Comparison of the Ultrastructure of Several Rickettsiae, Ornithosis Virus, andMycoplasmain Tissue Culture

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Abstract
In an effort to make a valid comparison of the ultrastructure of several intracellular parasites, selected agents were propagated under identical conditions in a single type of tissue culture cell; such infected preparations were processed for examination by electron microscopy by use of a standardized procedure for fixation and embedding. The organisms studied were: the Breinl- and E-strains of epidemic typhus, Rickettsia prowazeki; the Bitter rot strain of R. rickettsii; the Karp strain of R. tsutsugamushi (R. orientalis); R. sennetsu; the P-4 strain of ornithosis virus; and the HEp-2 strain of Mycoplasma hominis type L Each of the rickettsial species examined had a cell wall and a plasma membrane, and contained ribosomes and deoxyribonucleic-acid (DNA) in a ground substance. However, certain differences were noted. Both strains of R. prowazeki contained numerous intracytoplasmic electron-lucent spherical structures (4 to 10 m/i), not previously described R. sennetsu, unlike the other rickettsiae, was not free in the host cytoplasm but was always enclosed in a vacuole; R. rickettsii was observed intranuclearly and in digestive organelles of the host cell as well as in the cytoplasm. Cells infected with ornithosis virus contained several forms representing the stages in its life cycle. The "initial bodies," made up of ribosomes and DNA strands, were morphologically similar to the rickettsiae. In cultures infected with M. hominis, most of the cells became large and multinucleate. Although the Mycoplasma organisms were readily cultivated from these cultures, only a few could be found in the electron microscope preparations. These organisms were extracellular and lacked a cell wall, being bound only by a unit membrane. Again, the internal components were ribosomes and DNA strands. Under the uniform preparative conditions employed here, the three groups of organisms were morphologically distinguishable from one another.