Immunogenicity of R factor-carrying Salmonella.

An increased frequency of rough cultures with associated reductions in mouse virulence was detected after R-factor transmission into strains of Salmonella choleraesuis var. kunzendorf and S. typhimurium. All rough strains tested, with or without R factors, conferred protective immunity against challenge with homologous or heterologous species of Salmonella, some of which contained R factors. Although slight protection was afforded against challenge with Staphylococcus aureus, the immunity was specific to the extent that immunized mice were fully susceptible to infections with Klebsiella pneumoniae and Pasteurella multocida.