The physics of sessile droplets rediscovered in bacterial biofilms*

Bacteria spend much of their life time in biofilms – surface-attached, densely packed microbial communities encapsulated in an extracellular polymeric matrix (EPS). This mode of living provides many benefits: Biofilms facilitate social interaction between cells and provide protection against external influences. Yet, surface-attached biofilm formation also involves many inevitable physical consequences. Frictional interaction between cells and substrate surface impacts the radial expansion of a growing colony. In addition, physical cell-cell interactions and their interaction with the encapsulating EPS controls biofilm growth and expansion. It remains often unclear how these microscopic interactions lead to the macroscopic properties of a biofilm, such as growth dynamics and macroscopic shape of a colony. In this talk, I will present our newest results showing that the growth of surface attached bacterial colonies can be captured by the physics of sessile droplets. In particular, we demonstrate that bacterial biofilms exhibit an effective surface tension. As colonies grow from a single founder cell we find that colonies achieve interfacial force balance at the biofilm boundary. These physical forces impact colony growth, expansion and interaction between merging colonies.