Emergent behaviors of motile bacteria in a dynamic oxygen gradient*

Despite decades of microbiology research, we lack a practical understanding of how bacteria migrate through host tissues in disease and through soils in bioremediation to consume environmental pollutants. Models of bacterial motility neglect how the impact of cells on their microenvironment dynamically influences subsequent motion of cells. We aim to unravel how the multi-scale dynamic feedback between bacterial motility and nutrient consumption gives rise to emergent behaviors. We find that E. coli suspension droplets in two-dimensional confinement spontaneously separate into two populations according to a self-generated oxygen gradient: immotile cells cluster in a droplet center and motile cells congregate near an outer air interface. We show how cluster formation and size depend on the interplay between oxygen consumption, diffusion, and bacterial motility. Understanding the impact of gradient-consumption feedback on population scale dynamics will better inform population level microbial behaviors like biofilm formation and migration.