The behaviour of mixtures of streptomycin-sensitive and -resistant tubercle bacilli in liquid medium sensitivity tests.

A method for testing the sensitivity to streptomycin of strains of Mycobacterium tuberculosis has been recommended by the Streptomycin in Tuberculosis Trials Committee of the Medical Research Council (1948a). The tests are carried out in a modified Tween 80-albumen liquid medium (Dubos and Davis, 1946) containing various concentrations of streptomycin. It seems probable that strains derived from patients during or after treatment with streptomycin may consist of a mixture of organisms of different degrees of sensitivity (Pyle, 1947). An attempt has therefore been made to determine the proportion of resistant to sensitive organisms that could appear as resistant in the test. Since it has been shown that the presence of Tween 80 in the medium decreases the apparent concentration of streptomycin necessary to inhibit any particular strain (Fisher, 1948a and b; Williston and Youmans, 1949) media with and without Tween 80 have been used.