Acquired resistance against Listeria monocytogenes in red mice and CF1 mice immunized with strains of BCG or Mycobacterium tuberculosis.

Groups of red mice and CF1 mice immunized intravenously with varying doses of a weak or a strong strain of BCG or a strain of Mycobacterium tuberculosis were challenged 3 weeks after immunization with 0.1 or 0.2 ml 10(-2) Listeria monocytogenes injected intravenously, simultaneously with a non-immunized control group. The acquired resistance was determined on the basis of the number of survivors and the survival times of the animals that died spontaneously. In the red mice, the strong BCG strain induced a definitely higher resistance than the weak strain, and the M. tuberculosis strain a slightly higher resistance than the BCG strains. As in red mice, the resistance of CF1 mice was higher in animals immunized with M. tuberculosis than in those immunized with the BCG strains. However, the difference in the survival times of mice immunized with the two strains of BCG was much less than in red mice, and was only clearly significant as regards one of the doses used. The relationship between the virulence of a mycobacterial strain and its ability to induce acquired resistance against an infection with listeria or against an infection with virulent tubercle bacilli is discussed. It is concluded that red mice are more suitable than CF1 mice for evaluation of the protective potency of a BCG strain.