Bacterial active matter

Bacteria are among the oldest and most abundant species on Earth. Bacteria successfully colonize diverse habitats and play a significant role in the oxygen, carbon, and nitrogen cycles. They also form human and animal microbiota and may become sources of pathogens and a cause of many infectious diseases. Suspensions of motile bacteria constitute one of the most studied examples of active matter: a broad class of non-equilibrium systems converting energy from the environment (e.g., chemical energy of the nutrient) into mechanical motion. Concentrated bacterial suspensions, often termed active fluids, exhibit complex collective behavior, such as large-scale turbulent-like motion (so-called bacterial turbulence) and swarming. The activity of bacteria also affects the effective viscosity and diffusivity of the suspension. This work reports on the progress in bacterial active matter from the physics viewpoint. It covers the key experimental results, provides a critical assessment of major theoretical approaches, and addresses the effects of visco-elasticity, liquid crystallinity, and external confinement on collective behavior in bacterial suspensions.

细菌是地球上最古老且数量最多的物种之一。细菌成功地在各种不同的栖息地定殖，并在氧、碳和氮循环中发挥重要作用。它们还构成人类和动物的微生物群，并且可能成为病原体的来源以及许多传染病的病因。游动细菌的悬浮液是活性物质中被研究最多的例子之一：活性物质是一大类非平衡系统，它将来自环境的能量（例如，营养物质的化学能）转化为机械运动。浓缩的细菌悬浮液，通常被称为活性流体，表现出复杂的集体行为，例如大规模的类湍流运动（所谓的细菌湍流）和群聚。细菌的活动也会影响悬浮液的有效粘度和扩散率。这项工作从物理学的角度报告了细菌活性物质的研究进展。它涵盖了关键的实验结果，对主要的理论方法进行了批判性评估，并阐述了粘弹性、液晶性以及外部限制对细菌悬浮液集体行为的影响。