Morphodynamics of growing bacterial communities in polymeric environments

Many bacteria live in polymeric environments, such as mucus in the body, exopolymers in the ocean, and cell-secreted extracellular polymeric substances (EPS) that encapsulate biofilms. However, most studies of bacteria focus on cells in polymer-free fluids. How do interactions with polymers influence the behavior of bacterial communities? To address this question, we experimentally probe the growth of non-motile Escherichia coli in solutions of inert polymers. We find that, when the polymer is sufficiently concentrated, the cells grow in striking “cable-like” morphologies—in stark contrast to the compact morphologies that arise in the conventionally-studied polymer-free case. Experiments and agent-based simulations show that these unusual community morphologies arise from an interplay between polymer-induced entropic attraction between pairs of cells and their hindered ability to diffusely separate from each other in a viscous solution. These results suggest a pivotal role of polymers in regulating microbe-host interactions, and more broadly, this work helps to uncover quantitative principles governing the morphogenesis of diverse forms of growing active matter in polymeric environments.