NEW ASPECTS IN THE LIFE-CYCLE OF PNEUMOCYSTIS-CARINII, REVEALED BY DNA MEASUREMENTS

P. carinii is the most important opportunistic pathogen causing severe and often fatal infection in the immunocompromised host. Certain part s of the life cycle are still undetermined, including the existence an d the fusion of gametes, the course of meiosis and the fate of the mei otic products. We studied the different developmental stages of P. car inii cytophotometrically, using Schiff's reagent as a staining dye. On the basis of the DNA measurements and light microscopical studies, we present evidence for different types of trophozoites, including isoga metes. The proposed meiosis was confirmed and is considered to be a on e-step meiosis. The subsequent sporogony involves several mitotic divi sions, until the mature cyst with eight intracystic bodies is formed, which start immediately with a binary fission after the rupture of the cyst wall. This additional multiplication step leads to a dramatic in crease of parasites in the lungs of the host and may explain the often fatal course of infection.