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General Characteristics of Pneumocystis jiroveci

المؤلف:  Patricia M. Tille, PhD, MLS(ASCP)

المصدر:  Bailey & Scotts Diagnostic Microbiology

الجزء والصفحة:  13th Edition , p768

2025-12-04

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 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 organ ism 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 mem brane 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.