More is different: Spatiotemporal distribution of aerotactic bacteria under confinement

The response of bacteria to heterogeneous environments is a key question in understanding and controlling microorganisms. For instance, due to their active characteristics and hydrodynamic interactions with boundaries, motile bacteria often concentrate near confining surfaces. Here, we investigate the aerotactic response of bacteria confined within a thin liquid film subjected to controlled oxygen-concentration gradients. Our experimental observations reveal that the confinement effect dominates over aerotactic response at low total bacterial density, whereas the combined effects of aerotaxis and confinement are observed at high densities. To interpret these findings, we develop a diffusion-advection model of oxygen and motile bacteria, in which the bacteria interact hydrodynamically with the confining wall and exhibit Keller-Segel-type chemotaxis in response to the oxygen gradient. Our model reproduces the experimental results and provides insights into the collective behavior of confined bacteria under external stimuli, such as oxygen gradients under spatial constraints. These findings contribute to a deeper understanding of microorganisms-environment interactions, with implications for microbial ecology and the early stages of biofilm formation.