Pneumocystis carinii: taxing taxonomy
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1Department of Infectious Diseases, St. Jude Children's Research Hospital, Memphis, TN 38101.
Abstract:
The detailed taxonomy of Pneumocystis carinii has not been decided. Most experts agree that it is either a fungus or a protozoan. In favor of a fungal designation are the recent reports that 16s and 18s rRNA sequences of P. carinii have greater homology to Saccharomyces cerevisiae and Neurospora crassa than to selected protozoa. Other studies show that sequences of the 18s and 26s rRNA, microtubular ultrastructure, membrane fusion potential and DNA content per cell resemble protozoa more than fungi. Therapeutic responsiveness to drugs effective against protozoa, namely pentamidine (Trypanosoma, Leishmania, sp), trimethoprim-sulfamethoxazole (Isospora sp), pyrimethamine-sulfadiazine (Toxoplasma, Plasmodium sp) and nonresponsiveness to antifungal drugs such as amphotericin B and ketoconazole indicate characteristics similar to protozoa. Taxonomic placement is based upon multifactional determinants, not a single attribute. Data are not yet available to make such a determination for P. carinii. Furthermore, detailed rules on qualifying criteria for both protozoan and fungal classifications are lacking, especially in the utilization of methods in molecular genetics for this purpose.
Insights
The classification of Pneumocystis carinii remains uncertain, with conflicting evidence regarding its fungal or protozoan nature. Further research is needed to definitively place this organism taxonomically.
Area of Science:
- Microbiology
- Mycology
- Parasitology
Background:
- The taxonomic classification of Pneumocystis carinii is debated, with arguments for both fungal and protozoan designations.
- Existing data present conflicting characteristics when compared to known fungi and protozoa.
Purpose of the Study:
- To review the evidence for and against the fungal or protozoan classification of Pneumocystis carinii.
- To highlight the limitations in current data and taxonomic criteria for Pneumocystis carinii.
Main Methods:
- Comparative analysis of rRNA gene sequences (16s and 18s rRNA) between P. carinii, fungi (Saccharomyces cerevisiae, Neurospora crassa), and protozoa.
- Evaluation of ultrastructural features, membrane fusion potential, and DNA content.
- Assessment of therapeutic responses to antimicrobial and antifungal agents.
Main Results:
- rRNA sequence homology data show greater similarity to fungi, while other molecular and cellular characteristics resemble protozoa.
- P. carinii exhibits therapeutic responsiveness to antiprotozoal drugs (pentamidine, trimethoprim-sulfamethoxazole, pyrimethamine-sulfadiazine) but not to common antifungal agents (amphotericin B, ketoconazole).
Conclusions:
- Current data are insufficient for a definitive taxonomic placement of Pneumocystis carinii.
- Multifactorial determinants are crucial for classification, and clear criteria, especially incorporating molecular genetics, are lacking for both protozoan and fungal classifications.