Natural bacterial populations are subjected to constant predation pressure by bacteriophages. Bacteria use a variety of molecular mechanisms to defend themselves from phage predation. However, since phages are nonmotile, perhaps the simplest defense against phage is for bacteria to move faster than phages. In particular, chemotaxis, the active migration of bacteria up attractant gradients, may help the bacteria escape slowly diffusing phages. Here we study phage infection dynamics in migrating bacterial populations driven by chemotaxis through low viscosity agar plates. We find that expanding phage-bacteria populations supports two moving fronts, an outermost bacterial front driven by nutrient uptake and chemotaxis and an inner phage front at which the bacterial population collapses due to phage predation. We show that with increasing adsorption rate and initial phage population, the speed of the moving phage front increases, eventually overtaking the bacterial front and driving the system across a transition from a regime where bacterial front speed exceeds that of the phage front to one where bacteria must evolve phage resistance to survive. Our data support the claim that this process requires phage to hitchhike with moving bacteria. A deterministic model recapitulates the transition under the assumption that phage virulence declines with host growth rate which we confirm experimentally. Finally, near the transition between regimes we observe macroscopic fluctuations in bacterial densities at the phage front. Our work opens a new, spatio-temporal, line of investigation into the eco-evolutionary struggle between bacteria and phage.

中文翻译：

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迁移细菌种群的噬菌体感染中的搭便车、崩溃和偶然性。

天然细菌种群受到噬菌体的持续捕食压力。细菌使用多种分子机制来保护自己免受噬菌体的捕食。然而，由于噬菌体是非运动性的，因此抵御噬菌体的最简单的方法可能是让细菌比噬菌体移动得更快。特别是趋化性，即细菌向上引诱梯度的主动迁移，可以帮助细菌逃离缓慢扩散的噬菌体。在这里，我们研究了通过低粘度琼脂平板由趋化性驱动的迁移细菌种群的噬菌体感染动力学。我们发现不断扩大的噬菌体 - 细菌种群支持两个移动前沿，一个由营养吸收和趋化性驱动的最外层细菌前沿和一个内部噬菌体前沿，由于噬菌体捕食，细菌种群在该前沿崩溃。我们表明，随着吸附率和初始噬菌体数量的增加，移动噬菌体前沿的速度增加，最终超过细菌前沿并推动系统从细菌前沿速度超过噬菌体前沿的状态过渡到细菌前沿的状态。必须进化出噬菌体抗性才能生存。我们的数据支持这一过程需要噬菌体与移动的细菌搭便车的说法。确定性模型概括了假设噬菌体毒力随宿主增长率下降的转变，我们通过实验证实了这一点。最后，在制度之间的过渡附近，我们观察到噬菌体前沿细菌密度的宏观波动。我们的工作为细菌和噬菌体之间的生态进化斗争开辟了一条新的时空研究路线。