Active segregation in binary mixtures under flow

The spatiotemporal organization of bacteria within complex heterogeneous communities has broad functional impacts that affect many natural and industrial processes. From the gut to the soil, many bacterial environments feature pores and narrow ducts where shear flow is a ubiquitous external physical constraint on motile and sessile bacterial behaviors that might influence the community structure. It is therefore essential to understand how the physics of shear flow contributes to the structuring of complex, phenotypically heterogeneous communities. Here, we investigate experimentally how a model heterogeneous bacterial community of motile and non-motile Escherichia coli organizes in a confined environment under Poiseuille flow. We discovered that the mixture actively segregates under flow, with the non-motile population accumulating on one side of the channel, opposite the vorticity direction at the bottom. We demonstrate experimentally and in simulations that this segregation, which depends on motile cell density and flow rate, originates from the rheotactic drift of motile cells that stems from shear acting on their chiral helical flagella. The drifting motile cells collectively induce a conveyer-belt-like backflow advecting the non-motile cells. We also show that non-motile cell accumulation requires sedimentation, which counters flow incompressibility, to take effect. Finally, the segregation of non-motile cells under flow leads at long times to asymmetric mixed biofilm formation in the channel. Our findings reveal a novel mechanism by which physical interactions in flowing environments can drive the spatial organization and long-term dynamics of complex microbial communities.