School of Biomedical Sciences and the Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, United Kingdom.
School of Biomedical Sciences and the Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, United Kingdom
mBio. 2019 Jan 15;10(1):e02490-18. doi: 10.1128/mBio.02490-18.
Exoelectrogenic bacteria are defined by their ability to respire on extracellular and insoluble electron acceptors and have applications in bioremediation and microbial electrochemical systems (MESs), while playing important roles in biogeochemical cycling. MR-1, which has become a model organism for the study of extracellular respiration, is known to display taxis toward insoluble electron acceptors, including electrodes. Multiple mechanisms have been proposed for MR-1's tactic behavior, and, here, we report on the role of electrochemical potential by video microscopy cell tracking experiments in three-electrode electrochemical cells. MR-1 trajectories were determined using a particle tracking algorithm and validated with Shannon's entropy method. Tactic response by MR-1 in the electrochemical cell was observed to depend on the applied potential, as indicated by the average velocity and density of motile (>4 µm/s) MR-1 close to the electrode (<50 µm). Tactic behavior was observed at oxidative potentials, with a strong switch between the potentials -0.15 to -0.25 V versus the standard hydrogen electrode (SHE), which coincides with the reduction potential of flavins. The average velocity and density of motile MR-1 close to the electrode increased when riboflavin was added (2 µM), but were completely absent in a Δ/Δ mutant of MR-1. Besides flavin's function as an electron mediator to support anaerobic respiration on insoluble electron acceptors, we propose that riboflavin is excreted by MR-1 to sense redox gradients in its environment, aiding taxis toward insoluble electron acceptors, including electrodes in MESs. Previous hypotheses of tactic behavior of exoelectrogenic bacteria are based on techniques that do not accurately control the electrochemical potential, such as chemical-in-plug assays or microscopy tracking experiments in two-electrode cells. Here, we have revisited previous experiments and, for the first time, performed microscopy cell-tracking experiments in three-electrode electrochemical cells, with defined electrode potentials. Based on these experiments, taxis toward electrodes is observed to switch at about -0.2 V versus standard hydrogen electrode (SHE), coinciding with the reduction potential of flavins.
异化金属还原菌的定义是其能够在细胞外和不溶性电子受体上进行呼吸，并在生物修复和微生物电化学系统（MESs）中有应用，同时在生物地球化学循环中发挥重要作用。MR-1 已成为研究细胞外呼吸的模式生物，已知其对不溶性电子受体（包括电极）表现出趋化性。已经提出了多种机制来解释 MR-1 的趋性行为，在这里，我们通过三电极电化学电池中的视频显微镜细胞跟踪实验报告了电化学势的作用。使用粒子跟踪算法确定了 MR-1 的轨迹，并使用 Shannon 熵方法进行了验证。通过电化学电池中的 MR-1 轨迹确定，发现其趋化反应取决于施加的电势，这表明靠近电极的（>4 μm/s）MR-1 的平均速度和密度（<50 μm）。在氧化电势下观察到了 MR-1 的趋化行为，在标准氢电极（SHE）的-0.15 至-0.25 V 之间的电势之间存在强烈的切换，这与黄素的还原电位相吻合。当添加核黄素（2 μM）时，靠近电极的运动性 MR-1 的平均速度和密度增加，但在 MR-1 的 Δ/Δ 突变体中完全不存在。除了黄素作为电子介体支持不溶性电子受体上的厌氧呼吸的功能外，我们还提出，MR-1 排泄核黄素以感测其环境中的氧化还原梯度，有助于向不溶性电子受体（包括 MESs 中的电极）趋化。先前关于异化金属还原菌趋性行为的假设是基于不能准确控制电化学势的技术，例如化学插塞测定或在双电极细胞中的显微镜跟踪实验。在这里，我们重新审视了以前的实验，并首次在具有定义电极电势的三电极电化学电池中进行了显微镜细胞跟踪实验。基于这些实验，观察到向电极的趋化性在大约-0.2 V 左右切换，与黄素的还原电位相吻合。
