Scaling Crossover in Biofilm Surface Roughening in the Absence of Poly-γ-glutamic acid

The formation and development of the growing edge of bacterial colonies is a complex process influenced primarily by spatial heterogeneity in cellular division rates, adhesion to the underlying substrate, nutrient availability, and the material properties of the biofilm. In this study, we systematically investigate how the presence or absence of self-produced polymers affects the edge growth dynamics using the model biofilm-forming bacterium Bacillus subtilis. By analyzing time-lapse images of biofilm development across different matrix knockouts, we find that the inhibition of poly-γ-glutamic acid (PGA) production causes the biofilm edge to transition from a smooth profile to a significantly rugged one. We hypothesize that this shift in edge profile is linked to PGA’s capacity to fluidize the biofilm, promoting it to behave like a liquid droplet that seeks to minimize surface tension. To explore this transition, we employed a co-culture assay to reveal a monotonic increase in the edge roughness with decreasing PGA producer fraction. Furthermore, we developed a discrete 2D lattice-based mathematical model to elucidate this observed transition by using an Eden competitive growth model, incorporating random and weighted site occupation probabilities. This paper demonstrates how the self-engineered cellular environment contributes to phase transitions in stochastic growth.