The biophysical basis of bacterial colony growth

Bacteria often attach to surfaces and grow densely packed communities called biofilms. As biofilms grow, they expand across the surface, increasing their surface area and access to nutrients. Thus, the overall growth rate of a biofilm is directly dependent on its range expansion rate. A direct trade-off between horizontal and vertical growth impacts the range expansion rate and, crucially, the overall biofilm growth rate. The biophysical connection between horizontal and vertical growth remains poorly understood, in large part due to the difficulty in resolving the biofilm shape with sufficient spatial and temporal resolutions from small length scales to macroscopic sizes. Here we show that the horizontal expansion rate of bacterial colonies is strongly coupled to vertical expansion via the contact angle at the biofilm edge. Using white light interferometry, we measure the three-dimensional surface morphology of growing colonies, and find that small colonies are well described as spherical caps. At later times, nutrient diffusion and uptake prevent the tall colony centre from growing exponentially. We further show that a simple model connecting vertical and horizontal growth dynamics can reproduce the observed phenomena, suggesting that the spherical cap shape emerges due to the biophysical consequences of diffusion-limited growth.