SUMMARY: Mutational studies on the PAS (sodium -aminosalicylate) resistance of var. , strain H37RV have been carried out. It has been assumed that one type of population structure of clones with relation to drug resistance corresponds to one genotype. Clones derived from single colonies obtained on different PAS concentrations produced three types of population structure; the first type is similar to the population structure of the parent strain which has not been previously exposed to the drug and its degree of resistance is up to 0·2 g. PAS/ml. The second and the third types are different from the parent strain and their degrees of resistance are up to 3 and 1000 g. PAS/ml., respectively. Neither intermediate population structures nor intermediate degrees of resistance were clearly observed. From these results, it may be concluded that there are only two genotypes responsible for PAS resistance, one determining a lower, and the other a higher degree of resistance.


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