Bacterial Swarming: Role of Flagella in Emergent Behavior

鞭毛细菌表现出独特的环境依赖运动模式。在液体环境中，个体（浮游）细胞表现出所谓的游泳运动。另一种不同的运动模式被称为细菌群，细菌在表面（在生物膜中）集体迁移，并能够形成稳定的聚集体，这可以变得高度运动性。细菌群密集排列，并表现出大规模的旋转和流动运动。不同菌株在蜂群模式下表现出明显不同于游泳细胞的形态，由于细胞分裂受到抑制，它们更加细长，鞭毛数量显著增加。因此，鞭毛除了简单地为细菌提供推进力外，还在群体行为中扮演着重要的角色。然而，到目前为止，人们对细菌如何利用鞭毛在表面上迁移以及它们对群体的影响知之甚少。在这个项目中，我们想要阐明细菌组装中鞭毛的性质及其在其紧急集体行为中的作用。此外，我们想要解开粘弹性流体对单个细胞的游泳特性以及对群体的影响。因此，我们将应用中尺度流体动力学模拟，将（粘弹性）流体的多粒子碰撞动力学方法与嵌入细菌的分子动力学模拟相结合。为了解决粘弹性效应，我们将系统地增加微游泳者的复杂性，从一个球形蠕动体到一个游泳的大肠杆菌细胞，再到蜂拥细胞，并解决它们的特异性。为了阐明鞭毛数量对群体的影响，我们将研究两种鞭毛细胞：大约有20根鞭毛的大肠杆菌样的鞭毛细胞和大约有200根鞭毛的奇异单胞菌样的鞭毛细胞。特别注意鞭毛束的形成。在这里，我们将系统地研究鞭毛驱动力矩对小细胞鞭毛束和细胞迁移的影响。大肠杆菌型细胞的集体迁移行为与非常细长的细胞明显不同。对于大肠杆菌型细胞，我们将研究大细胞群（筏）的迁移行为，特别强调身体形状，鞭毛和流体动力学相互作用的作用。细胞间鞭毛的相互作用对P. mirabilis样细胞来说是最重要的，如之前的资助期所示。我们将系统地分析相邻细胞之间出现的交织鞭毛束。我们想要了解这些束的动力学以及细胞集体推进的方式。此外，我们对筏子和更大的集合的集体行为感兴趣，并希望解决鞭毛在筏子结构和运动中的作用。这包括单个细胞与木筏的合并（分裂）和细胞间束的形成。

Flagellated bacteria exhibit distinct environmental dependent modes of locomotion. In a liquid environment, individual (planktonic) cells exhibit the so-called swimming motility. A different mode of motion is denoted as bacterial swarming, where bacteria migrate collectively over surfaces (in biofilms) and are able to form stable aggregates, which can become highly motile. Bacteria swarms are densely packed and exhibit large-scale swirling and streaming motions. Various bacteria strains show distinctly different morphologies in the swarming mode compared to swimmer cells as they are more elongated by suppression of cell division and their number of flagella is significantly increased. Hence, it is expected that flagella play a major role in the swarming behavior, aside from simply providing the propulsion of a bacterium. However, so far little is known on how bacteria use flagella to migrate across surfaces and their impact on swarming.In this project, we want to elucidate the properties of flagella in bacteria assemblies and the role in their emergent collective behavior. Moreover, we want to unravel the influence of a viscoelastic fluid on the swimming properties of individual cells as well as on swarming. Thereby, we will apply mesoscale hydrodynamic simulations, combining the multiparticle collision dynamics approach for the (viscoelastic) fluid with molecular dynamics simulations for the embedded bacteria. To address viscoelastic effects, we will systematically increase the complexity of a microswimmer from a spherical squirmer through a swimming E. coli cell up to swarmer cells and resolve their specificities. Two types of flagellated cells will be studied in order to shed light on the influence of the flagellum number on swarming: E. coli-like swarmer cells with on the order of 20 flagella and P. mirabilis-like cells with about 200 flagella. Specific attention will be paid to bundle formation of flagella. Here, we will perform systematic studies on the influence of the flagella-driving torque on flagella bundles and cell migration for small cells. The collective migration behavior of E. coli-type cells is distinctly different from that of very elongated cells. For E. coli-type cells, we will study the migration behavior of large assemblies of cells (rafts) with particular emphasise on the role of body shape, flagella, and hydrodynamic interactions. Intercellular flagella interactions are most important for P. mirabilis-like cells, as shown in the previous funding period. We will systematically analyse the appearing interwoven flagella bundles between adjacent cells. We want to understand the dynamics of the bundles and the way cells are collectively propelled. In addition, we are interested in the collective behavior of rafts and larger assemblies and want to resolve the role of flagella on the raft structure and motility. This comprises merging (splitting) of individual cells with rafts and the formation of intercellular bundles.