Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B29: Focus Session: Micro-Organism Motility |
Hide Abstracts |
Sponsoring Units: DBP GSNP Chair: Udo Erdman, Humboldt University Room: Baltimore Convention Center 326 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B29.00001: Direct measurement of the propulsion efficiency of bacterium Escherichia Coli Suddhashil Chattopadhyay, Radu Moldovan, Chuck Yeung, Xiao-lun Wu Swimming of flagellated bacterium in an aqueous medium is studied in vivo with the aid of optical tweezers and an imposed external flow. By trapping the bacterium perpendicular to the optical axis, the beating frequency of the flagellum $\omega $and the counter rotation of the cell body $\Omega $ can be measured simultaneously. Measurements of the trapping force $F$ and the two rotation speeds, $\omega $ and $\Omega $, enable us to determine all the elements of the propulsion matrix for individual cells as well as their statistics in a population. Further we obtain the propulsion efficiency, defined as the ratio of the input rotational power provided by the motors to the output propulsive power, to be $\sim \quad 0.2\% .$ Significant heterogeneities are observed among the bacteria despite their starting from a single colony. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B29.00002: Modeling the motion of microscale synthetic swimmers Alexander Alexeev, Victor Yashin, Rolf Verberg, Anna Balazs By direct numerical simulations, we study the motility of synthetic micro-swimmers in a quiescent fluid. The micro-swimmers examined here are composed of an elastic polymeric material and comprise one or more wiggling tails, which propel the swimmer through its low-Reynolds number environment. The tails are driven to move by a reactive gel, which constitutes part of the swimmer's body. The gel undergoes a periodic swelling and deswelling due to a reaction or an external stimulus. To model the interaction of this micro-swimmer with the surrounding fluid, we adopt our recently developed LBM/LSM approach, which integrates the lattice Boltzmann model (LBM) for fluid dynamics with the lattice spring model (LSM) for the micromechanics of an elastic solid. Using this model, we investigate different aspects of the swimmer's propulsion. In particular, we determine how the properties of the swimmer and the oscillatory gel affect the swimmer's velocity and efficiency. Our goal is to design autonomous micro-swimmers that can be guided along a particular direction in order to perform a specified task. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B29.00003: Detecting the gravitational sensitivity of \textit{Paramecium caudatum} using magnetic forces Karine Guevorkian, James M Valles Jr. Under normal conditions, \textit{Paramecium} cells regulate their swimming speed in response to the pN level mechanical force of gravity. This regulation, known as gravikinesis, is more pronounced when the external force is increased by methods such as centrifugation. Here we present a novel technique that simulates gravity fields using the interactions between strong inhomogeneous magnetic fields and cells. We are able to achieve variable gravities spanning from 10$\times g$ to -8$\times g$; where $g$ is earth's gravity. