Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H39: Cell Motility: From Single Cell to Collective Dynamics IIIFocus
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Sponsoring Units: DBIO GSOFT Chair: Thomas Gregor, Princeton University Room: 342 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H39.00001: Expulsion of swimming bacteria by a circular flow. Andrey Sokolov, Igor Aronson Macroscopic shear flow alters swimming trajectories in a highly nontrivial way and results in dramatic reduction of viscosity and heterogeneous bacterial distributions. We report on experimental and theoretical studies of rapid expulsion of microswimmers, such as motile bacteria, by a circular flow created by a rotating microparticle. We observed a formation of a macroscopic depletion area in a high-shear region, in the vicinity of a microparticle. The rapid migration of bacteria from the shear-rich area is caused by a circular structure of the flow rather than intrinsic random fluctuations of bacteria orientations, in stark contrast to planar shear flow. Our mathematical model revealed that expulsion is a combined effect of motility and alignment by a vortical flow. Our findings offer a novel approach for manipulation of motile microorganisms and shed new light on bacteria-flow interactions. [Preview Abstract] |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H39.00002: Bacterial haptotaxis: Effect of auto-attraction and bacterial motility on microcolony formation Bernard Beckerman, Kun Zhao, Gerard C. L. Wong, Erik Luijten Recent work\footnote{K. Zhao \emph{et al.}, Nature \textbf{497}, 388 (2013)} has demonstrated that surface-adhered Pseudomonas aeruginosa tend to self-organize into microcolonies using a positive-feedback mechanism mediated by the exopolysaccharide Psl, which the bacteria secrete as they traverse the surface. We elucidate this colony-nucleation process and explore how it is influenced by the deposition rate of Psl and by bacterial motility. A detailed analysis of the data presented in our earlier study, in combination with additional simulations, provides further insight into the exploratory strategy of P. aeruginosa. Specifically, the isogenic bacterial population is found to exhibit polyphenic motility. As a result, the bacterial population splits into two distinct subpopulations when depositing Psl, those that become trapped in their self-deposited Psl and those that move sufficiently quickly to escape their Psl beds and explore the surface. We perform computer simulations in which we adjust the relative prevalence of these subpopulations by varying the Psl deposition rate and find that there is a trade-off between surface exploration, microcolony diversity and microcolony fortification. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H39.00003: Invariant manifolds as barriers to the motion of bacteria in vortex flows Minh Doan, Katie Lilienthal, Tom Solomon We present experiments that study the motion of swimming bacteria (bacillus subtilis) in a time-independent flow in a microfluidic T-channel. Experiments are done with both wild-type and a genetically-mutated ``smooth swimming'' \footnote{R. Rusconi, J.S. Guasto and R. Stocker, Nature Physics {\bf 10}, 212 (2014).} bacillus subtilis. We analyze the behavior of these bacteria in terms of invisible barriers, based on a theory of ``burning invariant manifolds'' \footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} that act as one-way barriers that impede the motion of reaction fronts in a fluid flow. We explore whether similar one-way barriers impede the motion of bacteria. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H39.00004: Tethered motion of uniflagellated bacteria at the liquid-solid surface Jordan Bell, Jay Tang Direct evidence of the bacterial flagellar motor’s rotation was first noted when multiflagellated bacterial cells were observed (under the optical microscope) to rotate when tethered to glass by a single flagellum. The tethered cell assay has continued to play a significant role throughout the subsequent studies of motor characteristics and behavior. Such studies have expanded to include uniflagellated bacteria, such as \textit{Vibrio alginolyticus, Pseudomonas aeruginosa,} and \textit{Caulobacter crescentus}. Here we show that such cells are not necessarily tethered by their flagellum, but rather elsewhere on the cell body. The observed cell body rotation is actually due to the flagellum either “rolling” against the glass surface, or pushing the cell body at the flagellar base. These motions are directly observed for \textit{Vibrio alginolyticus} with darkfield microscopy. Additionally, our recently measured distributions of intervals between motor switches for tethered \textit{Caulobacter crescentus} also confirm this more complicated mode of tethering. Therefore, the rotational speed of tethered uniflagellated bacteria may not equate to that of the motor itself, as is commonly assumed. [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H39.00005: Two-dimensional swimming behavior of bacteria Ye Li, He Zhai, Sandra Sanchez, Daniel Kearns, Yilin Wu Many bacteria swim by flagella motility which is essential for bacterial dispersal, chemotaxis, and pathogenesis. Here we combined single-cell tracking, theoretical analysis, and computational modeling to investigate two-dimensional swimming behavior of a well-characterized flagellated bacterium \textit{Bacillus subtilis} at the single-cell level. We quantified the 2D motion pattern of \textit{B. subtilis} in confined space and studied how cells interact with each other. Our findings shed light on bacterial colonization in confined environments, and will serve as the ground for building more accurate models to understand bacterial collective motion. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H39.00006: Swimming of bacteria under dielectrophoresis Ngoc Phu Tran, Marcos Marcos In this work, we present a model to predict the response of a swimming helically flagellated bacterium to a unidirectional dielectrophoretic (DEP) force with its strength varying linearly in space. We employ resistive force theory to compute the hydrodynamic force on the flagellar bundle, and the effects of DEP force and rotational diffusion are examined using the Fokker-Planck equation. The DEP force greatly contributes to the reorientation of the bacterium such that the bacterium's primary axis is aligned with the direction of the force. Interestingly, when the DEP strength varies perpendicularly to the direction of the force, the bacterium's primary axis is no longer aligned with the DEP force, which results in a translation of the bacterium perpendicular to its primary axis. Finally, we show the feasibility to utilize this phenomenon to achieve bacterial focusing. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 4:18PM |
