Bacteria regulate collective behaviour by mechanosensing cell-cell collisions.

Dense bacterial populations often navigate complex and heterogeneous environments using collective behaviours including group motility. Whether local interactions within a population can regulate collective order is unclear. Here we show using live imaging, single-cell tracking and simulations that Pseudomonas aeruginosa regulates collective order by mechanosensing cell-cell collisions, enabling adaptive surface exploration. Collision-induced reversals suppress collective cell alignment in dense bacterial populations. While a non-reversing mutant remains locked in cohesive near-linear motion regardless of density, wild-type cells adapt to local crowding, transitioning from isotropic motion in crowds to unidirectional exploration at the colony front. With increasing density, collisions generate information that feeds back on single-cell motility to regulate collective order, providing a navigational advantage. This advantage extends to spatially structured environments, such as micromazes, where collision-induced reversals prevent cells from becoming trapped at physical boundaries. These findings reveal that bacterial collective organization is not merely an emergent consequence of physical interactions, but a feedback-regulated state.