Active density fluctuations in bacterial binary mixtures

Abstract

In wild environments, physical and biochemical interactions between intermixed motile and sessile microorganisms give rise to spatial organization that is key for the functioning and ecology of complex communities. However, how motility-driven physical interactions contribute to shaping multispecies communities remains little understood. To address this gap, we investigated model binary mixtures of motile and non-motile Escherichia coli bacteria. We discovered a new type of non-equilibrium self-organization, wherein large-scale density fluctuations of non-motile bacteria emerge when mixed with motile ones under physiologically relevant conditions. Systematically exploring the phase diagram in microfluidics experiments and combining them with modeling and simulations, we uncovered the two-pronged physical mechanism of emergence: Circular swimming of motile cells close to surfaces generates recirculating hydrodynamic flows that advect non-motile cells, while sedimentation, by breaking the vertical symmetry, is essential for their local accumulation. This active self-organization behavior in mixed bacterial populations appears crucial for complex microbial community structuration.