Bacterial Swimming and Pattern Formation in Fluids

* 技术摘要 * 本项目研究远程流体动力学相互作用（LRHI）对单个细菌和彼此接近的一组细菌游泳的影响。在单细胞水平上，回流如何影响创造它的细胞的游泳行为引起了数学家和物理学家的关注。重要的理论预测，但他们还没有在实验室中进行严格的测试。这部分是由于迄今为止的大多数研究使用大肠杆菌，其具有难以数学建模的多个鞭毛，部分是由于难以在宽范围内改变物理参数，例如流体动力学负荷。利用海洋细菌溶藻弧菌，具有一个极性鞭毛，其细胞体可以分化成不同的大小，这些困难可以克服。海洋细菌还有其他有趣的属性，可以为这项研究做出重大贡献，其中包括：（i）由流体中的钠离子提供动力的可调马达，（ii）大的游泳速度，最重要的是（iii）只在向前（推动者）和向后（拉动者）方向游泳的可用突变体。从理论上讲，两个推进器、两个牵引器以及一个推进器和一个牵引器之间的相互流体动力相互作用是不同的。我们使用最先进的光学捕获技术来定量测量这些不同的力。在宏观层面上，我们研究这些不同的游泳者形成的流体动力学模式。最近，在这一领域进行了大量的理论研究，对这种“活”流体做出了有趣的预测。这些包括预测新的流变行为，非平衡有序/无序相变，和液晶类流体动力学结构。这项研究培养研究生在新兴领域的生物物理学。* 非技术摘要 * 常见的经验是，流体中的机械扰动以明确定义的速度传播到其相邻的流体，即，声速，而其强度随距离而减小。游动的细菌可以被认为是机械扰动的局部来源，并且附近的流体运动可以影响细菌自身的运动以及其附近的其他细菌。这就是所谓的流体动力学相互作用，可以用莱特希尔发展的数学理论来描述;它似乎对孤立的游泳细菌相当有效。然而，其应用于一个大组的游泳细菌在高浓度还没有得到很好的理解。流体动力学相互作用对单个和一大群细菌游动的影响是显著的，因为它影响它们在水性环境中螯合营养物质和在表面上形成生物膜的能力;这两者对于生态和生物医学原因都是重要的。该项目使用最先进的激光捕获技术来操纵细菌，以确定不同配置的细菌对之间的相互流体动力学相互作用。这些测量比较和完善了预测大量细菌集体行为的理论模型。鉴于研究的多学科性质，受过培训的学生在学术界和工业界的需求很高。

****Technical Abstract****This project investigates the effect of long-range hydrodynamic interactions (LRHI) on swimming of a single bacterium and a group of bacteria in proximity of each other. At a single cell level, how the backflow influences the swimming behavior of the cell who creates it has attracted attentions from mathematicians and physicists. Important theoretical predictions were made, but they have not been rigorously tested in the laboratory. This is in part due to most studies so far used Escherichia coli, which have multiple flagella that are difficult to model mathematically and in part due to the difficulty in changing the physical parameters, such as the hydrodynamic load, over a broad range. Using marine bacteria V. alginolyticus that possess a single polar flagellum and whose cell body can differentiate into different sizes, these difficulties can be overcome. The marine bacteria also have other interesting attributes that can contribute significantly to this research, which include: (i) a tunable motor powered by the sodium ions in the fluid, (ii) a large swimming speed, and most importantly (iii) the available mutants that swim exclusively in the forward (pushers) and backward (pullers) directions. Theoretically the mutual hydrodynamic interactions between two pushers, two pullers, and a pusher and a puller are expected to be different. We use a state-of-the-art optical trapping techniques to measure these different forces quantitatively. At a macroscopic level, we examine hydrodynamic patterns formed by these different swimmers. Recently, there have been intense theoretical efforts in this area, resulting in interesting predictions for this type of "living" fluids. These include the prediction of novel rheological behaviors, nonequilibrium order/disorder phase transitions, and liquid-crystalline-like hydrodynamic structures. This research trains graduate students in the emerging field of biophysics. ****Non-Technical Abstract****It is a common experience that a mechanical disturbance in a fluid spreads to its neighbors with a well-defined speed, i.e., the sound velocity, while its intensity decreases with distance. A swimming bacterium can be considered as a local source of mechanical disturbance and fluid motion in the vicinity can influence the bacterium's own motion as well as others in its neighborhood. This is what is called hydrodynamic interaction and can be described by a mathematical theory developed by Lighthill; it appears to work reasonably well for isolated swimming bacterium. However, its application to a large group of swimming bacteria in high concentrations is not well understood. The effect of hydrodynamic interaction on swimming of individual and a large group of bacteria is significant because it influences their ability to sequester nutrients in aqueous environments and to form biofilms on surfaces; both are important for ecological and biomedical reasons. The project uses the state-of-the-art techniques of laser trapping to manipulate bacteria to determine mutual hydrodynamic interactions between pair of bacteria in different configurations. These measurements compare and refine theoretical models that make predictions about collective behaviors of large number of bacteria. Given the multidisciplinary nature of the research, students trained are in high demand in academia and in industry.