Influence of Magnetic Fields on Magneto-Aerotaxis

The response of cells to changes in their physico-chemical micro-environment is essential to their survival. For example, bacterial magnetotaxis uses the Earth's magnetic field together with chemical sensing to help microorganisms move towards favoured habitats. The studies of such complex responses are lacking a method that permits the simultaneous mapping of the chemical environment and the response of the organisms, and the ability to generate a controlled physiological magnetic field. We have thus developed a multi-modal microscopy platform that fulfils these requirements. Using simultaneous fluorescence and high-speed imaging in conjunction with diffusion and aerotactic models, we characterized the magneto- aerotaxis of Magnetospirillum gryphiswaldense. We assessed the influence of the magnetic field (orientation; strength) on the formation and the dynamic of a micro-aerotactic band (size, dynamic, position). As previously described by models of magnetotaxis, the application of a magnetic field pointing towards the anoxic zone of an oxygen gradient results in an enhanced aerotaxis even down to Earth's magnetic field strength. We found that neither a ten-fold increase of the field strength nor a tilt of 45° resulted in a significant change of the aerotactic efficiency. However, when the field strength is zeroed or when the field angle is tilted to 90°, the magneto-aerotaxis efficiency is drastically reduced. The classical model of magneto-aerotaxis assumes a response proportional to the cosine of the angle difference between the directions of the oxygen gradient and that of the magnetic field. Our experimental evidence however shows that this behaviour is more complex than assumed in this model, thus opening up new avenues for research.

细胞对其物理化学微环境变化的响应，是其生存的必要前提。例如，细菌趋磁作用（magnetotaxis）借助地球磁场与化学感知系统，协助微生物向适宜的栖息环境移动。当前针对这类复杂响应的研究，尚缺乏可同时测绘化学环境与生物体响应的方法，也无法生成可控的生理级磁场。为此，我们开发了一套可满足上述全部需求的多模态显微成像平台。我们结合同步荧光成像与高速成像技术，并辅以扩散模型和趋氧模型，对格氏磁螺菌（Magnetospirillum gryphiswaldense）的磁趋氧作用（magneto-aerotaxis）开展了表征研究。我们评估了磁场参数（取向、强度）对微趋氧带的形成与动态变化（包括其尺寸、动态特性与空间位置）的影响。正如此前趋磁作用模型所描述的，当磁场指向氧梯度的缺氧区域时，即便磁场强度低至地球磁场水平，趋氧作用也会得到增强。我们的实验结果显示，将磁场强度提升十倍，或是将磁场倾斜45°，均不会使趋氧效率发生显著变化。然而，当磁场强度归零，或是磁场倾角调整至90°时，磁趋氧效率会大幅下降。经典磁趋氧作用模型假设，生物体的响应与氧梯度方向和磁场方向之间的夹角差的余弦值呈正比。但我们的实验证据表明，该实际行为比模型假设更为复杂，这为相关研究开辟了全新的方向。