Development of a rickettsia isolated from an aborted bovine fetus.

An obligate intracellular rickettsial organism isolated from an aborted bovine fetus was studied in bovine turbinate and mouse macrophage cell cultures with light and electron microscopy. Development of the organism was similar in both cell types. The organism replicated within cytoplasmic vacuoles in a developmental cycle that resembled that of both the ehrlichiae and chlamydiae. The inoculum contained only electron-dense forms, which infected cells within 2 h postinoculation by adhering to cell membranes at thickened areas that appeared to be coated pits and then being endocytosed. A striking feature occurred next as the organisms became surrounded by host cell mitochondria and, by light microscopy, appeared to have halos. During this intimate association with mitochondria, the electron-dense organisms changed into large reticulated forms that began to divide by binary fission. These large forms were often in direct contact with mitochondrial membranes. The organisms continued to divide by binary fission, and host cells contained large cytoplasmic inclusions of reticulated organisms. The reticulated organisms gradually changed into electron-dense forms that were released from degenerated host cells.