Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session Q8: Bacteria LocomotionBio Fluids: External
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Chair: Debasish Das, University of Cambridge Room: 501 |
Tuesday, November 21, 2017 12:50PM - 1:03PM |
Q8.00001: Swimming invariant manifolds and the motion of bacteria in a fluid flow Helena Yoest, Kevin Mitchell, Tom Solomon We present experiments on the motion of both wild-type and smooth-swimming bacillus subtilis in a hyperbolic, microfluidic fluid flow. Passive invariant manifolds crossing the fixed point in the flow act as barriers that block inert tracers in the flow. Self-propelled tracers can cross these passive manifolds, but are blocked by and attracted to swimming invariant manifolds (SWIMs) that split from the passive manifolds with larger and larger non-dimensional swimming speed $v_0 \equiv V_0/U$, where $V_0$ is the swimming speed in the absence of a flow and $U$ is a characteristic flos speed. We present the theory that predicts these SWIMs for smooth-swimming tracers, along with experiments that we are conducting to test these theories. We also discuss potential effects of rheotaxis and chemotaxis on the phenomena. [Preview Abstract] |
Tuesday, November 21, 2017 1:03PM - 1:16PM |
Q8.00002: Active motility in bimodular bacterial aggregates Yu Zeng, Bin Liu Dispersal capability is essential for microorganisms to achieve long-distance translocation, thus crucial for their abundance in various environments. In general, active dispersals are attributed to the movements of self-powered planktonic cells, while sessile cells that live a colonial life often disperse passively through flow entrainments. Here, we report another means of active dispersal employed by aggregates of sessile cells. The spherical rosette colonies of the bacterium \textit{Caulobacter crescentus} are aggregates of sessile stalked cells, of which a small proportion undergo cell division, grow active flagella and effect whole-rosette motility. We show that these rosettes actively disperse both in bulk water and near the solid-liquid interface. In particular, the proximity of a self-powered rosette to the solid surface promotes a rolling movement, leading to its persistent transportation along the solid boundary. The active dispersal of these rosettes demonstrated a novel mode of colonial transportation that is based on the division of labor between sessile and motile cells. [Preview Abstract] |
Tuesday, November 21, 2017 1:16PM - 1:29PM |
Q8.00003: Effects of elasticity and geometry on the locomotion of a model bacterium Frank Nguyen, Michael Graham The locomotion of flagellated bacteria in viscous fluid provides the blueprint for a number of micro-scale engineering applications. The elasticities of both the hook protein (connecting cell body and flagellum) and the flagella themselves play a key role in determining the stability of locomotion. We use a coarse-grained discretization of elastic flagella connected to a rigid cell body to examine trajectories and flow fields for free swimmers. We indeed find that hook and/or flagellar buckling occurs above a critical flexibility relative to the swimmer’s torque input. This renders straight swimming ineffective, though not necessarily undesirable in practice. Simulations with multiple flagella show bundling may partially stabilize the buckling effect. For a single flagellum or single bundle, we define a parameter space of characteristic angles tracking the overall time-averaged shape of the swimmer while also delineating stability boundaries between different modes of buckling. Ultimately our results may provide insight on how swimmers move through complex environments and how to design microrobotic swimmers for specific applications. [Preview Abstract] |
Tuesday, November 21, 2017 1:29PM - 1:42PM |
Q8.00004: The double positive effect of the swimming strategy of E-Coli bacteria in a flow Adama Creppy, Harold Auradou, Eric Clement, Carine Douarche, Veronica d'Angelo Active matters have been studied extensively in various regimes (from diluted to dense) in recent decades. More recently, it has been shown that the activity of the bacteria induces a rather significant measurable effect on the reduction of the viscosity of the carrier fluid. This effect is explained by the reorientation of the bacteria under the effect of shearing, the rheotaxis. In diluted regime, studies have shown the accumulation of microorganisms on the walls by an hydrodynamic mechanism. The experimental studies on the subject therefore consisted in putting the microorganisms under flow in tubes of circular or rectangular section. On the other hand, few is known about the effect of this coupling between their swimming and the flow in a more complex flow. In order to do this, we have developed a channel with random obstacles of different sizes in which the {\it E. coli} strain RP437 has been flowed with different velocities. At the scale of a porous medium, our experiments show that the fluid-bacterial coupling has a double effect (i) the activity of motile (active) bacteria favors trapping between and around the grains which is not the case for non-motile (inactive) bacteria and (ii) as a bonus some motile bacteria progressing more rapidly in the medium. [Preview Abstract] |
