Mechanical Interactions Govern Self-Organized Ordering in Bacterial Colonies on Surfaces
Bacterial colonies growing on surfaces are shaped by mechanical stresses transmitted through the community, governed by the balance between cell growth and steric and cell-substrate interactions. Using overdamped dynamics simulations of nonmotile, stress-responsive bacteria, we examine how purely mechanical interactions determine colony morphology and internal organization. Growth-induced extensile stresses compete with steric constraints, giving rise to the spontaneous formation of microdomains composed of highly aligned cells. We characterize this self-organization through the distribution of microdomain areas and a nematic order parameter that quantifies colony-wide alignment. Mechanosensitivity does not systematically alter domain structure, but increasing substrate friction reduces the mean domain size and broadens the diversity of orientations. Shifting the balance toward steric interactions, by lengthening the cell division size, slows the relaxation of colony shape toward isotropy and broadens the distribution of contact forces, producing a slower exponential decay. In dense colonies, strong forces are transmitted anisotropically through chains of aligned neighbors within microdomains. These findings demonstrate that colony-level morphology and stress organization can emerge from local mechanical interactions alone, even without requiring biochemical signaling. By linking microscopic force transmission to macroscopic growth dynamics, our study provides a physical framework for understanding how mechanical interactions shape the self-organization of bacterial communities under surface confinement.