Dynamic clustering in suspension of motile bacteria

Abstract

-Bacteria suspension exhibits a wide range of collective phenomena arsing 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 counter-clockwise 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.Active systems are composed of self-propelled particles that can produce motion by taking in and dissipating energy [1][2][3][4]. Examples exist at different length scales, from bacteria suspension [5][6][7][8][9][10] to flocks of birds [11][12][13]. Being far from thermal equilibrium, active systems are not subject to thermodynamic constraints, such as detailed balance or fluctuation-dissipation theorem [14][15][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][24][25][26]. On the experimental side, dynamic clusters have been observed in Janus particles (platinum-coated [27] and Carbon-coated 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 flagella [32]. For cultivation, small amount of bacteria from frozen stock is put in 4 ml of Luria Broth (LB) growth p-1

Transition to bound states for bacteria swimming near surfaces

“…Finally, we note that the surface-binding phenomena have been observed with at least two different bacterial species, namely, E. coli [34,35] and Serratia marcescens [36], both 10 times smaller than T. majus cells but possessing an elongated sphero-cylindrical cell body. Furthermore, in the case of Serratia marcescens, the cells became bound to an air-liquid interface in the same way as T. majus cells, suggesting that the surface binding mechanism described here is not just restricted to solid surfaces.…”

Active Particles in Complex and Crowded Environments

“…The formation of active crystals was also observed as the result of collective dynamics of fast swimming bacteria (Chen et al, 2015a;Petroff et al, 2015).…”

Nucleation of rotating crystals byThiovulum majusbacteria

“…The flow causes cells to aggregate near the surface [22] and pulls nearby cells together [20,23]. As in other examples of active crystals [24][25][26][27][28] and fluids [29][30][31][32][33][34][35][36][37][38][39][40], the power of the rotating and swimming cells is dissipated by their large-scale motion [41][42][43][44][45].…”