Abstract
Secondary chicken embryo fibroblasts infected with the Sheila Smith strain of Rickettsia rickettsii and grown in monolayer culture undergo rapid morphological alterations. Transmission electron microscopic examination of cells at intervals after infection showed several progressive host cell lesions, including widespread dilatation of the rough endoplasmic reticulum and outer nuclear envelope and the accumulation of electron-dense material within the cisternae of intracellular membranes. Dilatation of the rough endoplasmic reticulum is a common, early reversible manifestation of other forms of cell injury. However, the severity of the damage to the host cell resulting from the progressive distention of intracellular membranes and the subsequent formation of small segments of membrane-bound host cytoplasm within the cisternae of these membranes is unknown. Early in the infection cycle, the rickettsiae were found free in the host cell cytoplasm, within invaginations of the nuclear envelope, occasionally free in the space between the outer and inner nuclear membranes, and in the host nucleoplasm, but not within cisternae formed by swollen endoplasmic reticulum. As a consequence of intracisternal swelling and fusion of intracellular membranes later in the infection cycle, the majority of the rickettsiae were found surrounded by host cytoplasm bound by host-derived internal membranes and appeared to persist in this state until cell lysis. The overall cytopathological changes in cells infected with R. richettsii appear dramatic and, from other studies in our laboratory, are significantly different from those observed in cells infected with Rickettsia prowazekii.