Pneumocystis carinii continues to surprise us:
Its new name and the features underlying the controversies about its taxonomy
The genus
Pneumocystis, first reported by Chagas in 1909 as a morphologic form of
Trypanosoma cruzi, was later clarified to be a distinct microorganism and named as
Pneumocystis carinii. Until the late 1980s,
Pneumocystis carinii was accepted as a protozoon based on these findings:
- Morphologic features typical for protozoa,
- Absence of ergosterol in the cell membrane and resistance to inhibitors of ergosterol biosynthesis,
- Susceptibility to some drugs used to treat protozoal infections, and
- Lack of in vitro growth on media used for isolation of fungi.
All seemed clear until 1988 when rRNA analysis interestingly revealed that
Pneumocystis was a fungus rather than a parasite. In addition to rRNA and mitochondrial DNA sequence homology to fungi, there were other data suggesting the validity of this new taxonomy for
Pneumocystis:
- Ultrastructural and staining features similar to fungi,
- Existence of ß-1,3-glucan in its cell wall and susceptibility to agents that inhibit glucan synthesis,
- Poorly developed mitochondria containing lamellar cristae (in contrast to protozoan mitochondria with tubular cristae),
- Existence of translocation elongation factor-3 gene which is unique to fungi,
- Existence of thymidylate synthase (TS) and dihydrofolate reductase (DHFR) as distinct proteins (in contrast to TS and DHFR being a single protein in protozoa) and homologous to Saccharomyces cerevisiae,
- Cross-reactivity of its monoclonal antibodies with fungi, and
- Similarity between its sporogenous state and ascospore formation in some yeasts.
Not surprisingly, the acceptance of
Pneumocystis as a fungus took some time. But, we are not yet to the end of the taxonomic road for
Pneumocystis carinii as it now appears that there are multiple species in this genus. The first evidence suggesting the presence of phenotypic differences between the
Pneumocystis isolated from different mammals came decades ago. This was followed by suggestion of the name
Pneumocystis jiroveci (pronounced as ""yee row vet zee") in 1976 by Frenkel for the
Pneumocystis species isolated from humans. However, this name did not gain acceptance at that time. Molecular studies and DNA analysis found that
Pneumocystis isolated from humans and other animals were quite different and human-derived samples of
Pneumocystis proved to contain the DNA of only
P. jiroveci. In addition,
P. jiroveci DNA was not detected in any samples obtained from other mammals, including non-human primates. These data all established
P. jiroveci as a distinct species unique to humans and the name
P. jiroveci rather than
P. carinii should be used to refer to the
Pneumocystis of humans. This new name first appeared in 1999 and is gradually gaining acceptance. The name was given in honor of the Czech parasitologist Otto Jirovec who described the microorganism in humans. Despite the change in the name of the organism, the acronym PCP is retained and now refers to
Pneumocystis pneumonia.
Regardless of its new taxonomy and name,
Pneumocystis remains as a significant cause of pneumonia and extrapulmonary pneumocystosis particularly in patients with cellular immunodeficiency, including AIDS.
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