Different ultrastructural morphology of Pneumocystis carinii derived from mice, rats, and rabbits.

Pneumocystis carinii (PC) is a fungus present in the lungs of many mammal species. Even though studies of the genome, the isoenzymes, and the antigens have proved some host-species-linked heterogeneity, the existence of distinct Pneumocystis species or subspecies has still not been accepted. Comparative studies of the ultrastructural morphology of pneumocysts derived from several host species may support evidence of host-species-linked heterogeneity. We have compared the ultrastructural morphology of pneumocysts derived from mice, rats, and rabbits. The density of membrane-limited electron-dense cytoplasmic granules was found to be higher in mouse-derived pneumocysts than in rabbit-derived pneumocysts, and furthermore the average diameter of the granules from mouse pneumocysts was larger than that of granules from rabbit-derived pneumocysts. The average diameter of the filopodia of mouse-derived pneumocysts was smaller than that of filopodia from rat-derived pneumocysts, which was smaller than that of filopodia from rabbit-derived pneumocysts. Globular electron-dense bulbous dilatations at the tip of the filopodia were described for the first time and they were only found on filopodia of mouse-derived pneumocysts. These distinct host-species-linked morphological differences of pneumocysts from mouse, rat, and rabbit may support previous biochemical data indicating the existence of different Pneumocystis species or subspecies.