Role of surface interactions in bacterial biofilms*

Bacteria often live in surface attached and densely packed communities called biofilms. As they grow, biofilms experience various internal and external forces. For example, the expanding edge of a biofilm experiences friction as it slides across a surface. Within biofilms, mechanical stresses from reproduction push cells apart while intercellular adhesion pulls cells together. These forces are unavoidable; yet it remains unclear how they impact biofilm morphology and the rate of colony expansion, especially for relatively small colonies, which are common in nature, that have not formed wrinkly or rugose morphologies. Here, we use interferometry to look at the three-dimensional surface morphology of biofilms and investigate the effect of different surface friction and adhesive forces. We show that biofilms look and act similar to sessile drops, and, accordingly, that the growth and morphology of surface attached biofilms depends on the balance between these different forces in a manner reminiscent of the Young equation. Finally, we demonstrate the importance of surface-tension-like adhesive forces when separate colonies merge.