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 parts of the life cycle are still undetermined, including the existence and the fusion of gametes, the course of meiosis and the fate of the meiotic products. We studied the different developmental stages of P. carinii cytophotometrically, using Schiffs 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 isogametes. The proposed meiosis was confirmed and is considered to be a one-step meiosis. The subsequent sporogony involves several mitotic divisions, 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 increase of parasites in the lungs of the host and may explain the often fatal course of infection.