Collective morphology of bacteria growing in complex fluids

Many bacteria live in complex fluids, such as mucus in the body, exopolymers in the ocean, and cell-secreted extracellular polymeric substances (EPS) that encapsulate biofilms. However, lab studies of bacteria typically focus on cells in simple homogeneous fluids that do not mimic real microbial habitats. How does inhabiting a complex fluid environment influence the behavior of bacterial communities? To address this question, we experimentally probe the growth of non-motile Escherichia coli cells in polymer and liquid crystalline solutions. When grown in a polymer solution or in a nematic liquid crystal, the cells grow in striking “cable-like” morphologies—in stark contrast to the random dispersions that are observed in simple fluids. Experiments and mathematical modeling elucidate how polymer-induced depletion attraction forces or liquid crystalline nematic elasticity, coupled with bacterial growth, lead to the emergence of cable structures in polymer and liquid crystalline environments, respectively. Our work uncovers quantitative principles governing the morphogenesis of bacterial colonies in diverse environments.