The structure of five parasitic strains of Bdellovibrio bacteriovorus was studied by electron microscope after negative staining and in shadow-case and etched freezefractured preparations. Special attention was paid to the cell wall and the flagellar sheath which is continuous with the wall or part of it. These structural components reveal distinct features which are induced by certain staining substances; they are exceedingly susceptible to disruption by physical treatments, and in old cells

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... ten appear impaired. In freeze-fractured cells the wall shows characteristic fracturing tendencies not known in other microorganisms. These structural properties and features are distinct to Bdellovibrio wall and flagellar sheath, the structural integrity of which is a fundamental requirement for the infectivity and survival of this organism. The anterior end of Bdellovibrio is differentiated: 6 to 12 ring-like structures (9 to 12 nm, outer diameter) are built into its wall and several fibers (7 to 10 nm wide, up to 1.5 ,um long) emerge from it. Intracellular structures, which are revealed as compact oval bodies bulging from the cell border and have internal laminated organization, are characteristic of Bdellovibrio after negative staining with certain compounds. These findings on the structure of parasitic Bdellovibrio substantiate previous observations indicating the uniqueness of this organism and add criteria for the identification of this genus. Proc., p. 41, 1967). MATERIALS AND METHODS Organisms. Five strains of B. bacteriovorus were employed: 109 and D (obtained from M. Shilo, Hebrew University Medical School, Jerusalem, Israel), and ATCC 15143, ATCC 15362, and ATCC 15360 (obtained from the American Type Culture Collection). Escherichia coli B (obtained from M. Shilo) was used as host organism to maintain stock cultures of Bdellovibrio 109 and D. E. coli ATCC 15144 and Proteus mirabilis ATCC 15363 were used as host organisms to maintain stock cultures of Bdellovibrio 15143 and 15362, and Bdellovibrio 15360, respectively. Enterobacter aerogenes ATCC 15361, Spirillum serpens NCIB 9510 (obtained from the National Col-948 on May 8, 2020 by guest