Anomalous statistics of collectively moving bacteria with different aspect ratios

不同纵横比集体运动细菌的异常统计

• 批准号: 396653815
• 负责人: Professor Dr. Markus Bär
• 金额: --
• 依托单位: Blaustein Institutes for Desert Research
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2018
• 资助国家: 德国
• 起止时间: 2017-12-31 至 2020-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/396653815?language=en
• 关键词: Anomalous; statistics; collectively; moving; bacteria

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.

成群的细菌是一种自我推进的细胞，它们在表面集体运动，形成由数百万个个体组成的动态漩涡和喷流。由于一些成群的物种在实际运动之前拉长，细胞纵横比被认为在集体动力学中发挥着重要作用。对自行推进的棒状颗粒的广泛研究证实，伸长可以促进排列，从而强烈影响动力学。此外，最近对长度不同的成群枯草芽孢杆菌变种的集体动力学实验表明，成群统计数据以一种关键的、基本的方式依赖于长宽比，这是当前理论没有预测的：野生型和长度相似的变种获得了最快的运动。令人惊讶的是，较短和较长的单元都表现出异常的、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表明，当前的活性粒子理论缺乏一些必要的成分。先前实验的主要困难之一是，所考虑的蜂群是多层的。这里，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相应模型的模拟的定量比较。为此，我们提出了一种新的方法，该方法基于细菌在单层中生长和聚集的实验。初步结果显示了与在多层群中观察到的定性相似的行为。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。其中包括运动性、关联函数、局域有序和密度涨落的局域分析。在建模方面，我们提出了基于代理的模拟来研究不同长宽比粒子的局部有序和堆积分数之间的相互作用。这些模拟将耦合到包括密度变化的连续介质模型来描述活动流体。我们的方法的主要假设是非高斯涨落是由于密度涨落和局域有序之间的耦合。在大长宽比和小长宽比下，棒状晶胞的空间分布都允许出现大空洞，这要么是由于大长宽比下的空间斥力，要么是因为小长宽比下的低极序度。因此，在两种制度下，密度都不是均匀的。我们假设异常统计是由于出现了几个允许的亚稳态，每个亚稳态都显示了正常的统计。总而言之，对异常集群统计数据的观察，提供了一个独特的机会，让我们得以一窥活性物质背后的物理原理。最先进的细菌悬浮液理论并没有捕捉到这一点。建议的研究旨在对这一现象进行实验和理论相结合的研究。