Epidemiological and taxonomic impact of Pneumocystis biodiversity

A cluster of antigenic, genomic, karyotypic, isoenzymatic and morphological differences have been reported among Pneumocystis populations. Multilocus enzyme electrophoresis revealed strong linkage disequilibrium suggesting that Pneumocystis genotypes from different hosts have been genetically isolated from each other for a very long time. At least in some cases, genetic diversity is associated with phenotypic differences as revealed by in vitro, ultrastructural and cross infection studies. Thus, biodiversity in Pneumocystis has obvious epidemiological implications. Cross infection experiments revealed that Pneumocystis host species-related genetic differences are associated with close host species specificity, which suggests that transmission cannot take place between hosts of different species and that immunocompromised patients contract the infection primarily from infected humans. But these affirmations do not preclude other reservoirs for human pneumocystosis and research has to be extended to natural populations of synanthropic or wild mammals. Transmission of human pneumocystosis was also approached by typing human Pneumocystis isolates from patients or carriers, which should allow the follow up of parasite strains in human populations. As the strains of Pneumocystis found in different host species were considered for a long time to be morphologically indistinguishable, only one species of Pneumocystis was accepted for almost one century. At present, the scientific community is progressively accepting that the terminology 'P. carinii' is hiding a heterogeneous group of microorganisms. As available data made it impossible to establish if genetic divergence derives from clonal reproduction or speciation, no new species names have been attributed to Pneumocystis populations, but a trinomial nomenclature, including the Latin name of the host, was adopted in 1994. It has to be outlined finally that works on biodiversity of Pneumocystis populations are basically important as they have revealed a new group of eukaryotic, pathogenic, heterogeneous microorganisms with fungal affinities, difficult to cultivate until now and widely spread in ecosystems. These researches are opening a virgin field for microbiology research.