The growth and form of bacterial colonies

A simple method is described for measuring the profile of bacterial colonies. Profiles were determined for colonies of Bacillus cereus, Escherichia coli and Staphylococcus albus of different ages. In spite of differences in cell morphology, the colony profiles had a common basic structure consisting of steeply rising leading edge connected by a ridge to an interior region where height also rose, though less steeply, to a flat or domed centre. The colony mass increased exponentially through part of the growth phase. It is suggested that net colony growth consists of a combination of leading edge growth, which is unrestricted and approaches the maximum specific growth rate of the organism, and diffusion-limited growth in the colony interior. Common elements of profiles from each species may be a consequence of such differences in growth rate.