Center for Studies in Physics and Biology, The Rockefeller University, New York, NY 10021.
Proc Natl Acad Sci U S A. 2014 Feb 4;111(5):E537-45. doi: 10.1073/pnas.1322092111. Epub 2014 Jan 23.
The ecology and dynamics of many microbial systems, particularly in mats and soils, are shaped by how bacteria respond to evolving nutrient gradients and microenvironments. Here we show how the response of the sulfur-oxidizing bacterium Thiovulum majus to changing oxygen gradients causes cells to organize into large-scale fronts. To study this phenomenon, we develop a technique to isolate and enrich these bacteria from the environment. Using this enrichment culture, we observe the formation and dynamics of T. majus fronts in oxygen gradients. We show that these dynamics can be understood as occurring in two steps. First, chemotactic cells moving up the oxygen gradient form a front that propagates with constant velocity. We then show, through observation and mathematical analysis, that this front becomes unstable to changes in cell density. Random perturbations in cell density create oxygen gradients. The response of cells magnifies these gradients and leads to the formation of millimeter-scale fluid flows that actively pull oxygenated water through the front. We argue that this flow results from a nonlinear instability excited by stochastic fluctuations in the density of cells. Finally, we show that the dynamics by which these modes interact can be understood from the chemotactic response of cells. These results provide a mathematically tractable example of how collective phenomena in ecological systems can arise from the individual response of cells to a shared resource.
许多微生物系统的生态和动态，特别是在垫状和土壤中，受到细菌如何响应不断变化的营养梯度和微环境的影响。在这里，我们展示了硫氧化细菌硫单胞菌如何响应不断变化的氧气梯度，导致细胞大规模地形成前沿。为了研究这种现象，我们开发了一种从环境中分离和富集这些细菌的技术。利用这种富集培养物，我们观察到了氧气梯度中 T. majus 前沿的形成和动态。我们表明，这些动态可以理解为分两步发生。首先，向氧气梯度上升的趋化性细胞形成一个以恒定速度传播的前沿。然后，我们通过观察和数学分析表明，这个前沿对细胞密度的变化变得不稳定。细胞密度的随机波动会产生氧气梯度。细胞的反应放大了这些梯度，并导致毫米级的流体流动，将含氧水主动地拉过前沿。我们认为，这种流动是由细胞密度的随机波动激发的非线性不稳定性引起的。最后，我们表明，这些模式相互作用的动力学可以从细胞的趋化反应来理解。这些结果提供了一个在生态系统中集体现象如何由细胞对共享资源的个体反应产生的数学上易于处理的例子。
