Immunization of rabbits with Spirochaeta aurantia does not induce resistance to Treponema pallidum

Rabbits were immunized with viable Spirochaeta aurantia, a free-living, facultative anaerobic spirochete that is similar in some biochemical characteristics to Treponema pallidum, a parasitic, microaerophilic spirochete. Single and multiple immunizations with living S. aurantia, with or without Freund's incomplete adjuvant, Freund's complete adjuvant, or heat-killed T. pallidum, were carried out over a four-month period. Living S. aurantia was neither toxic nor virulent for rabbits. Immunized rabbits produced a high level of agglutinating antibody to S. aurantia but no antibody to T. pallidum, as determined by the T. pallidum hemagglutination test. Immunized rabbits were challenged with multiple intradermal inoculations of 100 viable T. pallidum (Nichols strain) and compared to unimmunized rabbits similarly infected. Immunization with S. aurantia did not protect against T. pallidum infection. Thus S. aurantia appears not to be suitable as a potential vaccine against infection with T. pallidum.