Phase Transition Driven Wrinkling in Bacillus subtilis Biofilms: The Role of γ-PGA and EPS

The emergence of three-dimensional morphologies in living tissues is governed by a delicate interplay between physio-chemical properties and genetic programs such as cell division and extracellular matrix production. It remains an open question how rates of cellular matrix production determine the material physics of tissues and how large-scale morphology emerges from those physics. We investigate this question by examining the formation of wrinkles in mature Bacillus subtilis biofilms. We find that two distinct self-secreted polymers—poly-γ-glutamic acid and exopolysaccharide—play complementary roles in the evolution of wrinkled biofilm morphology. Poly-γ-glutamic acid absorbs fluid from the substrate, halting radial expansion while promoting vertical swelling, whereas exopolysaccharide provides structural integrity. By varying the production rates of these polymers, we find a phase transition from thin, flat biofilms to thick, wrinkled biofilms. Using a percolation model, we find that this transition qualitatively resembles a sol-gel phase transition. Our results offer new insights into how phase transitions govern the morphology of bacterial communities.