Dynamic Clustering in Suspension of Motile Bacteria

Bacteria suspension exhibits a wide range of collective phenomena, arising from interactions between individual cells. Here we show Serratia marcescens cells near an air-liquid interface spontaneously aggregate into dynamic clusters through surface-mediated hydrodynamic interactions. These long-lived clusters translate randomly and rotate in the counterclockwise direction; they continuously evolve, merge with others and split into smaller ones. Measurements indicate that long-ranged hydrodynamic interactions have strong influences on cluster properties. Bacterial clusters change material and fluid transport near the interface and hence may have environmental and biological consequences. Copyright c © EPLA, 2015 Active systems are composed of self-propelled particles that can produce motion by taking in and dissipating energy [1–4]. Examples exist at different length scales, from bacteria suspension [5–10] to flocks of birds [11–13]. Being far from thermal equilibrium, active systems are not subject to thermodynamic constraints, such as detailed balance or fluctuation-dissipation theorem [14–16]. This renders the physics of active systems much richer than that of thermal systems. For example, collective motion with extended spatio-temporal coherence has been reported in many active systems [5–9,11–13,17–19]. Such coherent motion can arise from local interactions that align a particle’s motion with its neighbors through biological coordination [11,12] or physical interactions [17,19]. Active systems without alignment interactions also exhibit interesting collective behavior. Theoretical models have shown that systems with a density-dependent motility phase separate into dense dynamic clusters and a dilute gas phase [14,15,20]. Numerical simulations of repulsive self-propelled disks confirmed the theoretical prediction of phase separation [21,22]. Effects of motility, attractive interaction, and hydrodynamic forces have been extensively explored in simulations [23–26]. On the experimental side, dynamic clusters have been observed in Janus particles (a)E-mail: hepeng zhang@sjtu.edu.cn (platinum-coated [27] and carbon-coated [28]) and colloidal particles with an embedded hematic cube [29]. Schwarz-Linek et al. observed clusters of motile bacteria when they added polymers to bacteria suspension to induce depletion attraction between bacteria [30]. In a very recent paper [31], Petroff et al. reported that Thiovulum majus bacteria form two-dimensional crystals near a liquid-solid interface. Understanding the origins and properties of these dynamic clusters may provide new insights into emergent behaviors of active matters and open up possibilities to build novel materials [15]. In this letter, we report experimental results for a new type of bacterial clusters formed near an air-liquid interface in a pure suspension without depletant agents. Fluid dynamic calculation and flow visualization are used to show surface-mediated hydrodynamic interactions can explain the formation of these clusters. We further quantify the statistical and dynamic properties of bacterial clusters and show long-ranged hydrodynamic forces have important influences on cluster properties. We conclude with discussions on related research and on possible technological and environmental implications of our work. Experiments. – Our experiments are carried out in drops of wild-type Serratia marcescens (ATCC 274) bacteria, which are propelled by a bundle of a few rotating