发表机构
Global College, Shanghai Jiao Tong University; School of Physics, Georgia Institute of Technology; Department of Mechanical Engineering, Johns Hopkins University; Intelligent Medicine Institute, Shanghai Medical College, Fudan University（上海交通大学全球学院; 佐治亚理工学院物理学院; 约翰霍普金斯大学机械工程系; 复旦大学上海医学院智能医学研究所）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究通过倾斜琼脂表面探究重力方向对细菌集群扩张的影响，发现扩张呈非单调依赖，并通过扩展两相薄膜模型揭示切向与法向重力分量的不同作用机制。
AI 中文摘要
细菌集群运动是密集运动细胞群体在水合软表面上的集体扩张，但在标准水平实验中，重力通常被视为固定的背景条件。我们利用表面倾斜来研究琼脂表面相对于重力的方向如何影响肠杆菌属 SM3 菌株的集群扩张。菌落扩张对倾斜角度呈非单调依赖，在中等倾斜角度时扩散最强，而在浅倾斜或接近倒置条件下扩散较弱。具有相同倾斜幅度但分别朝上的锐角和朝下的钝角配置表现出不同的扩张响应，表明集群扩张既取决于倾斜幅度，也取决于琼脂表面相对于重力哪一侧朝上。倾斜还产生了偏向低处的菌落形态。粒子图像测速（PIV）分析显示，倾斜条件下的平均活动普遍低于水平参考条件，且角响应呈非单调性，速度和空间速度相关长度向内增加。低处尖端区域表现出比上部区域更慢的运动、更短的空间相关性和更小的涡旋核心。为了解释宏观趋势，我们扩展了一个两相薄膜模型，引入了投影到琼脂表面法向和切向的有效重力耦合。模拟结果表明，表面切向分量主要驱动横向重新分布，而表面法向分量则改变薄膜压力，从而影响依赖压力的水分补充，这有助于区分朝上与朝下的配置。
英文摘要
Bacterial swarming is the collective expansion of dense populations of motile cells across hydrated soft surfaces, yet gravity is usually treated as a fixed background condition in standard horizontal assays. We used surface inclination to examine how agar-surface orientation relative to gravity affects the expansion of Enterobacter sp. SM3 swarms. Colony expansion depended non-monotonically on inclination, with the strongest spreading at intermediate inclinations and weaker expansion under shallow or near-inverted conditions. Paired up-facing acute and down-facing obtuse configurations with the same tilt magnitude showed different expansion responses, suggesting that swarm expansion depended on both tilt magnitude and which side of the agar surface faced upward relative to gravity. Inclination also produced downhill-biased colony morphologies. PIV analysis showed generally lower mean activity under inclination than in the flat references, with a non-monotonic angular response and inward increases in speed and spatial velocity-correlation length. The downhill tip showed slower motion, shorter spatial correlations and smaller vortex cores than the upper region. To interpret the macroscopic trends, we extended a two-phase thin-film model by introducing an effective gravity coupling projected normal and tangential to the agar surface. Simulations suggest that the surface-tangential component mainly drives lateral redistribution, whereas the surface-normal component modifies film pressure and thereby pressure-dependent water replenishment, helping distinguish up-facing from down-facing configurations.
Commentsmain: 19 pages, 8 figures; SI: 16 pages, 7 figures
