP), was changed from Pneumocystis carinii to Pneumocystis jiroveci. (The causative organism for the rodent form of pneumocystis is still called P. carinii.) P. jiroveci is an opportunistic, atypical fungus that infects immunocompromised hosts and mostly manifests as PCP.

P. jiroveci originally was thought to be a trypanosome, but its precise taxonomic categorization remains challenging. Several factors supported the notion that P. jiroveci was a protozoan parasite: its morphology is similar to that of microbes and protozoa, and clinically it responds to antiprotozoal drugs but not to antifungal drugs in patients with pneumocystosis. Inability to maintain and propagate the organism in routine culture has further limited its characterization, although cultivation is possible under special conditions. P. jiroveci exists as three forms in its life cycle: trophozoite, precyst (sporocyte), and cyst (the latter is the diagnostic form).

Although P. jiroveci has been shown to be a fungus, it differs from other fungi in various aspects. Its cell membrane contains cholesterol rather than ergosterol. The flexible-walled trophozoite is susceptible to osmotic disturbances. Also, P. jiroveci contains only one or two copies of the small ribosomal subunit gene, whereas most other fungi contain numerous copies of this gene. DNA sequence analysis of the small ribosomal subunit gene in P. jiroveci has disclosed a greater sequence homology with the fungi than with the protozoa. Two independent analyses that compared the DNA sequences of P. jiroveci with those of other fungi confirmed the placement of P. jiroveci in the fungal kingdom, somewhere between the ascomycetes and the basidiomycetes (the closest yeast is the fission yeast, Schizosaccharomyces pombe). DNA analysis also confirmed the difference between the rodent and human forms of pneumocystis.


P. jiroveci has a worldwide distribution and most commonly presents as pneumonia in an immunocompromised host. As mentioned, infection appears to be species specific, with P. carinii causing disease in rodents and P. jiroveci causing human disease. The exact transmission of disease is still not known. Some speculate that pneumocystis is transmitted person to person. Immunocompetent mammals may be the reservoir for P. jiroveci, which is transmitted to immunodeficient individuals as a pathogen. Most children by age 2 to 4 have antibodies to pneumocystis. Vargas et al. showed that pneumocystis DNA was present in 24 of 72 infants, as determined from nasopharyngeal specimens, and that seroconversion occurred in 85% of infants by 20 months of age.

Since the onset of the human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic in the 1980s, pneumocystis has been defined as the most common opportunistic infection among those with HIV or AIDS in the United States. The introduction of highly active antiretroviral therapy (HAART) for patients with HIV has reduced the incidence of disease. However, PCP remains a significant medical problem, because numerous patients with HIV do not respond to therapy, do not comply with therapy, or do not know they are infected.