By making use of the increased phagocytosis which follows the exposure of HeLa cells to tissue culture media containing selected horse sera, it was possible to introduce all of the mycobacterial species studied into the cells, where many of them proceeded to grow.

Fully virulent strains of tubercle bacilli filled much of the cytoplasm in a few days and formed characteristic cords not seen with other strains. The strains said to be less virulent, R1Rv, BCG, H37Ra, and R1Ra, grew less rapidly and in characteristic patterns. Their rates of multiplication in HeLa cells were in the order named and correlated well with their reported pathogenicity for mice and guinea pigs. Six INH-resistant strains grew at rates characteristic of fully virulent strains.

Among the "rapidly growing" species, M. phlei and M. smegmatis did not show evidence of growth in the cells, although M. fortuitum did.

Some strains with optimal temperatures on bacteriological media below 37°C, M. balnei, M. marinum, and M. platypoecilus, grew rapidly in HeLa cells, especially at temperatures of 31 to 35°C.

The growth patterns of the bacilli in HeLa cells appear sufficiently specific to be useful in differentiation among the mycobacteria.

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