Anomalous statistics of collectively moving bacteria with different aspect ratios

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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.