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 a combination of polymer-induced entropic attraction between 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, e.g. by promoting bacterial exposure to external biochemicals that protect the host against pathogens. More broadly, this work helps to uncover quantitative principles governing the morphogenesis of diverse forms of growing active matter in polymeric environments