Ultrastructure of Rickettsiae with Special Emphasis on Ehrlichiae

Abstract

Basic morphological features of rickettsiae and ehrlichiae are similar to those of gram-negative bacteria. The uptake and intracellular fates of the rickettsiae in eukaryotic cells are markedly different. For example, Rickettsia tsutsugamushi cells are individually phagocytized by polymorphonuclear leukocytes in tightly enclosed phagosomal membranes. Lysosomes, labeled with electron-dense tracers or by acid phosphatase cytochemistry, do not fuse with the bacteria-laden phagosomes. Subsequently, the microorganisms are released from phagosomes into the host cytoplasm. Ehrlichia risticii, the etiologic agent of Potomac horse fever, infects macrophages and glandular epithelial cells of the intestines of horses. The microorganisms are pleomorphic and exist as two distinct morphological forms. The small forms (0.2 to 0.4 µm in diameter) are electron-dense and some were undergoing binary fission. Multiple small forms of the microorganisms occur in loosely enclosed host membrane vacuoles. On the other hand, larger forms (0.6 to 1.5 µm in diameter) are less electron-dense and individual microorganisms are tightly enveloped by host membranes. In contrast, Cowdria ruminantium, which belongs to the tribe Ehrlichiae, was found in loosely enclosed host membrane vacuoles. Lysosomal fusion with bacteria-laden vacuoles was not detectable. Rickettsial antigens were demonstrated in the capsular layer by immunoferritin labeling. The capsular component appeared extremely abundant in c. ruminantium. The outer membrane of rickettsiae has a thick outer leaflet (R. tsutsugamushi) or inner leaflet (R. prowazekii and R. rickettsii). The outer membrane of ehrlichiae does not show this variation in thickness. Rickettsiae, ehrlichiae, and C. ruminantium produce outer membrane vesicles of variable sizes in the host cell. A peptidoglycan layer in the periplasmic space is not obvious in rickettsiae or ehrlichiae. However, C. ruminantium had a large amount of peptidoglycan-like material between the outer and inner membrane. Although the rickettsial and ehrlichial microorganisms show similarities in their basic morphology, they vary markedly in their capsule, outer membrane, and peptidoglycan structure.