Bacterial swimming and oxygen transport near contact lines

Abstract

Aerobic bacteria often live in thin fluid layers near solid-air-water contact lines, in which the biology of chemotaxis, metabolism, and cell-cell signaling is intimately connected to the physics of buoyancy, diffusion, and mixing. Using the geometry of a sessile drop, we demonstrate in suspensions of Bacillus subtilis the self-organized generation of a persistent hydrodynamic vortex that traps cells near the contact line. Arising from upward oxygentaxis and downward gravitational forcing, these dynamics are related to the Boycott effect in sedimentation and are explained quantitatively by a mathematical model consisting of oxygen diffusion and consumption, chemotaxis, and viscous fluid dynamics. The vortex is shown to advectively enhance uptake of oxygen into the suspension, and the wedge geometry leads to a singularity in the chemotactic dynamics near the contact line.bioconvection ͉ chemotaxis ͉ singularity ͉ Bacillus subtilis T he interplay of chemotaxis and diffusion of nutrients or signaling chemicals in bacterial suspensions can produce a variety of structures with locally high concentrations of cells, including phyllotactic patterns (1), filaments (2), and concentrations in fabricated microstructures (3). Less well explored are situations in which concentrating hydrodynamic f lows actually arise from these ingredients. Here we report a detailed experimental and theoretical study of an intriguing mechanism termed the ''chemotactic Boycott effect.'' Described brief ly before (4), it is intimately associated with buoyancy-driven f lows, metabolite diffusion, and slanted air-water menisci. The ubiquity of contact lines and their transport singularities (5) suggest importance of these observations in biofilm formation (6). The large-scale stirring created by these f lows illustrate important advective contributions to intercellular signaling, as in quorum sensing (7).The chemotactic Boycott effect takes its name from a phenomenon in sedimentation (8) that occurs when the chamber containing a fluid with settling particles is tilted from vertical. Settling depletes the fluid near the upper wall, making it buoyant relative to nearby fluid, whereupon it rises. This boundary flow stirs up the entire medium, greatly accelerating the settling process. In the chemotactic version, negatively buoyant aerobic bacteria swim up to the free surface of a sessile drop and slide down the slanted meniscus, producing high concentrations of cells near the three-phase contact line. In earlier work where this was observed (4), the detailed nature of hydrodynamic flows near the contact line was unclear. Here, by direct visualization and particle-imaging velocimetry (PIV), we show that the sliding surface layer drives a circulating hydrodynamic vortex in the meniscus region that is central to the microecology. Although counterintuitive in viscous flows, persistent circulation driven by forcing at the free surface is consistent with the classic analysis for vortex generation in wedge geometry (9).The initial discussion of the chemota...

Batchelor Prize Lecture Fluid dynamics at the scale of the cell

“…Later, Simha & Ramaswamy (2002) noted that for the case of swimming (pusher) microorganisms in suspension, hydrodynamic interactions between them would lead to a long-wavelength instability, frustrating such order. Experiments by Wu & Libchaber (2000) on E. coli in soap films saw hints of the lack of such order, and later we discovered (Dombrowski et al 2004;Tuval et al 2005) suspensions of swimming Bacillus subtilis exhibit a dynamical state resembling turbulence, with transient recurring vortices and jets of collective swimming on length scales large compared to the individual cells, as shown in figure 19. This was experimental verification of the prediction of an intrinsic instability in pusher suspensions.…”

Stabilization in a two-dimensional chemotaxis-Navier–Stokes system

“…Indeed, striking experimental findings indicate that such a mutual chemotaxis-fluid interaction may lead to quite complex types of collective behavior, even in markedly simple settings such as present when populations of Bacillus subtilis are suspended in sessile drops of water ( [7], [33], [20]). …”

The Aquatic Dance of Bacteria

“…These movies were taken immediately after each other with a $3 min time lag between subsequent pairs. During the $10 min imaging period for each device, the motility of B. subtilis cells decreased markedly due to oxygen depletion [29]. The experimental setup yields quasi-2D projected velocities of 3D suspension motion (see Fig.…”

“…The experiments reported here, performed in closed 3D microfluidic chambers, allowed near-simultaneous measurements of cell and tracer motion, and exploit a natural reduction in bacterial swimming activity due to oxygen depletion [8,29,36] to obtain data spanning 2 orders of magnitude in fluid kinetic energy. Combined with extensive 3D numerical simulations of the model, this data allows robust parameter estimates.…”

“…[3,4,29,30], freestanding films [5,8,27,31], on surfaces [6,32,33], or quasi-2D microfluidic chambers [7] focused separately on the bacterial and fluid components, leaving uncertain how accurately passive tracers [34,35] reflect collective bacterial dynamics. The experiments reported here, performed in closed 3D microfluidic chambers, allowed near-simultaneous measurements of cell and tracer motion, and exploit a natural reduction in bacterial swimming activity due to oxygen depletion [8,29,36] to obtain data spanning 2 orders of magnitude in fluid kinetic energy. Combined with extensive 3D numerical simulations of the model, this data allows robust parameter estimates.…”