Resistance to reinfection in experimental mouse typhoid

Mice surviving infection with a strain of Salm. typhimurium of reduced virulence developed a progressive resistance to reinfection with virulent strains. The degree of resistance was greater than that of vaccinated mice, although the primary infection had not caused any significant degree of natural selection or higher serum titres of O antibody. The previously infected animals responded to reinfection with a virulent strain of Salm. typhimurium by gradually restraining bacterial growth. Many of the survivors eradicated the reinfecting strain. The possible reasons for the difference between vaccinated mice and survivors of previous infection have been discussed. I am indebted to the Medical Research Council for the provision of a grant towards the expenses of this investigation and to Mr T. Norris and Miss Italia S. Alderton for technical assistance.