Active bulging promotes biofilm formation in a bacterial swarm

Microbial communities such as biofilms are commonly found at interfaces. However, it is unclear how the physical environment of interfaces may contribute to the development and behavior of surface-associated microbial communities. Combining multi-mode imaging, single-cell tracking and numerical simulations, here we discovered that an interfacial process denoted as "active bulging" promotes biofilm formation. During this process, an initially two-dimensional layer of swarming bacteria spontaneously develops scattered liquid bulges; the bulges have a higher propensity to transit from motile to sessile biofilm state, presumably due to the enrichment of pre-existing immotile cells in the colony. We further demonstrate that the formation of liquid bulges can be controlled reversibly by manipulating the speed and local density of cells with light. Our findings reveal a unique physical mechanism of biofilm formation and provide a new strategy for biofilm patterning in engineered living materials as well as for directed self-assembly in active fluids.