On the Production of Variant Colonies by Certain of the Intestinal Bacteria.

Abstract

Much of the literature on microbic dissociation has emphasized the occurrence of rough (R) colonies, in contrast to the smooth (S) form, with, on the whole, relatively little mention of other colony types. In contrast to this, it has been the writer's experience that frequently many variant colonies of different character are encountered.

The results presented in the accompanying table were obtained in connection with a study of factors which might stimulate the process of dissociation. They are typical of many other similar experiments and illustrate the changes in colony form which were found in cultures of certain of the colon-paratyphoid-dysentery organisms. Smooth forms, previously carried through a series of colony isolations, were used for inoculation of the broth or peptone media. The appearance of variant colonies was followed by streaking agar plates of standard composition at intervals. The changes thus represent those occurring in “ageing” cultures.

The change from S to R appeared to be a gradual one and was not accomplished by distinct or abrupt changes from one colony type to another. All gradations in colony form from the typical white homogeneous colonies with even margins, characteristic of the S form, to the flat, dull, dry-looking, coarsely-granular and irregular colonies of the R form were found. Colonies appearing to fall between these two extremes are referred to as intermediate (I) forms in the table. It will be seen that in ageing cultures such colonies were usually present in large numbers and often far out-numbered the true R forms. At times the change went no farther than the intermediate stage and true rough colonies were never seen, though there was nevertheless a decided departure from the typical smooth form.