Determination of drug resistance of Mycobacterium tuberculosis cultures

A total of 3,303 strains of Mycobacterium tuberculosis were tested for sensitivity to streptomycin (SM), isoniazid (INH), and p-aminosalicylic acid (PAS) by the Steenken modified minimal inhibitory concentration (MIC) test. A simultaneous double blind comparison was carried out on 277 selected strains by the Steenken MIC test and the Canetti proportion method. Agreement between the results for the two tests was 82% for SM, 95% for INH, and 89% for PAS. A small number of strains appeared to be sensitive when tested by one method but resistant by the other. MIC determinations were carried out on 83 strains by using Steenken-Smith, Lowenstein-Jensen, and Middlebrook 7H10 media containing a more extended range of concentrations of the test drugs. The MIC values for both SM and dihydrostreptomycin increased on Steenken-Smith medium compared with the other two. INH did not show any medium effect, whereas PAS showed increased MIC values in 7H10 agar. The significance of the comparisons of the MIC values on the various media is discussed in terms of possible changes in the drug sensitivity testing methods used at present in this laboratory.