Potassium response and homeostasis in Mycobacterium tuberculosis modulates environmental adaptation and is important for host colonization

Successful host colonization by bacteria requires sensing and response to the local ionic milieu, and coordination of responses with the maintenance of ionic homeostasis in the face of changing conditions. We previously discovered that Mycobacterium tuberculosis (Mtb) responds synergistically to chloride (Cl-) and pH, as cues to the immune status of its host. This raised the intriguing concept of abundant ions as important environmental signals, and we have now uncovered potassium (K+) as an ion that can significantly impact colonization by Mtb. The bacterium has a unique transcriptional response to changes in environmental K+ levels, with both distinct and shared regulatory mechanisms controlling Mtb response to the ionic signals of K+, Cl-, and pH. We demonstrate that intraphagosomal K+ levels increase during macrophage phagosome maturation, and find using a novel fluorescent K+-responsive reporter Mtb strain that K+ is not limiting during macrophage infection. Disruption of Mtb K+ homeostasis by deletion of the Trk K+ uptake system results in dampening of the bacterial response to pH and Cl-, and attenuation in host colonization, both in primary murine bone marrow-derived macrophages and in vivo in a murine model of Mtb infection. Our study reveals how bacterial ionic homeostasis can impact environmental ionic responses, and highlights the important role that abundant ions can play during host colonization by Mtb.细菌成功定殖宿主，需感知并响应局部离子微环境，同时在环境条件变化时协调各类应答以维持离子稳态（ionic homeostasis）。我们此前发现，结核分枝杆菌（Mycobacterium tuberculosis，Mtb）可协同响应氯离子（chloride，Cl⁻）与pH值，以此感知宿主免疫状态。这一发现提出了一个引人深思的概念：丰度较高的离子可作为重要的环境信号；本研究进一步证实，钾离子（potassium，K⁺）便是可显著影响结核分枝杆菌定殖的关键离子之一。该细菌对环境钾离子水平的变化存在独特的转录应答，且存在独特且共有的调控机制，共同调控Mtb对钾离子、氯离子及pH值这三类离子信号的应答过程。我们证实，巨噬细胞吞噬体成熟过程中，吞噬体腔内的钾离子水平会升高；同时通过构建新型响应钾离子的荧光报告Mtb菌株开展实验，发现巨噬细胞感染过程中钾离子并未成为限制因素。通过敲除Trk钾离子摄取系统（Trk K+ uptake system）破坏Mtb的钾离子稳态，会削弱该细菌对pH值与氯离子的应答能力；且无论是在小鼠原代骨髓源巨噬细胞中，还是在结核分枝杆菌感染小鼠模型的体内实验中，该细菌的宿主定殖能力均出现显著减弱。本研究揭示了细菌离子稳态如何影响其对环境离子信号的应答，并阐明了丰度较高的离子在结核分枝杆菌宿主定殖过程中发挥的关键作用。