Tuesday, November 21, 2017 1:42PM - 1:55PM |
Q8.00005: Impact of hydrodynamic stresses on bacterial flagella Debasish Das, Emily Riley, Eric Lauga The locomotion of bacteria powered by helical filaments, such as Escherichia coli, critically involves the generation of flows and hydrodynamic stresses which lead to forces and moments balanced by the moment applied by the bacterial rotary motor (which is embedded in the cell wall) and the deformation of the short flexible hook. In this talk we use numerical computations to accurately compute these hydrodynamic stresses, to show how they critically lead to fluid-structure instabilities at the whole-cell level, and enquire if they can be used to rationalise experimental measurements of bacterial motor torques. [Preview Abstract] |
Tuesday, November 21, 2017 1:55PM - 2:08PM |
Q8.00006: Aquatic bacteria elongate and wobble their bodies for flagellar performance Bin Liu, Joanna Valenzuela, Pooja Chopra Bacteria are endowed with well-regulated sizes and shapes. A bacillus has a rod-like cell body, achieving swimming motility by rotating a single flagellum or multiple flagella. Along with other shapes, this elongated cell is often viewed as a payload, and its movements are regarded as passive responses to its flagellar propulsion. Here, we simultaneously measured the morphology and movement of individual free-swimming bacteria using an automated tracking microscope and 3D reconstruction techniques. These cells were found to consistently precess, based on reconstructions of the apparent wobbling movements viewed from a microscope. Through a hydrodynamic model that incorporates such precessing cell bodies and rod-like geometries, we found that there is a critical cell size below which wobbling movement is beneficial for flagellar performance. This critical cell size is consistent with the cellular morphologies of Caulobacter crescentus and Escherichia coli, as examples of single- and multi-flagellated species that are known for wobbling movements in aquatic environments. We also showed that the moderate cell sizes of these species could be attributed to a compromise between dispersal speed and power consumption. [Preview Abstract] |
Tuesday, November 21, 2017 2:08PM - 2:21PM |
Q8.00007: A computational model of amoeboid cell swimming in unbounded medium and through obstacles Eric Campbell, Prosenjit Bagchi Pseudopod-driven motility is commonly observed in eukaryotic cells. Pseudopodia are actin-rich protrusions of the cellular membrane which extend, bifurcate, and retract in cycles resulting in amoeboid locomotion. While actin-myosin interactions are responsible for pseudopod generation, cell deformability is crucial concerning pseudopod dynamics. Because pseudopodia are highly dynamic, cells are capable of deforming into complex shapes over time. Pseudopod-driven motility represents a multiscale and complex process, coupling cell deformation, protein biochemistry, and cytoplasmic and extracellular fluid motion. In this work, we present a 3D computational model of amoeboid cell swimming in an extracellular medium (ECM). The ECM is represented as a fluid medium with or without obstacles. The model integrates full cell deformation, a coarse-grain reaction-diffusion system for protein dynamics, and fluid interaction. Our model generates pseudopodia which bifurcate and retract, showing remarkable similarity to experimental observations. Influence of cell deformation, protein diffusivity and cytoplasmic viscosity on the swimming speed is analyzed in terms of altered pseudopod dynamics. Insights into the role of matrix porosity and obstacle size on cell motility are also provided. [Preview Abstract] |
Tuesday, November 21, 2017 2:21PM - 2:34PM |
