SUMMARY: Colonies of four strains of var. and three strains of var. investigated may develop rough outgrowths while growing on a medium containing no fermentable carbohydrate. Colonies do not form visible outgrowths when growing on the same medium containing more than 0·5% (w/v) glucose. Rough elements may be present in the latter colonies, but because of lack of a selective advantage do not show phenotypically. The rough outgrowths possess the characteristics of R variants. The biochemical reactions of the R variants are identical to those of the parent S form. The mutational origin of the R variants from wild S types is demonstrated by a method involving the direct observation of rough clones. The mutation rate involved is presented for six of the strains. In continuous culture in broth both the S and the R types are stable. On agar the R variant is also stable.


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