Spatial self-organization of confined bacterial suspensions

Abstract

Lab studies of bacteria usually focus on cells in spatially-extended, nutrient-replete settings, such as in liquid cultures and on agar surfaces. By contrast, many biological and environmental settings--ranging from mucus in the body to ocean sediments and the soil beneath our feet--feature multicellular bacterial populations that are confined to tight spots where essential metabolic substrates (e.g., oxygen) are scarce. What influence does such confinement have on a bacterial population? Here, we address this question by studying suspensions of motile Escherichia coli confined to quasi two-dimensional (2D) droplets. We find that when the droplet size and cell concentration are both large enough, the initially-uniform suspension spatially self-organizes into a concentrated, immotile inner "core" that coexists with a more dilute, highly-motile surrounding "shell". By simultaneously measuring cell concentration, oxygen concentration, and motility-generated fluid flow, we show that this behavior arises from the interplay between oxygen transport through the droplet from its boundary, uptake by the cells, and corresponding changes in their motility in response to oxygen variations. Furthermore, we use theory and simulations to develop quantitative principles describing this interplay--establishing a bio-physical framework that unifies all our experimental observations. Our work thereby sheds new light on the rich collective behaviors that emerge for bacterial populations, and other forms of chemically-reactive living and active matter, in confined environments, and provides a way to predict and control these behaviors more broadly.