The outer envelope (OE) of Treponema is a membranelike structure that surrounds the flagella (axial filaments) and protoplasmic cylinder (PC). Morphological and chemical studies (1; 2; J. Pillot, Ph.D. Thesis, Univ. of Paris, 1965) have shown that the O E is a triple-layered structure that has a complex chemical structure including proteins, lipids, and carbohydrates. It is resistant to lysozyme but sensitive to a variety of lipid solvents and surface-acting agents, including sodium dodecyl sulfate (SDS).The O E of T. pallidum and other parasitic spirochetes appears to play an important role in the host-parasite interaction. Infected hosts produce antibodies that immobilize or kill the infecting spirochete in vitro in the presence of complement, and in parasitic leptospires it has been demonstrated that the site of antibody-complement action is the OE (3). The OE of parasitic leptospires is an easily extractable immunogen (4). Animals vaccinated with purified leptospiral OE are immune to challenge (4, 5) and possess antibodies that are leptospiricidal and protective.It is not yet known if the O E of T. pallidurn plays a similar protective role in syphilitic infections, but if so, then the potential exists for the development of an O E vaccine for syphilis. Cultural techniques are not presently available for the in vitro propagation of T. pallidurn, but it is possible to study the nonvirulent cultivatable treponemes in order to learn more about the physical, chemical, and antigenic nature of treponemal OE. A method for the solubilization and reaggregation of the O E of T. phagedenis biovar Kazan 5 has been described previously (6). In the present study, O E was obtained by this method from T. phagedenis biovar Kazan 5 and the chemical com-
Exposure of the avirulent Kazan 5 treponeme to low concentrations of sodium dodecyl sulfate (1.4 m
m
) results in the solubilization of the outer envelope. Maximal reaggregation of the outer envelope preparation requires cations, and cation concentration affects both the yield and structure of the aggregates. The reaggregated outer envelope preparation can be resolubilized with the chelating agent, ethylenediaminetetraacetic acid, or with sodium dodecyl sulfate.
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