Magneto-aerotaxis in magnetotactic bacteria

趋磁细菌在一个专门的细胞器--磁小体的帮助下在磁场中进行定向。磁小体通常以链的形式组织起来，起到细胞内指南针的作用。在这种情况下，它们的游泳由鞭毛提供动力，由磁场引导；细菌可以被理解为自我推进的指南针。趋磁作用通常与对氧气梯度的战术反应交织在一起，因为趋磁细菌利用地球磁场的垂直分量导航到分层水生环境底部附近的缺氧-缺氧过渡区。在这个项目中，我们使用理论和实验相结合的方法来定量描述磁-空气趋向性行为，以了解对磁场和氧气梯度的响应的复杂相互作用。此外，我们的目的是了解这种相互作用如何依赖于细胞磁矩和运动装置的结构。在第一个资助期，我们发现了几种新的磁航向行为，并用基于氧气梯度或磁场或两者兼有的定向游泳理论模型对这些行为进行了分类。此外，我们还发展了单菌体三维运动的理论和实验研究方法。在第二个资助期，我们将结合这些方法，对磁悬浮和基本的游泳特性进行定量的了解。我们将在三个维度上跟踪单个细菌，并定量描述它们在不同条件下(不同的氧气浓度、均匀和梯度、不同的磁场强度和相对于氧气梯度的倾角)和不同菌株的趋磁细菌的运动性(速度、方向变化的速度和角度、轨迹的形状)。这些结果将被用来提供一个基于具有多个内态的活性布朗粒子的磁气浮模型。该模型将被用来预测不同磁场强度下氧梯度(向气带)中细菌密度的空间分布。这些预测将与实验密度分布进行对比检验。此外，我们将使用Stokesian动力学模拟来模拟趋磁细菌在不同细胞结构下的游泳，以深入了解鞭毛的协调以及磁矩与鞭毛或鞭毛的相对取向的影响。这些模拟的轨迹将与3D跟踪的实验轨迹进行比较。我们的实验方法和多层次的理论描述相结合，将导致磁-空气导向运动性的全面量化图景，并更广泛地阐明生物和合成微泳者定向运动的不同信号的整合。

Magnetotactic bacteria orient in magnetic fields with the help of a dedicated organelle, the magnetosome. Magnetosomes are typically organized in a chain, which acts as an intracellular compass needle. In this way, their swimming, powered by their flagella, is guided by the magnetic field; the bacteria can be understood as self-propelled compass needles. Magnetotaxis is usually intertwined with the tactic response to oxygen gradients, as magnetotactic bacteria use the vertical component of the Earth magnetic field to navigate towards the oxic-anoxic transition zone near the bottom of layered aquatic environments. In this project, we use a combined theoretical and experimental approach for the quantitative characterization of magneto-aerotactic behaviors to understand the complex interplay of the responses to magnetic field and to oxygen gradients. In addition, we aim at understanding how this interplay depends on the architecture of the cells magnetic moment and motility apparatus. In the first funding period, we have discovered several new magneto-aerotactic behaviors and classified these behaviors with a theoretical model of directional swimming based on either the oxygen gradient or the magnetic field or both. Moreover, we have developed methods for the theoretical and experimental study of the motion of single bacteria in 3D. In the second funding period, we will combine these methods towards a quantitative understanding of magneto-aerotaxis and the underlying swimming properties. We will track individual bacteria in three dimensions and quantitatively characterize their motility (speed, rate and angle of directional changes, shape of the trajectories) under different conditions (different oxygen concentrations, homogeneous and gradient, different magnetic field strength and inclination relative to the oxygen gradient) and for different strains of magnetotactic bacteria. These results will be used to inform a model for magneto-aerotaxis based on active Brownian particles with multiple internal states. The model will be used to predict the spatial profiles of the bacterial density in oxygen gradients (aerotactic band) for different magnetic field strengths. These predictions will be tested against experimental density profiles. Moreover, we will use Stokesian dynamics simulations to simulate the swimming of magnetotactic bacteria for different cellular architectures to get insight into the coordination of flagella and the effect of the relative orientation of the magnetic moment and the flagellum or flagella. The trajectories from these simulations will be compared to the experimental ones from the 3D tracking. The combination of our experimental approaches and theoretical description on multiple levels will lead to a comprehensive quantitative picture of magneto-aerotactic motility and more generally shed light on the integration of different signals for directional motility of biological and synthetic microswimmers.