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

A delicate interplay between physio-chemical properties and genetic programs such as cell division and extracellular matrix production governs the emergence of three-dimensional morphologies in living tissues. It remains an open question how cellular matrix production rates determine tissues' material physics 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 gelation phase transition. Our results offer new insights into how phase transitions govern the morphology of bacterial communities.