The Effect of Penicillin on the Growth of Mycobacterium tuberculosis in Dubos' Medium

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Abstract
Depending upon the size of inoculum, human, bovine, and avian types of M. tuberculosis were inhibited by concs. of penicillin varying from 1 to> 200 U./ml. When inocula were adjusted to permit 90% transmission of light at 620 m/x through a depth of 19 mm., representing an inoculum of approximately 106 organisms /10 ml. of test medium, growth of the tubercle bacillus was inhibited by drug concs. of 40-200 U./ml. Lower concs. of penicillin were effective when smaller inocula were used. With a 1:10 dilution of standard cell suspension as inoculum, 1/2 the original inhibiting conc. frequently produced complete inhibition of visible growth. This trend was maintained with higher dilutions of inoculum. Among the human strains, no marked difference in sensitivity was observed between a standard laboratory strain (H37Rv) and strains recently isolated from sputum. The strains of bovine and avian type were more sensitive to penicillin than human ones.