Pneumocystis carinii: taxing taxonomy.

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.