Genetic heterogeneity in Pneumocystis carinii: an introduction.

Molecular techniques have played an important role in demonstrating a high level of heterogeneity among the different types of organisms which are collectively known as Pneumocystis carinii. Genetic heterogeneity has been observed in Pneumocystis organisms isolated from different host species, suggesting that Pneumocystis infection is host-species specific. On the basis of this genetic divergence a provisional trinomial nomenclature has been adopted, in which different types of Pneumocystis organisms are designated a 'special form'. Multiple special forms of Pneumocystis have been detected in some host species, for example in infected rat and ferret lungs, but not in human lungs. Lower levels of genetic heterogeneity have been observed within each Pneumocystis special form, and in particular in human-derived Pneumocystis. Analysis of the genetic heterogeneity of populations of Pneumocystis is contributing to the understanding of the epidemiology and pathophysiology of this infection.