Localization of the protective antigen in Salmonella typhimurium.

Active protection to challenge with 100 LD50 of Salmonella typhimurium could be induced in white mice by inoculation with 10(3) live bacteria (0.1 LD50) or 10(8) heat killed bacteria (56 degrees C for 30 min), or 20 gamma ribosomal fraction. Passive protection could be given by antisera taken from hyperimmune rabbits inoculated with either the heat killed bacteria or by the ribosomal fraction. No direct correlation between antibody titre to 'O' antigen and degree of protection was observed. Absorption of any antisera with 'O' antigen decreases its protective power. The protective antibodies could be completely absorbed from both antisera by heat killed bacteria and released afterwards. Abosrption of 'O' antibodies from this preparation decreases the ability of the mice to survive the challenge. Our data suggests that the antigens which induce protective antibodies are located on the surface of the bacteria. The protection achieved by this antisera is probably the result of a synergistic effect between the antibodies to the 'O' antigen and other antigen(s).