Microbial communities such as biofilms are commonly found at interfaces. However, it is unclear how the physical environment of interfaces may contribute to the development and behavior of surface-associated microbial communities. Combining multimode imaging, single-cell tracking, and numerical simulations, here, we found that activity-induced interface bulging promotes colony biofilm formation in Bacillus subtilis swarms presumably via segregation and enrichment of sessile cells in the bulging area. Specifically, the diffusivity of passive particles is ~50% lower inside the bulging area than elsewhere, which enables a diffusion-trapping mechanism for self-assembly and may account for the enrichment of sessile cells. We also uncovered a quasilinear relation between cell speed and surface-packing density that underlies the process of active interface bulging. Guided by the speed–density relation, we demonstrated reversible formation of liquid bulges by manipulating the speed and local density of cells with light. Over the course of development, the active bulges turned into striped biofilm structures, which eventually give rise to a large-scale ridge pattern. Our findings reveal a unique physical mechanism of biofilm formation at air–solid interface, which is pertinent to engineering living materials and directed self-assembly in active fluids.

中文翻译：

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枯草芽孢杆菌群中的活性界面膨胀促进自组装和生物膜形成


生物膜等微生物群落常见于界面处。然而，目前尚不清楚界面的物理环境如何促进表面相关微生物群落的发育和行为。结合多模式成像、单细胞跟踪和数值模拟，我们发现活动引起的界面膨胀促进了菌落生物膜的形成枯草芽孢杆菌群体可能是通过凸出区域中无柄细胞的分离和富集而形成的。具体来说，凸出区域内被动粒子的扩散率比其他地方低约 50%，这使得自组装的扩散捕获机制成为可能，并可能解释固着细胞的富集。我们还发现了细胞速度和表面堆积密度之间的拟线性关系，这是活性界面膨胀过程的基础。在速度-密度关系的指导下，我们通过用光操纵细胞的速度和局部密度来证明液体凸起的可逆形成。在发育过程中，活跃的凸起变成条纹生物膜结构，最终形成大规模的脊状图案。我们的研究结果揭示了空气-固体界面生物膜形成的独特物理机制，这与工程活性材料和活性流体中的定向自组装有关。