Breathe in, breathe out: The density-dependent collective migration of E. coli in oxygen gradients

Swimming bacteria navigate their environment by biasing their motion through a run-and-tumble behavior, migrating toward beneficial chemicals and away from harmful ones. Out of all the chemicals bacteria respond to, oxygen stands out due to its ubiquitous presence, distinct influence on bacterial motility, and key role in the historical understanding of chemotaxis. Here, we track the migration of E. coli at both the individual and population levels in a capillary channel under an oxygen gradient. We demonstrate that the direction of bacterial migration depends on the population density. At high densities, E. coli migrates toward higher oxygen concentrations, whereas at low densities, the direction reverses, with bacteria moving toward lower oxygen concentrations. We develop a kinetic theory that quantitatively explains our findings. A continuum model has also been constructed, illustrating the possible signaling pathway of E. coli that allows them to accumulate at an optimal oxygen concentration. Our study sheds light on the dynamics of bacteria in response to oxygen gradients and resolve a long-standing controversy on the conflicting chemotactic behaviors of bacteria in environment with non-uniform oxygen distributions.