HeLa, monkey kidney, and human amnion cells in tissue cultures were compared as sites for the multiplication of strains of tubercle bacilli or original and reduced pathogenicity, and for several other species of mycobacteria capable of causing disease in humans.

The arrangement of the pathogenic species inorder of their growth rates in HeLa cells was Mycobacterium fortuitum, Mycobacterium balnei, and the "yellow bacillus," followed closely by the tubercle bacillus. This order was also correct for these species in monkey kidney and human amnion cells, and is the same as that seen in bacteriological media.

The arrangement of the strains of tubercle bacilli in order of their growth rates in all three types of cells was: H37Rv, then R1Rv, and lastly H37Ra, which multiplied about as slowly as BCG. An INH-resistant strain grew about as rapidly as H37Rv.

Growth of the pathogenic species occurred at about the same rates in HeLa and monkey kidney cells, but was distinctly slower in human amnion cells, which are less active metabolically.

Irradiation of the cells in doses up to 5000 r did not affect the subsequent growth of mycobacteria in them.

Preliminary experiments with human leprosy bacilli indicate that they can be introduced into these cells in high numbers and that the bacilli then persist for the life of the cells.

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