Immunization Studies with Living Vaccine of Salmonella typhimurium.

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Abstract
A streptomycin-dependent mutant was isolated from a virulent strain of Salmonella typhimurium for evaluation as a living vaccine in mice. In vivo and in vitro studies indicated that this strain was unsafe as an immunizing agent due to reversions to streptomycin independence. A mutant of the streptomycin-dependent strain was obtained nequiring purine, a nutritional factor absent in mouse peritoneum. Reversions of this double mutant strain to the purine-strep-tomycin-independent state could not be detected. The results of experiments comparing this living vaccine with heat-killed preparation indicated that the living vaccine was not superior in protecting mice.