The biology of Pneumocystis carinii

Some basic biologic aspects of Pneumocystis are reviewed. From the available data, it is evident that only two stages, the trophozoite and the cyst, have been described, but that the complete life cycle is unknown. Both of these two stages are found in the alveoli and other tissues of hosts with some forms of immune suppression, but rarely in healthy ones. The current views of how trophozoites evolve into cysts is outlined. No agreement about the classification of Pneumocystis has been reached, and its protozoan or fungal nature has not been fully confirmed. Recent data obtained with electron microscopy, and biochemical and molecular biologic studies, indicate that Pneumocystis has characteristics more akin to a fungus than to a protozoan. Moreover, the existence of one or of several species of Pneumocystis has not been determined. The mode of transmission of the parasite in nature, between different hosts, remains unconfirmed, although the cyst is believed to play a role.