Center for Nonlinear Dynamics and Department of Physics, University of Texas at Austin, Austin, Texas, USA.
Biophys J. 2011 Sep 7;101(5):1017-24. doi: 10.1016/j.bpj.2011.07.019.
Swarming bacteria move on agar surfaces in groups, using flagella as motive organelles. Motility depends critically on surface wetness, which is enabled by osmotic agents and surfactants secreted by the bacteria. In a recent study, the upper surface of an Escherichia coli swarm was found to be stationary, as determined from the motion of MgO particles deposited on the swarm. This led to the remarkable conclusion that the bacteria move between two stationary surfaces-the agar gel below and the liquid/air interface above. That study suggested that secreted surfactants may contribute to immobilizing the upper surface of a swarm. Here, we test this proposition using two robust surfactant-producing bacteria. We find antithetically that the upper surfaces of both these swarms are mobile, showing a superdiffusive behavior in swarms with stronger surfactant activity. Superdiffusive behavior was not observed on the surface of a drop of bacterial culture, on bacteria-free culture supernatant, or on nonswarming surfactant-producer colonies, which suggests that superdiffusion is an emergent property resulting from the interaction of the collective motion of the bacteria within the swarm with the surfactant layer above. Swarming not only allows bacteria to forage for food, but also confers protective advantages against antimicrobial agents. Our results are therefore relevant to superdiffusive strategies in biological foraging and survival.
群体游动细菌在琼脂表面成群移动，使用鞭毛作为运动器官。运动能力取决于表面的湿度，这是由细菌分泌的渗透剂和表面活性剂实现的。在最近的一项研究中，发现大肠杆菌群体的上表面是静止的，这是通过在群体上沉积的氧化镁颗粒的运动来确定的。这得出了一个惊人的结论，即细菌在两个静止的表面之间移动——下面的琼脂凝胶和上面的液体/空气界面。该研究表明，分泌的表面活性剂可能有助于固定群体的上表面。在这里，我们使用两种稳健的表面活性剂产生细菌来检验这一假设。我们发现，这两种群体的上表面都是可移动的，在表面活性剂活性更强的群体中表现出超扩散行为。在细菌培养物的液滴表面、无细菌的培养上清液或不进行群体游动的表面活性剂产生菌的菌落上均未观察到超扩散行为，这表明超扩散是由群体内细菌的集体运动与表面活性剂层之间的相互作用产生的一种涌现性质。群体游动不仅使细菌能够觅食，还赋予其对抗菌剂的保护优势。因此，我们的研究结果与生物觅食和生存中的超扩散策略有关。
