The disease of CBA and BALB/c mice that follows inoculation of a small number of Mycobacterium lepraemurium into the hind foot pad

To learn if the lack of an immune response in mice infected with Mycobacterium lepraemurium (MLM) was a consequence of the organisms, we studied the disease that followed inoculation of ≤ 5000 organisms into the hind foot pads of CBA and BALB/c mice. The mice of both strains demonstrated a rapid increase of bacterial numbers soon after inoculation, with a slowing of the rate of multiplication once the number of organisms per foot pad passed 3 x 10⁷. By 1 year after inoculation, the numbers of organisms had reached levels ≥ 10¹¹ in the spleen and liver, and ≥ 10⁸ in the femoral bone marrow. In mice that had been inoculated with as few as 5 MLM or 50 MLM, the organisms had multiplied to numbers > 10⁸ in the foot pads and to ≥ 10⁹ in the spleens, suggesting that the ID₅₀ of viable MLM may be ≤ 5 organisms per foot pad. No protection against superinfection could be demonstrated. On the other hand, initial multiplication of MLM in the foot pads was followed virtually immediately by the death of at least 97% of the organisms.