Magneto-aerotaxis in magnetotactic bacteria

趋磁细菌在一种专门的细胞器——磁小体的帮助下在磁场中定向。磁小体通常呈链状组织，就像细胞内的指南针一样。这样，它们在磁场的引导下，以鞭毛为动力游泳；这种细菌可以被理解为自我推进的罗盘针。趋磁性通常与对氧梯度的策略反应交织在一起，因为趋磁细菌利用地球磁场的垂直分量来导航到靠近分层水生环境底部的氧-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表征磁致航致行为，以了解磁场和氧梯度响应的复杂相互作用。此外，我们旨在了解这种相互作用如何取决于细胞磁矩和运动装置的结构。在第一个资助阶段，我们发现了几种新的磁致气动行为，并将这些行为分类为基于氧梯度或磁场或两者的定向游泳理论模型。此外，我们已经开发了三维单个细菌运动的理论和实验研究方法。在第二个资助期，我们将把这些方法结合起来，对趋磁性和潜在的游泳特性进行定量理解。我们将在三维空间中跟踪单个细菌，并定量表征它们在不同条件下（不同氧浓度、均匀性和梯度、不同磁场强度和相对于氧梯度的倾斜度）和不同趋磁细菌菌株的运动性（方向变化的速度、速率和角度、轨迹形状）。这些结果将用于建立一个基于具有多个内部状态的活跃布朗粒子的趋磁性模型。该模型将用于预测不同磁场强度下氧梯度（气动带）细菌密度的空间分布。这些预测将根据实验密度曲线进行测试。此外，我们将使用Stokesian动力学模拟方法模拟趋磁细菌在不同细胞结构下的游动，以深入了解鞭毛的协调以及磁矩与鞭毛或鞭毛的相对方向的影响。这些模拟的轨迹将与3D跟踪的实验轨迹进行比较。我们在多个层面上的实验方法和理论描述的结合将导致磁致气动运动的全面定量图像，并更广泛地阐明生物和合成微游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.