(65n) Spatial Self-Organization of Confined Bacterial Suspensions

Lab studies of bacteria typically focus on cells in spatially extended, nutrient-rich environments such as liquid cultures or agar surfaces. In contrast, many biological and environmental contexts—ranging from bodily mucus to ocean sediments and the soil beneath our feet—host multicellular bacterial populations confined to tight spaces where essential metabolic substrates (e.g., oxygen) are limited. What impact does such confinement have on bacterial populations? Here, we explore this question by studying suspensions of motile Escherichia coli confined within quasi–two-dimensional (2D) droplets. We find that when both the droplet size and cell concentration exceed critical thresholds, the initially uniform suspension spontaneously self-organizes into a concentrated, immotile inner “core” surrounded by a more dilute, highly motile “shell”. By simultaneously measuring cell concentration, oxygen concentration, and motility-induced fluid flows, we demonstrate that this spatial organization emerges from a dynamic interplay between oxygen transport through the droplet boundary, oxygen uptake by the bacteria, and changes in bacterial motility in response to local oxygen availability. In addition, we employ theory and simulations to establish quantitative principles governing this feedback—yielding a biophysical framework that captures and unifies all of our experimental observations. Our findings shed new light on the rich collective behaviors that can emerge in confined bacterial populations and other forms of chemically reactive living and active matter, and offer a foundation for predicting and controlling these dynamics in broader biological and environmental systems.