IMMUNIZATION AGAINST BRUCELLA INFECTION I

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Abstract
This study was based on the hypothesis that a constant antigenic stimulus over a lengthy period would best immunize against brucelloses and that such a stimulus would be best afforded by a living avirulent vaccine. A streptomycin-dependent mutant (SMd) was isolated from a virulent Brucella melitensis culture for this purpose. It was similar to the parent strain in cellular and colonial morphology, antigenic characteristics, and growth in the presence of dyes. It differed from the wild type in its requirement for the antibiotic, slower growth rate, and avirulence. A concn. of 2.5 [mu]g. streptomycin/ml. supported delayed growth of the variant in broth while 5-1000 [mu]g./ml. yielded opt. cell crops. Concns. above 1000 [mu]g./ml. inhibited growth. The variant was capable of limited "residual growth" in the absence of streptomycin due to intracellularly stored drug. Populations of the variant exhibited small numbers of mutants which had "reverted" to drug-independence and sensitivity with some loss of virulence compared to the wild type. Large inocula (1010 cells) failed to infect the guinea pig or induce pyrexia in the monkey. Mice were infected with 108 cells, administered subcut., and the animals subsequently cleared themselves of the organisms. Admn. of streptomycin to the variant in the mouse failed to stimulate in vivo growth of the organism. Mice and guinea pigs inoculated with large numbers of SMd organisms did not yield antibiotic-independent organisms from their tissues. The SMd variant may be useful in the living form as an avirulent vaccine against virulent Brucella melitensis infection in the goat.