Studies of Morphology and Immunofluorescence of Pneumocystis carinii

Rat and human strains of Pneumocystis carinii were found to be identical in morphology of all the developmental stages. “Trophozoites” varied greatly in size and had paired nuclei located eccentrically. Transitional forms were also observed in addition to “cyst” forms with eight “sporozoites” which have been described previously. By these observations, life cycle of P. carinii inside the pulmonary alveoli was proposed. Uninfected rats were immunized with the rat strains of P. carinii. The pooled rat antisera contained specific antibody to homologous P. carinii through dilution of 1:2048 as detected by indirect fluorescent antibody (IFA) method. Eight rat and five human strains of P. carinii were then studied by IFA method. All of the rat strains fluoresced while none of human strains gave fluorescence which suggested antigenic dissimilarity between these two strains. Both cysts and trophozoite forms identified by Giemsa staining fluoresced by IFA method.