The host specificity of Pneumocystis carinii

Pneumocystis carinii was discovered by Carlos Chagas at 1909. For almost one century this microorganism, frequently found in the lung of mammals, including humans, was considered as a unique widespread eukaryotic species. Only recently, molecular epidemiology studies revealed a highly important host species-related genetic diversify among Pneumocystis isolates from laboratory animals, domestic or wild mammals or humans. Markers were genes of mtRNAr, thymidylate synthase or other genes. DNA-Pulse Field Gel Electrophoresis karyotyping, antigens or multilocus enzyme electrophoresis (MEE) were also used. MEE researches should lead to the development of Pneumocystis population genetic studies. On the whole, although intraspecific variation was found in rats and humans, the dominant fact was that Pneumocystis isolates from different host species represent drastically distinct genotypes, which are most probably genetically isolated from each other. This result has obvious epidemiological implications, for it supports the view that P. carinii from different hosts cannot be infective to each other. This was a crucial point in so far as neither the sources of the human P. carinii infection, nor how humans become infected by this agent have been clearly established. Phenotypical differences, correlated to genomic, karyotypic or isoenzymatic host-related polymorphisms, were then investigated. Cross infection experiments revealed that Pneumocystis from a given mammal can not infect other host species. The absence of parasites in hosts inoculated with organisms from another host species was verified by using molecular methods, which were also used in order to identify accurately parasite isolates with DNA probes specific for Pneumocystis from each tested mammal species. Furthermore, differences in the in vitro behaviour were observed among Pneumocystis isolates from rat and mouse. In addition, a few but neat ultrastructural differences between rabbit- and mouse-derived Pneumocystis were found. Significative advances were recently made in the development of in vitro systems. Now, efforts should be made for obtaining viable Pneumocystis organisms from humans arid other mammals. Thus, the correlation between biological properties of Pneumocystis (capacity to attach to cultivated epithelial cells, drug sensitivity, infectivity to experimental hosts) and genetic typing could be investigated. Endly, as host specificity is directly linked to pneumocystosis epidemiology, particularly to factors involved in transmission and infectivity, we are investigating the cell and molecular bases of the Pneumocystis host specificity. Progresses in the knowledge of Pneumocystis taxonomy and in the development of efficient, reproductible experimental models, are providing new bases to understand the transmission of P. carinii pneumonia and to develop new prevention and control strategies.