Pneumocystis carinii. Taxonomy as viewed by electron microscopy.

Pneumocystis carinii is an opportunistic unicellular organism that can cause serious pulmonary infection in immunosuppressed patients. The taxonomy and classification of P. carinii has not yet been settled. The authors present transmission and scanning electron microscopic (TEM and SEM) observations of tissue from two patients with pulmonary Pneumocystis infections. The infectious organisms display marked variability in shape and size. They appear to divide by binary fission and lack motility organelles, Golgi apparatus, phagosomes, and lysosomes. The mitochondria and endoplasmic reticulum were poorly developed. The nucleus was rather ill defined, and there appeared to be asynchrony in the development of nuclear membranes and cytoplasm. The authors contend that there are firm ultrastructural evidences against the claim for a protozoan nature of Pneumocystis and in favor of its being a fungus, albeit of a primitive form, in which the mycelium is reduced to a unicellular state but the ability to sporulate is preserved.