Anomalous statistics of collectively moving bacteria with different aspect ratios

群集细菌是自我推进的细胞，它们在表面上集体移动，形成由数百万个体组成的动态漩涡和射流。由于一些群集物种在实际运动之前伸长，因此细胞长宽比被认为在集体动力学中起着重要作用。对自推进杆状颗粒的广泛研究证实，伸长促进对齐，强烈影响动力学。此外，最近的实验表明，成群的枯草芽孢杆菌的变种，不同的只是在长度上的集体动力学表明，群集统计取决于纵横比在一个关键的，基本的方式没有预测到目前的理论：最快的运动是获得野生型和变种是相似的长度。令人惊讶的是，较短和较长的细胞都表现出自相关函数的异常、非高斯统计和非指数衰减。反常统计的出现意味着目前的活性粒子理论缺乏一些必要的成分。以前实验的主要困难之一是所考虑的群是多层的。在此，不同层的体积分数不能被足够好地控制以允许测量结果与对应模型的模拟的定量比较。出于这个原因，我们提出了一种新的方法，这是基于实验中，细菌生长和群集在一个单层。初步结果表明，在多层集群中观察到的定性相似的行为。此外，2D设置允许对视场内的细胞配置进行全面分析，从而可以与模拟进行详细比较。这些包括局部分析的运动，相关函数，局部秩序和密度波动。对于建模，我们提出了基于代理的模拟来研究不同纵横比的颗粒的局部顺序和包装分数之间的相互作用。这些模拟将耦合到连续模型，包括密度变化，以描述活动流体。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。无论是在大的和小的纵横比，棒状细胞的空间分布承认大的孔，无论是由于在大的纵横比的空间排斥或由于在低的纵横比的低极性顺序。因此，密度在两种状态下是不均匀的。我们假设，异常的统计是由于发生了几个允许的亚稳态，每个正常的统计。 总而言之，对异常群集统计的观察，这是最先进的细菌悬浮液理论所不能捕捉到的，提供了对活性物质物理学的独特一瞥。拟议的研究旨在结合实验和理论研究这一现象。

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.