While most quantitative studies of the motion of magnetotactic bacteria rely on the premise that the cells' magnetic dipole moment is aligned with their direction of motility, this assumption has so far rarely been challenged. Here we use phase contrast microscopy to detect the rotational diffusion of non-motile cells of Magnetospirillum magneticum AMB-1 around their magnetic moment, showing that in this species the magnetic dipole moment is, in fact, not exactly aligned with the cell body axis. From the cell rotational trajectories, we are able to infer the misalignment between cell magnetic moment and body axis with a precision of better than 1°, showing that it is, on average, 6°, and can be as high as 20°. We propose a method to correct for this misalignment, and perform a non-biased measurement of the magnetic moment of single cells based on the analysis of their orientation distribution. Using this correction, we show that magnetic moment strongly correlates with cell length. The existence of a range of misalignments between magnetic moment and cell axis in a population implies that the orientation and trajectories of magnetotactic bacteria placed in external magnetic fields is more complex than generally assumed, and might show some important cell-to-cell differences.

中文翻译：

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趋磁细菌Magnetospirillum magneticum AMB-1中的磁偶极矩与细胞轴之间未对准。

尽管对趋磁细菌运动的大多数定量研究都以细胞的磁偶极矩与其运动方向对齐为前提，但迄今为止，这种假设很少受到质疑。在这里，我们使用相差显微镜检测磁动螺旋菌AMB-1的非运动细胞在其磁矩附近的旋转扩散，表明在该物种中，磁偶极矩实际上与细胞体轴不完全对齐。从细胞旋转轨迹，我们能够推断出细胞磁矩与体轴之间的失准，其精度优于1°，这表明它平均为6°，甚至可以高达20°。我们提出了一种纠正这种偏差的方法，并根据其方向分布的分析对单个单元的磁矩进行无偏测量。使用该校正，我们表明磁矩与晶胞长度密切相关。群体中磁矩和细胞轴之间存在一系列不对准现象，这意味着放置在外部磁场中的趋磁细菌的方向和轨迹比通常假定的更为复杂，并且可能显示出一些重要的细胞间差异。