Force balance and surface interactions play an important role in determining bacterial colony growth rate

Bacteria often live in surface attached, densely packed communities called biofilms. Though biofilms expand both across and perpendicular to the surface they sit on, we lack a systematic framework that would allow us to assess the impact of vertical growth on horizontal expansion. Here, we use interferometry to measure the three-dimensional surface morphology of biofilms grown on different surfaces. We find that colonies that grow at different rates exhibit different shapes. In particular, we observe that as the colony contact angle increases the horizontal expansion rate decreases; this observation holds across many different strains and species. More cells are necessary to increase the radius of a steep biofilm than the radius of a shallow biofilm, so higher contact angle biofilms expand at a slower rate than lower contact angle biofilms. We show that contact angle depends on the balance between three forces at the biofilm edge in a manner reminiscent of the Young equation for sessile drops. Thus, we find that mechanical interactions and shape of a colony play a crucial role in determining colony growth rate.