Active segregation in bacterial binary mixtures under flow

Abstract Confined flow is a major physical constraint, from the soil to the gut, for bacteria whose complex spatial organization within heterogeneous multispecies communities can be determinant for their environmental success. In particular, non-equilibrium spatial patterns emerge at rest in mixed motile - non-motile populations, but how flow affects these structures is unknown. Using a model community of mixed motile and non-motile Escherichia coli in channels under Poiseuille flow, we discovered a mode of active self-organization where the motile bacteria induce the rapid segregation of the non-motile ones to one channel side, which eventually cements in asymmetric biofilm formation. Our experiments and modeling identified the purely physical mechanism driving segregation: the rheotactic drift of the motile cells, resulting from shear on their chiral flagella, induces a conveyor-belt-like backflow advecting the non-motile cells. The latter then accumulate due to sedimentation countering flow incompressibility. This unexpected long-lasting structural consequence of motility may influence many bacterial communities colonizing confined-flow environments.