Active segregation in binary mixtures under flow

Many bacterial habitats, from the gut to the soil, feature narrow channels where confined flow is a key constraint that might influence the spatial organization, and thus the functioning, of the complex, phenotypically heterogeneous communities these microbes form. Here, we investigate how a model heterogeneous bacterial community of motile and non-motile Escherichia coli organizes under confined Poiseuille flow. We discovered a new mode of active self-organization, where the motile bacteria induce the rapid sideways segregation of the non-motile cells to one side of the channel, eventually resulting in asymmetric biofilm formation. Our experiments and modeling elucidated the purely physical segregation mechanism: the rheotactic drift of the motile cells, which stems from shear acting on their chiral flagella, induces a conveyer-belt-like backflow advecting the non-motile cells. The latter then accumulate thanks to sedimentation countering flow incompressibility. This unexpected consequence of motility can affect the organization of complex bacteria communities colonizing environments under confined flow.