Zhu Xuejun, Ge Xin, Li Ning, Wu Long-Fei, Luo Chunxiong, Ouyang Qi, Tu Yuhai, Chen Guanjun
Center for Quantitative Biology and Peking-Tsinghua Center for Life Sciences at Peking University, Academy for Advanced Interdisciplinary Studies, Peking University, Beijing, 100871, China.
Integr Biol (Camb). 2014 Jul 24;6(7):706-13. doi: 10.1039/c3ib40259b. Epub 2014 May 30.
The mechanism of how magnetotactic bacteria navigate along the magnetic field has been a puzzle. Two main models disagree on whether the magnetotactic behavior results from passive alignment with the magnetic field or active sensing of the magnetic force. Here, we quantitatively studied the swimming patterns of Magnetospirillum magneticum AMB-1 cells to understand the origin of their magnetotactic behaviors. Single-cell tracking and swimming pattern analysis showed that the cells follow a mixed run-reverse-tumble pattern. The average run time decreased with the angle between the cell's moving velocity and the external magnetic field. For mutant cells without the methyl-accepting chemotaxis protein (MCP) Amb0994, such dependence disappeared and bacteria failed to align with magnetic field lines. This dysfunction was recovered by complementary Amb0994 on a plasmid. At high magnetic field (>5 mT), all strains with intact magnetosome chains (including the Δamb0994-0995 strains) showed alignment with the external magnetic field. These results suggested that the mechanism for magnetotaxis is magnetic field dependent. Due to the magnetic dipole moment of the cell, the external magnetic field exerts a torque on the cell. In high magnetic fields, this torque is large enough to overcome the random re-orientation of the cell, and the cells align passively with the external magnetic field, much like a compass. In smaller (and biologically more relevant) external fields, the external force alone is not strong enough to align the cell mechanically. However, magnetotactic behaviors persist due to an active sensing mechanism in which the cell senses the torque by Amb0994 and actively regulates the flagella bias accordingly to align its orientation with the external magnetic field. Our results reconciled the two putative models for magnetotaxis and revealed a key molecular component in the underlying magneto-sensing pathway.
趋磁细菌如何沿着磁场导航的机制一直是个谜。关于趋磁行为是由与磁场的被动对齐还是对磁力的主动感知导致的，两种主要模型存在分歧。在此，我们定量研究了趋磁螺菌AMB-1细胞的游动模式，以了解其趋磁行为的起源。单细胞追踪和游动模式分析表明，细胞遵循混合的前进-后退-翻滚模式。平均前进时间随着细胞移动速度与外部磁场之间的夹角而减少。对于没有甲基接受趋化蛋白（MCP）Amb0994的突变细胞，这种依赖性消失，细菌无法与磁力线对齐。通过在质粒上互补Amb0994，这种功能障碍得以恢复。在高磁场（>5 mT）下，所有具有完整磁小体链的菌株（包括Δamb0994-0995菌株）都显示出与外部磁场对齐。这些结果表明，趋磁机制是依赖磁场的。由于细胞的磁偶极矩，外部磁场会对细胞施加一个扭矩。在高磁场中，这个扭矩足够大，能够克服细胞的随机重新定向，细胞被动地与外部磁场对齐，就像指南针一样。在较小（且在生物学上更相关）的外部磁场中，仅靠外力不足以机械地使细胞对齐。然而，趋磁行为仍然存在，这是由于一种主动感知机制，在这种机制中，细胞通过Amb0994感知扭矩，并相应地主动调节鞭毛偏置，以使其方向与外部磁场对齐。我们的结果调和了两种关于趋磁的假定模型，并揭示了潜在磁传感途径中的一个关键分子成分。
