Kinetics of 2 genetically distinct Pneumocystis carinii populations in rat colonies.

The natural history of prototype and variant Pneumocystis carinii populations in several colonies of rats were monitored over time periods ranging from 1 to 4 years. Karyotypic profiles of organism populations from individual rats were evaluated for the proportion of prototype:variant P. carinii and the number of animals harboring co-infections were enumerated. Throughout the observation periods: (1) each P. carinii population retained its distinctive karyotype; (2) the proportions of prototype:variant within the lungs fluctuated; and (3) the number of rats co-infected within a colony varied. These surveys suggest a competition between 2 similarly adapted populations.