External layers of Rickettsia prowazekii and Rickettsia rickettsii: occurrence of a slime layer.

Using a simple specific-antibody stabilization procedure on organisms gently liberated from their host cells, we have demonstrated by electron microscopy that Rickettsia prowazekii and Rickettsia rickettsii possess a coat of variable thickness, external to the outer leaflet of the cell wall and the structure designated by others as a "microcapsule," which corresponds most closely to the slime layer of certain other bacteria. Reactions in the methenamine silver and ruthenium red staining procedures and the failure to be visualized by standard procedures suggest that the slime layer is largely polysaccharide in nature. It is postulated that this slime layer accounts in large part for the large, electron-lucent, halo-like zone which is found by electron microscopy to surround organisms of the typhus and spotted fever groups in the cytoplasm of their host cells, that it may be the locus of some major group-specific antigens, and that it may function as an antiphagocytic mechanism, as an aid for attachment of rickettsiae to potential host cells, or both. Moreover, because the attenuated E strain of R. prowazekii has been shown to possess a substantial slime layer, the basis for attenuation is not likely to be a simple smooth-to-rough variation.