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 systematically investigate how these mechanical factors 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. We find that substrate friction plays a key role in controlling domain size and orientational diversity, whereas mechanosensitive growth, despite introducing a feedback between stress and proliferation, does not systematically alter domain structure within the biologically relevant regime. Increasing the division length enhances steric effects, slows the relaxation of colony shape toward isotropy, and broadens the distribution of contact forces. In dense colonies, strong forces are transmitted anisotropically through chains of aligned neighbors within microdomains. These results demonstrate that colony-level morphology and stress organization can emerge from local mechanical interactions alone. More broadly, our study establishes how substrate interactions, proliferation dynamics, and stress-growth coupling jointly govern the self-organization of bacterial communities under surface confinement.