Tata Institute of Fundamental Research Hyderabad, Telangana 500046, India.
Soft Matter. 2021 Aug 21;17(31):7322-7331. doi: 10.1039/d1sm00311a. Epub 2021 Jul 21.
Self-propelled bacteria can exhibit a large variety of non-equilibrium self-organized phenomena. Swarming is one such fascinating dynamical scenario where a number of motile individuals group into dynamical clusters and move in synchronized flows and vortices. While precedent investigations into rod-like particles confirm that an increased aspect-ratio promotes alignment and order, recent experimental studies in bacteria Bacillus subtilis show a non-monotonic dependence of the cell-aspect ratio on their swarming motion. Here, by computer simulations of an agent-based model of self-propelled, mechanically interacting, rod-shaped bacteria under overdamped conditions, we explore the collective dynamics of a bacterial swarm subjected to a variety of cell-aspect ratios. When modeled with an identical self-propulsion speed across a diverse range of cell aspect ratios, simulations demonstrate that both shorter and longer bacteria exhibit slow dynamics whereas the fastest speed is obtained at an intermediate aspect ratio. Our investigation highlights that the origin of this observed non-monotonic trend of bacterial speed and vorticity with the cell-aspect ratio is rooted in the cell-size dependence of motility force. The swarming features remain robust for a wide range of surface density of the cells, whereas asymmetry in friction attributes a distinct effect. Our analysis identifies that at an intermediate aspect ratio, an optimum cell size and motility force promote alignment, which reinforces the mechanical interactions among neighboring cells leading to the overall fastest motion. Mechanistic underpinning of the collective motions reveals that it is a joint venture of the short-range repulsive and the size-dependent motility forces, which determines the characteristics of swarming.
自主运动的细菌可以表现出多种多样的非平衡自组织现象。群体运动就是这样一种引人入胜的动力学场景，其中许多运动个体聚集成群，并以同步的流和涡旋运动。虽然先前对棒状颗粒的研究证实，增加长径比可以促进对齐和有序，但最近对枯草芽孢杆菌的实验研究表明，细胞长径比对其群体运动的依赖性是非单调的。在这里，我们通过对自主运动、机械相互作用的棒状细菌的基于代理的模型进行计算机模拟，研究了在各种细胞长径比下细菌群体的集体动力学。当用跨越各种细胞长径比的相同自推进速度对模型进行建模时，模拟表明，较短和较长的细菌都表现出较慢的动力学，而在中间长径比下获得最快的速度。我们的研究强调，观察到的细菌速度和涡度与细胞长径比的这种非单调趋势的起源，源于运动力的细胞大小依赖性。在广泛的细胞表面密度范围内，群体运动特征仍然很稳健，而摩擦力的不对称性则产生了明显的影响。我们的分析表明，在中间长径比下，最优的细胞大小和运动力促进了对齐，这增强了相邻细胞之间的机械相互作用，从而导致整体最快的运动。集体运动的机制基础表明，这是短程排斥力和尺寸依赖性运动力的共同作用，决定了群体运动的特征。
