Self-Concentration and Large-Scale Coherence in Bacterial Dynamics

Abstract

Suspensions of aerobic bacteria often develop flows from the interplay of chemotaxis and buoyancy. We find in sessile drops that flows related to those in the Boycott effect of sedimentation carry bioconvective plumes down the slanted meniscus and concentrate cells at the drop edge, while in pendant drops such self-concentration occurs at the bottom. On scales much larger than a cell, concentrated regions in both geometries exhibit transient, reconstituting, high-speed jets straddled by vortex streets. A mechanism for large-scale coherence is proposed based on hydrodynamic interactions between swimming cells.

Dynamics of self-propelled particles under strong confinement

“…Conversely, the theory may be used to construct the box shape that yields any desired density distribution on the boundary. When the curvature variations are small, we also predict the distribution of orientations at the boundary and the exponential decay of pressure as a function of box size recently observed in 3D simulations in a spherical box.Active fluids consisting of self-propelled units are found in biology on scales ranging from the dynamically reconfigurable cell cytoskeleton [1] to swarming bacterial colonies [2,3], healing tissues [4,5], and flocking animals [6]. Experiments have begun to achieve the extraordinary capabilities and emergent properties of these biological systems in nonliving active fluids of self-propelled particles, consisting of chemically [7][8][9][10][11][12] or electrically [13] propelled colloids, or monolayers of vibrated granular particles [14][15][16].…”

Dynamics of bacterial flow: Emergence of spatiotemporal coherent structures

“…These complicated, spatially coherent structures have been observed in Bacillus subtilis colony grown on an agar plate [6], in E. coli confined in a quasi-two-dimensional soap film [7], in Bacillus subtilis at the edge of a pendent drop [9]. It is estimated from direct visualization that these structures have a typical size of about ten times that of a bacterium and persist for a few seconds.…”

“…Recent experiments [6,7,8,9] show that when concentrated, the crowd of swimming bacteria creates arrays of transient jets and swirls whose size can be orders of magnitude larger than an individual bacterium. These complicated, spatially coherent structures have been observed in Bacillus subtilis colony grown on an agar plate [6], in E. coli confined in a quasi-two-dimensional soap film [7], in Bacillus subtilis at the edge of a pendent drop [9].…”

“…Yet, a large concentration of these microorganisms constitutes a state that is far from equilibrium, and exhibits self-organized collective motion with spatial and temporal patterns that are both physically fascinating and potentially of great biological significance [6,7,8,9]. As a first step towards understanding of how suspended cells generate coherent motion, we identify, in this Letter, two simple but central ingredients -the elongated shape of the self-propelled particles and the hardcore interactions among them, and demonstrate via computer implementation of these two ingredients that this system exhibits coherent jets and swirls with characteristics strikingly similar to those observed in experiments.Recent experiments [6,7,8,9] show that when concentrated, the crowd of swimming bacteria creates arrays of transient jets and swirls whose size can be orders of magnitude larger than an individual bacterium. These complicated, spatially coherent structures have been observed in Bacillus subtilis colony grown on an agar plate [6], in E. coli confined in a quasi-two-dimensional soap film [7], in Bacillus subtilis at the edge of a pendent drop [9].…”

“…With a typical size of a bacterium of the order of microns and a typical speed of the order of 10 µm/s, the Reynolds number R ≪ 1 is quite small. Yet, a large concentration of these microorganisms constitutes a state that is far from equilibrium, and exhibits self-organized collective motion with spatial and temporal patterns that are both physically fascinating and potentially of great biological significance [6,7,8,9]. As a first step towards understanding of how suspended cells generate coherent motion, we identify, in this Letter, two simple but central ingredients -the elongated shape of the self-propelled particles and the hardcore interactions among them, and demonstrate via computer implementation of these two ingredients that this system exhibits coherent jets and swirls with characteristics strikingly similar to those observed in experiments.…”