Numerical Relations of an Unstable Variant of Salmonella Aertrycke

Abstract

Although many bacteriologists have encountered variants which were difficult to stabilize, most have felt that with sufficient care and repeated selection, any variant could be obtained in a stable form. In our studies on colonial forms of Salmonella aertrycke certain variants were encountered which lacked stability. A variant of this type cannot be obtained free from another variant to which it constantly gives rise. The variation of these cultures was not haphazard but seemed to be governed by some fixed rule, the mechanism of which we have attempted to investigate.

We studied a strain MT2C-R, a rough variant derived from the typical smooth strain of S. aertrycke. A typical colony of this variant on infusion agar is low, rough and of medium size. When one was plated on infusion agar about 83% of the colonies derived from it were identical with the parent type. The other 17% were smooth and dome shaped and about 1/10 the diameter of the parent colony (MT2C-S). On continued daily subculture of these rough colonies on infusion agar plates, the percentage of rough colonies remained constant at 83% ± 5%. The S colonies when plated in the same way always yielded 100% S colonies.

When a 24-hour broth culture of an R colony was streaked on an agar plate, the percentage of R colonies fell to 33% or less; but each of these R colonies when subsequently plated directly, yielded the original 83 %. When, however, the broth culture from the R colony was inoculated into broth a second time the second broth culture, on plating, yielded only S colonies.

Thus the percentage of R forms obtained on streaking out a broth culture of an R colony is less than that obtained by streaking the R colony directly on a second agar plate. Nevertheless, the intermediate broth passage in no way affects the inherent tendency of an R organism to give rise to a colony containing a constant ratio of R and S cells (about 83% R and 17% S).