Doubling time of Mycobacterium lepraemurium in mouse footpads

The doubling time of Mycobacterium lepraemurium (MLM) was measured in CBA/Ca mice. In eight experiments, 5 × 10³ MLM were inoculated into the hind footpads of groups of mice, and the organisms were harvested from 1-45 days later. The harvested organisms were enumerated, and the doubling time was calculated assuming that MLM had multiplied without a lag phase and that multiplication continued at a constant rate from inoculation to harvest. Simultaneously, the proportions of viable organisms in the inocula were determined by inoculation of serially diluted suspensions into the footpads of other mice, harvesting 4 months later and calculating the most probable number. MLM were observed to multiply rapidly during the first several days, and more slowly thereafter: the mean initial doubling time was determined to be 0.5 days, a value much smaller than those previously reported by other workers.