抄録
Genetic studies on the viomycin resistance of Mycobacterium tuberculosis var. hominis, strain H37Rv, have been conducted and the following conclusions have been obtained:
1. Survivors appearing on media containing viomycin up to 10μg/ml can be produced by phenotypic variability within the wild genotype, and survivors appear-ing on media containing more than 20μg/ml of viomycin is produced by mutation.
2. Population structures of single clones obtained by the first-step selection of the parent strain, using more than 20μg/ml of viomycin, have been similar each other. Clones producing mutants resistant to higher levels of viomycin at higher ratios have been obtained by successive selections. However, increase of such mutants in ratio has remained to less extent even by multi-step selection.
3. The pattern of viomycin resistance is a multi-step type with a wide first-step like the pattern of furadroxyl resistance in Escherichia coli.
4. Viomycin resistance appears to be determined by multiple genotypes. It appears possible that these multiple genotypes are produced by mutations of a single gene (a main factor) and some conditional factors.
5. The emergence of viomycin resistance occurs within a narrow limitation. The upper limit of resistance remains within the tenfold increase of resistance from sensitive cells even after the multi-step selection.
6. It has been considered that the ability to survive on media containing 100μg/ml (the upper limit of survsval) is produced by phenotypic variability within any resistant genotype.