Q8.00008: Elasto-hydrodynamics of the gliding motion of myxobacteria Joel Tchoufag, Pushpita Ghosh, Connor Pogue, Beiyan Nan, Kranthi Mandadapu The mysterious "A-motility” of myxobacteria has long been a mystery, since no appendage is involved in its motion known as “gliding”. Several studies in molecular microbiology have identified a number of structural features of this motion: 1) A trail of a nanometer slime film secreted underneath the bacteria 2) the shape of this rod-like bacteria, and 3) the soft substrate over which the gliding motion occurs. Using the above mentioned features, we present a mechanism for the gliding of myxobacteria. In our theory, we consider a thin slime film bounded on the top by a bacterial membrane displaying a traveling wave and on the bottom by a deformable substrate. Enforcing the lift force on the bacteria to vanish, we obtain the velocity of bacteria to be dependent on the so-called “softness” parameter. Using the celebrated lubrication approximation for the slime coupled to linear elastic theory for the substrate, we show that the velocity of bacteria is proportional to the shear modulus in the limit of very stiff substrates. More surprisingly, we find that the velocity is independent of substrate stiffness for softer substrates. Our results are validated with experimental measures of the gliding speed of M. xanthus cells on agar pads at various concentrations. [Preview Abstract] |
Tuesday, November 21, 2017 2:34PM - 2:47PM |
Q8.00009: The dynamic instability in the hook/flagellum system that triggers bacterial flicks Mehdi Jabbarzadeh, Henry Fu Dynamical bending, buckling, and polymorphic transformations of the flagellum are known to affect bacterial motility, but run-reverse-flick motility of monotrichous bacteria also involves the even more flexible hook, which connects the flagellum to the cell body. Here, we identify the dynamic buckling mechanism that produces flicks in \textit{Vibrio alginolyticus}. Estimates of forces and torques on the hook from experimental observations suggest that flicks are triggered at stresses below the hook's static Euler buckling criterion. Using an accurate linearization of the Kirchoff rod model for the hook in a model of a swimming bacterium with rigid flagellum, we show that as hook stiffness decreases there is a transition from on-axis flagellar rotation with small hook deflections to flagellar precession with large deflections. When flagellum flexibility is incorporated, the precession is disrupted by significant flagellar bending -- $i.e.$, incipient flicks. The predicted onset of dynamic instabilities corresponds well with experimentally observed flick events. [Preview Abstract] |
Tuesday, November 21, 2017 2:47PM - 3:00PM |
Q8.00010: Hydrodynamic interaction between bacteria and passive sphere Bokai Zhang, Yang Ding, Xinliang Xu Understanding hydrodynamic interaction between bacteria and passive sphere is important for identifying rheological properties of bacterial and colloidal suspension. Over the past few years, scientists mainly focused on bacterial influences on tracer particle diffusion or hydrodynamic capture of a bacteria around stationary boundary. Here, we use superposition of singularities and regularized method to study changes in bacterial swimming velocity and passive sphere diffusion, simultaneously. On this basis, we present a simple two-bead model that gives a unified interpretation of passive sphere diffusion and bacterial swimming. The model attributes both variation of passive sphere diffusion and changes of speed of bacteria to an effective mobility. Using the effective mobility of bacterial head and tail as an input function, the calculations are consistent with simulation results at a broad range of tracer diameters, incident angles and bacterial shapes. [Preview Abstract] |
Tuesday, November 21, 2017 3:00PM - 3:13PM |
Q8.00011: Ciliary Locomotion in Varying Viscosity Flow Patrick Eastham, Kourosh Shoele Ciliary locomotion is a common method of transportation employed by bacteria. They must be able to move through their environment at will to seek nutrients as well as avoid dangers. While research into bacteria motility has received considerable attention, very little has been done to consider the effects of a spatially-varying viscosity environment on swimming. This presentation will discuss recent research into how bacteria can take advantage of nutrient-dependent viscosity to generate an asymmetric stress field around their body, potentially increasing free-swimming velocity. First, we analytically show that asymptotically small variations in viscosity due to nutrient concentrations can affect the free-swimming velocity of a bacteria. Then we extend our study to fully nonlinear coupling between nutrient concentration and viscosity and employ the Finite Element method to solve a system containing a convection-diffusion equation for nutrient concentration as well as Stokes flow for stress distribution on the swimmer. We will discuss how the free-swimming velocity profile changes for various nutrient Pecletnumbers and ciliary locomotion modes. [Preview Abstract] |
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