(17i) Bacterial Colonies Growing in Polymeric Fluids Form "Active Gels"

Many bacteria live in polymeric fluids, such as mucus, environmental polysaccharides, and extracellular polymers in biofilms. However, lab studies typically focus on cells in polymer-free fluids. Here, we show that interactions with polymers shape a fundamental feature of bacterial life—how they proliferate in multicellular colonies. Using experiments, we find that when the polymer is sufficiently concentrated, cells generically and reversibly form large serpentine "cables" as they proliferate. By combining experiments with biophysical theory and simulations, we demonstrate that this distinctive form of colony morphogenesis arises 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 polymer solution. Our work thus reveals a pivotal role of polymers in sculpting proliferating bacterial colonies, with implications for how they interact with hosts and the natural environment, and uncovers quantitative principles governing colony morphogenesis in such complex environments. It also opens up a new direction for research in soft matter physics: while polymer-induced entropic attraction is well-studied for passive particulate systems, our work highlights that fascinating new behaviors can emerge when the constituent particulates can also proliferate, which represents a new form of "activity" to be considered in active colloidal systems.