ULTRASTRUCTURAL VARIATION OF CULTURED EHRLICHIA-CHAFFEENSIS

The ultrastructure of Ehrlichia chaffeensis (Arkansas strain) was stud ied in nonirradiated and irradiated monolayers of mouse embryo, Vero, BGM and L929 cells, and in non-irradiated DH82 cells. Within the intra cellular parasitophorous vacuoles (morulae), two types of ehrlichial c ells were found regularly-those with uniformly dispersed nucleoid fila ments and ribosomes (reticulate cells) and smaller ones with centrally condensed nucleoid filaments and ribosomes (dense-cored cells), which represent the normal life cycle of ehrlichiae. In addition, large ret iculate cells were observed, forming long projections of the cell wall , protrusions of cytoplasmic membrane into the periplasmic space, or b udding of protoplast fragments (minute forms) into the periplasmic spa ce. Ehrlichiae with abnormalities of protoplast fission were found, ap parently leading to formation of giant, multilobular or elongated rod- like ehrlichiae. Morulae were usually surrounded by cisterns of granul ar endoplasmic reticulum and mitochondria and often contained vesicles , long tubules 25 nn in diameter, probably originating from the ehrlic hial cell wall, and fibrillar ehrlichial antigen apparently shed from the surface of the cell wall. Some cells contained, in addition to nor mal morulae, a whole morula that had become dense and contained degene rating ehrlichiae. These results indicate that as well as normal growt h and reproduction, ehrlichiae exhibit pathological events: they can b e remarkably damaged inside the host cell vacuoles, presumably phagoly sosomes, or enter a process morphologically similar to bacterial L-tra nsformation.