Emergent spatiotemporal structures in bacterial binary mixtures

Microbial communities often exhibit complex spatial structures that are important for their functioning, ecology and evolution. While the role of biochemical interactions has been extensively studied, it is unclear how physical interactions contribute to the structuring of multispecies and phenotypically diverse bacterial communities. Here, we investigate the physical effect of motility, a key bacterial trait, on the spatial organization of complex communities, using binary bacterial mixtures as a minimal model system, where one component is motile and the other is not. We observe that large spatial heterogeneities in the density of non-motile cells arise under the influence of the motile cells, which themselves remain homogeneous. Using a combination of experiments and quantitative modelling, we show that this patterning results solely from physical interactions and relies on two key ingredients: by swimming in circles on the surface, the motile bacteria generate recirculating hydrodynamic flows that advect non-motile cells, and the breaking of vertical symmetry by gravity allows local density accumulation. As the configuration of the motile cells swimming on the surface rearranges, the advection landscape also does, making the density patterns fluctuate. This new non-equilibrium mechanism for pattern formation in bacteria belongs to a different class compared to previous models for self-organization in self-propelled systems, which rely on localized traffic jamming in two dimensions. It is instead similar to fluctuation-dominated phase ordering in active nematics. As these density patterns form over a wide range of biologically relevant densities of both phenotypes, we also investigated their effect on the aggregation of adherent non-motile cells and on biofilm formation. We found that the activity of the motile cells enhances the aggregation of non-motile bacteria and that adhesive motile cells promote the formation of complex, threedimensional biofilm structures, suggesting that motility, together with the ability of motile cells to aggregate, is important for the early stages of biofilm formation. These results show how the activity of motile species can shape the spatial organization of complex microbial communities and highlight the importance of studying the physical interactions in these communities.