Mechanical Principles Underlying Development of Bacterial Biofilm Morphology

Surface-attached bacterial communities called biofilms display diverse morphologies. Our recent experiments demonstrated that growth-induced mechanical instabilities – including wrinkling and delamination – underlie the morphogenesis program of biofilms growing at the air-solid interface, and determine the characteristic wavelength of surface undulations. Yet, how the interplay between mechanics and biofilm growth determines colony expansion and morphogenesis remains unclear. Here, we define the physical mechanism underlying biofilm mechano-morphogenesis by combining microscopic and continuum models. We show that surface friction affects the thickness and edge propagation angle of colony biofilms. Moreover, this friction, along with a non-uniform pattern of growth due to nutrient depletion at the biofilm center, gives rise to anisotropic stress patterns. Interestingly, we also find that the combination of active exponential growth near the edge and linear increase of contour length contributes to a constant accumulation of tangential stress. This residual stress can be relaxed by subsequent mechanical instabilities. Finally, we propose a coarse-grained model to characterize the post-wrinkling/delamination expansion of biofilms.