Studies of Mycobacterium lepraemurium in cell culture. II. Pathogenicity of Mycobacterium lepraemurium maintained in mouse foot pad cell culture and interaction of the bacilli with the infected cells.

A serially diluted bacterial suspension of the Kurume-42 strain of Mycobacterium lepraemurium maintained for 1255 days in a mouse foot pad (MFP) cell culture was inoculated in mice subcutaneously. The ID50 value was estimated at more than 10.7 and less than 85 organisms, indicating that pathogenicity of the organism had been maintained well in a long-term cell culture. The cells infected and maintained for a long period in the cell culture showed all the stages of cell mitosis. This suggests that the bacterial increase in cell cultures of M. lepraemurium is not only due to rephagocytosis of the bacilli released from the infected cells but also to a constant intracellular growth cycle of the bacilli accompanied by mitosis of the infected cells. In acid phosphatase activity, no appreciable differences were noted between the infected and uninfected cells as far as the present cell culture system was concerned. Most of the bacilli within the cells were ultrastructurally normal. Solid bacilli in phagosomes were surrounded by less electron-dense clear zones.