Emergent bacterial dynamics in self-generated oxygen gradients

In many biological and environmental settings, e.g. mucus in the lungs and subsurface porous media, bacterial populations are strongly confined in tight spots where oxygen availability is spatially variable. How do such variations in oxygen supply influence the collective behavior of a population of motile bacteria? To address this question, here, we experimentally study how bacterial suspensions react to self-generated oxygen gradients in micro-confinement. We find that the coupling between cellular motility and the local oxygen concentration gives rise to the formation of a dense, immotile phase that coexists with a less dense, highly motile phase in a manner reminiscent of Motility-Induced Phase Separation (MIPS). Furthermore, we show how the active turbulence generated by the motile phase can result in large-scale fluctuations at the interface between both phases, enhancing oxygen influx and thus subsequent propagation of this interface. Finally, we show how these collective dynamics are markedly altered in a complex fluid akin to airway mucus. Altogether, our work reveals the rich collective dynamics that can emerge for bacterial populations in environments with physicochemical complexity.