Bacteria in biofilms secrete potassium ions to attract free swimming cells. However, the basis of chemotaxis to potassium remains poorly understood. Here, using a microfluidic device, we found that Escherichia coli can rapidly accumulate in regions of high potassium concentration on the order of millimoles. Using a bead assay, we measured the dynamic response of individual flagellar motors to stepwise changes in potassium concentration, finding that the response resulted from the chemotaxis signaling pathway instead of the motor response to changes in the proton motive force (PMF). To characterize the chemotactic response to potassium, we exposed the bacteria to a range of potassium concentrations and measured the dose-response curve and adaptation kinetics via a FRET assay, finding that the chemotaxis pathway exhibited a sensitive response and fast adaptation to potassium. We further found that the two major chemoreceptors Tar and Tsr respond differently to potassium. Tar receptors exhibit a biphasic response, whereas Tsr receptors respond to potassium as an attractant. These different responses were consistent with the responses of the two receptors to intracellular pH changes. Therefore, bacteria may sense the change in potassium concentration by sensing the change in intracellular pH. The sensitive response and fast adaptation allow bacteria to sense and localize small changes in potassium concentration. As the ratio of the two major chemoreceptors changes with bacterial growth stages, the differential responses of Tar and Tsr receptors to potassium suggest that cells at different growth stages respond differently to potassium and may have different requirements for potassium.

中文翻译：

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钾介导的细菌趋化反应

生物膜中的细菌分泌钾离子来吸引自由游动的细胞。然而，对钾的趋化作用的基础仍然知之甚少。在这里，使用微流体装置，我们发现大肠杆菌可以在毫摩尔量级的高钾浓度区域快速积累。使用珠测定，我们测量了单个鞭毛电机对钾浓度逐步变化的动态响应，发现该响应是由趋化信号通路引起的，而不是电机对质子动力（PMF）变化的响应。为了表征对钾的趋化反应，我们将细菌暴露于一系列钾浓度下，并通过 FRET 测定测量剂量反应曲线和适应动力学，发现趋化途径对钾表现出敏感的反应和快速的适应。我们进一步发现两种主要化学感受器 Tar 和 Tsr 对钾的反应不同。 Tar 受体表现出双相反应，而 Tsr 受体则对钾作为引诱剂做出反应。这些不同的反应与两种受体对细胞内 pH 变化的反应一致。因此，细菌可以通过感知细胞内pH值的变化来感知钾浓度的变化。敏感的反应和快速的适应使细菌能够感知并定位钾浓度的微小变化。由于两种主要化学感受器的比例随细菌生长阶段而变化，Tar和Tsr受体对钾的不同反应表明不同生长阶段的细胞对钾的反应不同，可能对钾有不同的需求。