Protective role of O antigen in Salmonella typhi-murium infection.

Mice were actively immunized with preparations produced from different Salmonella strains. They were challenged with 2 LD50 of a virulent S. typhi-murium strain and examined for viable cell counts in the liver 4, 7 and 11 days postinfection. Whole cell vaccines, ribosomal extracts and endotoxin preparations of the O antigen-deficient variant of S. typhi-murium strain LT2-MI failed to protect the mice or did so in a much lower degree than preparations of the corresponding O antigen-bearing variant. Preparations from other salmonellae exerted a protective action only if the strain had an O-antigen identical with that of S. typhi-murium. The results pointed to a considerable protective role of the O antigen.