Anomalous statistics of collectively moving bacteria with different aspect ratios

成群的细菌是自我推进的细胞，它们在表面上集体移动，形成由数百万个个体组成的动态漩涡和射流。由于一些群居物种在实际运动之前就拉长了，细胞宽高比被认为在集体动力学中起着重要作用。对自走杆状颗粒的广泛研究证实，伸长率促进对齐，强烈影响动力学。此外，最近对枯草芽孢杆菌（Bacillus subtilis）群居变异的集体动力学实验表明，只有长度不同的枯草芽孢杆菌（Bacillus subtilis）的群居统计数据以一种关键的、基本的方式依赖于长宽比，而不是当前理论所预测的：野生型和长度相似的变异获得了最快的运动。令人惊讶的是，较短和较长的细胞都表现出异常，非高斯统计和自相关函数的非指数衰减。反常统计数据的出现意味着目前的活性粒子理论缺少一些必要的成分。先前实验的主要困难之一是所考虑的群体是多层的。在这里，不同层的体积分数不能被很好地控制，从而不能将测量结果与相应模型的模拟进行定量比较。因此，我们提出了一种新的方法，该方法是基于细菌在单层中生长和聚集的实验。初步结果表明，在多层蜂群中观察到的定性行为相似。此外，2D设置允许对视野内的单元配置进行全面分析，允许与模拟进行详细比较。其中包括局部运动性分析、相关函数、局部秩序和密度波动。在建模方面，我们提出了基于智能体的模拟来研究不同纵横比粒子的局部阶数和堆积分数之间的相互作用。这些模拟将与包括密度变化在内的连续统模型相结合，以描述活跃流体。我们的方法的主要假设是，非高斯波动是由于密度波动和局部秩序之间的耦合。无论是在大长宽比还是小长宽比下，杆状细胞的空间分布都允许有大孔，这是由于大长宽比下的空间排斥力或低长宽比下的低极性序。因此，密度在两种情况下都不是均匀的。我们假设反常统计是由于几个允许的亚稳态的发生，每个显示正常的统计。总而言之，观察到的异常蜂群统计数据，这是没有被最先进的细菌悬浮液理论捕获的，提供了一个独特的视角来了解活性物质的物理基础。本研究旨在对这一现象进行实验与理论相结合的研究。

Swarming bacteria are self-propelled cells that move collectively on surfaces, forming dynamic whirls and jets that consist of millions of individuals. Because some swarming species elongate prior to actual motion, cell aspect ratio is thought to play a significant role in the collective dynamics. Extensive research on self-propelled rod-like particles confirms that elongation promotes alignment, strongly affecting the dynamics. Moreover, recent experiments on the collective dynamics of variants of swarming Bacillus subtilis that differ only in length show that the swarming statistics depends on the aspect ratio in a critical, fundamental fashion not predicted by current theory: the fastest motion was obtained for the wild-type and variants that are similar in length. Surprisingly, both shorter and longer cells exhibit anomalous, non-Gaussian statistics and non-exponential decay of the auto-correlation function. The appearance of anomalous statistics implies that the current theory of active particles is lacking some essential ingredients. One of the major difficulties with previous experiments is that the considered swarms were multi-layered. Here, the volume fraction of the different layers cannot be controlled sufficiently well to allow for a quantitative comparison of measurement results with simulations of corresponding models. For this reason, we propose a new approach which is based on experiments in which bacteria grow and swarm in a monolayer. The preliminary results show a qualitatively similar behavior as observed in multilayer swarms. In addition, the 2D setup allows a comprehensive analysis of cell configurations within the field of view, allowing detailed comparison with simulations. These include local analysis of motility, correlation functions, local order and density fluctuations. For modelling, we propose agent-based simulations to study the interplay between local order and packing fractions for particles of different aspect ratios. These simulations will be coupled to continuum models including density variations to describe the active fluid. The main assumption underlying our approach is that non-Gaussian fluctuations are due to coupling between density fluctuations and local order. Both at large and small aspect ratios, the spatial distribution of the rod-like cell admits large holes, either due to steric repulsion at large aspect ratios or due to low polar order at low aspect ratios. Therefore, the density is not homogeneous in both regimes. We hypothesize that anomalous statistics is due to the occurrence of several allowed metastable states, each showing normal statistics. To summarise, the observation of anomalous swarming statistics, which is not captured by the state-of-the-art theory of bacterial suspensions, offers a unique glimpse into the physics underlying active matter. The proposed research aims at a combined experimental and theoretical study of this phenomenon.