EPIDEMIOLOGY OF PNEUMOCYSTIS-CARINII INFECTION - APPLICATION OF MOLECULAR-GENETIC METHODS

Pneumocystis carinii causes a life threatening, diffuse, extracellular infection of the pulmonary alveoli in 20-70% of immunosuppressed subj ects with AIDS, organ transplantation or malignancy. Although animal m odel experiments favour air borne transmission of infection, failure t o culture P. carinii has impeded efforts to clarify its epidemiology. A high prevalence of positive serology to P. carinii in normal individ uals has suggested, by analogy with tuberculosis, that opportunistic p neumonia in the immunosuppressed individual results from recrudescence of a dormant pulmonary focus of organisms, the legacy of childhood in fection. We have cloned a series of mitochondrial genes from P. carini i as a basis for i) conducting comparative DNA sequence studies and ii ) providing phylogenetic information for the development of a highly s pecific and sensitive DNA amplification technique for recognizing the parasite in different samples. We have not found carriage of P. carini i in normal lungs and have shown that P. carinii organisms infecting d ifferent mammalian hosts are genetically distinct. Comparative DNA stu dies based on DNA amplification products using P. carinii specific pri mers at reduced annealing temperatures in the polymerase chain reactio n, suggest that P. carinii is most closely related to the ustomycetous red yeast fungi. These organisms are found freely in the environment and produce widely disseminated air-borne spores. These data suggest t hat opportunistic Pneumocystis pneumonia results from fresh infection, acquired from an environmental source.