Data from: Biofilms as self-shaping growing nematics

Active nematics are the nonequilibrium analog of passive liquid crystals
in which anisotropic units consume free energy to drive emergent behavior.
Similar to liquid crystal (LC) molecules in displays, ordering and
dynamics in active nematics are sensitive to boundary conditions; however,
unlike passive liquid crystals, active nematics, such as those composed of
living matter, have the potential to regulate their boundaries through
self-generated stresses.  Here, using bacterial biofilms confined
by a hydrogel as a model system, we show how a three-dimensional, living
nematic can actively shape itself and its boundary in order to regulate
its internal architecture through growth-induced stresses. We show that
biofilms exhibit a sharp transition in shape from domes to lenses upon
changing environmental stiffness or cell-substrate friction, which is
explained by a theoretical model considering the competition between
confinement and interfacial forces. The growth mode defines the
progression of the boundary, which in turn determines the trajectories and
spatial distribution of cell lineages. We further demonstrate that the
evolving boundary defines the orientational ordering of cells and the
emergence of topological defects in the interior of the biofilm. Our
findings reveal novel self-organization phenomena in confined active
matter and provide strategies for guiding the development of programmed
microbial consortia with emergent material properties.

活性向列相（active nematics）是被动液晶的非平衡类比体系，其中各向异性单元通过消耗自由能来驱动涌现行为。与显示器中的液晶（LC）分子类似，活性向列相的有序性与动力学特性对边界条件极为敏感；但与被动液晶不同，由活体物质构成的活性向列相可通过自身产生的应力调控自身边界。本研究以水凝胶约束的细菌生物被膜（bacterial biofilms）为模型体系，阐明了三维活体向列相如何通过生长诱导应力主动塑造自身及其边界，以调控内部结构。研究发现，当改变环境刚度或细胞-基底摩擦力时，生物被膜的形貌会从圆顶状发生突变转变为透镜状，这一现象可通过考虑约束与界面力竞争的理论模型得到解释。生长模式决定了边界的演化进程，而边界演化又进一步决定了细胞谱系的轨迹与空间分布。我们进一步证实，动态演化的边界可调控细胞的取向有序性，并在生物被膜内部催生拓扑缺陷。本研究发现了受限活性物质中全新的自组织现象，并为开发具有涌现材料特性的程序化微生物群落提供了可行策略。