Morphogenesis of bacterial colonies in liquid crystalline environments

Many bacteria live in liquid crystalline (LC) environments, from mucus to biofilms. We explore how such anisotropic, viscoelastic fluids can shape a fundamental feature of bacterial life - how they proliferate in space in multicellular colonies. Using experiments, we find that when grown in a LC, cells form large single-cell width chains that at sufficiently long lengths undergo a mechanical buckling instability. We then argue, using a continuum mechanical theory, that these dynamics emerge because cells are kept in alignment by the medium's elasticity, and that growth-induced stresses along the bacterial chain's length eventually promote its buckling. The critical buckling length is found to depend primarily upon an Ericksen number, which compares growth-induced viscous stresses to elastic stresses, and to a dimensionless LC anchoring strength. Our work thus reveals how a liquid crystalline environment sculpts proliferating bacterial colonies, with implications for how they interact with hosts and with the natural environment, and uncovers quantitative principles governing colony morphogenesis in complex fluids.