'Take-over'--an Unusual Selection Process in Steady-State Cultures of Escherichia coli

Abstract

When the proportion of prototrophic revertants in a continuous culture of the tryptophan-requiring strain wp2 of Escherichia coli was adjusted to exceed about 10-7, a large increase (‘take-over’) often occurred after a variable delay, the proportion becoming as high as 50% in some cases. The phenomenon appeared to be correlated with a selective advantage of revertants which became attached, with parent auxotrophs, to the wall of the culture tube. After some hours the film of growth on the wall was composed of approximately 107 bacteria. It was quite different from the massive ‘sticky’ growth sometimes encountered in continuous cultures of E. coli. The experimental observations of this ‘take-over’ are quantitatively consistent with a hypothesis of wall adsorption of rather rare and readily attached variant prototrophs.