SUMMARY: Three initially streptomycin-sensitive strains of Mycobacterium tuber-cuZosis, two from patients with tuberculosis, and one the laboratory strain H37Rv, were grown for 6 weeks on Herrold s agar medium containing graded concentrations of streptomycin. Discrete colonies of M. tuberculosis that grew on the streptomycin media were tested for streptomycin sensitivity in Tween 80 albumin medium.

In all three strains the distribution of resistance was discontinuous, the subcultures from the colonies being either sensitive, or resistant to streptomycin in three distinct ranges of minimal inhibitory concentrations, namely, 0-25-0.5,1-2 and 32-64, ug./ml.; and of initial populations of 04-1-0 x lo9 bacilli, about 1000 were in the first, slightly resistant group, about 100 in the second, and about 2 were in the third, highly resistant group.

Of the two patients from whom the two of the strains were isolated, one developed a highly resistant strain in his sputum during treatment, whereas the other developed a strain only of slight resistance, in spite of prolonged streptomycin treatment.


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