H39.00007: The bacterial gliding machinery Invited Speaker: Abhishek Shrivastava Cells of Flavobacterium johnsoniae, a rod-shaped bacterium, glide over surfaces with speeds reaching up to 2 micrometer's. Gliding is powered by a protonmotive force. The adhesin SprB forms filaments about 160 nm long that move on the cell-surface along a looped track. Interaction of SprB filaments with a surface produces gliding. We tethered F. johnsoniae cells to glass by adding anti-SprB antibody. Tethered cells spun about fixed points, rotating at speeds of about 1 Hz. The torques required to sustain such speeds were large, comparable to those generated by the flagellar rotary motor. Using a flow cell apparatus, we changed load on the gliding motor by adding the viscous agent Ficoll to tethered cells. We found that a gliding motor runs at constant speed rather than constant torque. We attached gold nanoparticles to the SprB filament and tracked its motion. We fluorescently tagged a bacterial Type IX secretion system (T9SS) protein and imaged its dynamics. Fluorescently tagged T9SS protein localized near the point of tether, indicating that T9SS localizes with the gliding motor. Based on our results, we propose a model to explain bacterial gliding. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H39.00008: Coordinated Beating of Algal Flagella is Mediated by Basal Coupling Kirsty Wan, Raymond Goldstein Cilia or flagella often exhibit synchronized behavior. This includes phase-locking, as seen in \textit{Chlamydomonas}, and metachronal wave formation in the respiratory cilia of higher organisms. Since the observations by Gray and Rothschild of phase synchrony of nearby swimming spermatozoa, it has been a working hypothesis that synchrony arises from hydrodynamic interactions between beating filaments. Recent work on the dynamics of physically separated pairs of flagella isolated from the multicellular alga \textit{Volvox} has shown that hydrodynamic coupling alone is sufficient for synchrony. However, the situation is more complex when considering multiple flagella on a single cell. We suggest that a mechanism, internal to the cell, provides an additional flagellar coupling. For instance, flagella of \textit{Chlamydomonas} mutants deficient in filamentary connections between basal bodies are found to display markedly different synchronization from the wildtype. Diverse flagellar coordination strategies found in quadri-, octo- and hexadecaflagellates reveal further evidence that intracellular couplings between flagellar basal bodies compete with hydrodynamic interactions to determine the precise form of flagellar synchronization in unicellular algae. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H39.00009: Non-Poissonian run-and-turn motions Francois Detcheverry Swimming bacteria exhibit a variety of motion patterns (run-and-tumble, run-reverse, run-reverse-flick), in which persistent runs are punctuated by sudden turning events. What are the properties of such random motions? If a complete answer has been given when the turning events follow a Poisson process, it has remained elusive outside this particular case. We present a generic framework for such non-Poissonian run-and-turn random motions. We obtain the generating function of moments by building on the framework of continuous time random walks and using non-commutative calculus. The approach is applied to a bimodal model of persistent motion that is directly applicable to swimming patterns and cell motility. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H39.00010: Directionality Time - New Analytical Treatment of Directionally Biased, Crawling Motility Jay Tang, Alexander Loosley Insights on crucial biological functions often emerge from measuring how animal cells crawl on surfaces, particularly in response to gradients of external cues that cause directionally biased motion. Most existing metrics commonly used to characterize directional migration, such as straightness index (or chemotactic index), persistence time, and turning angle distribution, tend to be sensitive to relatively large errors at short sampling times. In contrast, we recently introduced a new metric, called directionality time, to define the onset time by which a seemingly random motion becomes directionally biased (O’Brien et al., J Leukocyte Biol, 2014, 95:993–1004; Loosley et al., PLOS ONE, 2015, 10.1371). Directionality time is obtained by fitting the mean squared displacement as a function of time interval, in log-log coordinates, to a fit function based on biased and persistent random walk processes. We show that the fit function is approximately model invariant and is applicable to a variety of directionally biased motions. Simulations are performed to show the robustness of the directionality time model and its decoupling from measurement errors. Finally, we demonstrate as an example how to usefully apply the directionality time fit to trajectories of chemotactic neutrophils. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H39.00011: ABSTRACT WITHDRAWN |
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