Immunization with R mutants of Salmonella minnesota. II. Comparison of the protective effect of immunization with lipid A and the Re mutant.

The protective effect of active immunization with Salmonella minnesota Re bacilli or lipid A was assessed in the granulocytopenic rabbit model. Animals were immunized with Re bacilli, lipid A, or Pseudomonas aeruginosa as a nonspecific immunogen. After colonization with one of three enterobacterial strains (two Escherichia coli, one Enterobacter aerogenes), the immunized rabbits as well as controls given saline injections were made leukopenic with nitrogen mustard and monitored for fever, bacteremia, and death. Survival rates were significantly greater in Re-immunized animals than in saline controls or P. aeruginosa-immunized animals. Immunization with lipid A afforded no protection. In addition, rabbits immunized with Re mutant bacilli developed bacteremia less frequently than the others, indicating that antibody to Re may inhibit invasion of the intestinal mucosa as well as protect after bacteremia has developed.