Immunogenicity of a low-virulence vaccinal strain against Salmonella abortus-ovis infection in mice.

Strain Rv6, a streptomycin-independent reverse mutant of Salmonella abortus-ovis, multiplied and disseminated in vivo to a lesser extent than the virulent parental strain 15/5, as determined by subcutaneous inoculation of two outbred strains of mice with different susceptibilities to salmonella infection. This lower virulence of strain Rv6 was not modified by 10 passages in susceptible mice. Vaccination with strain Rv6 induced a dose-dependent capacity of mice to control subsequent infection with the virulent parental strain. Induced resistance began as early as 3 days after vaccination and lasted at least 11 months. Its efficiency was controlled by different routes and doses of challenge. Strain Rv6 induced a greater protection than that conferred by heat-killed bacteria or by tested commercial killed vaccines.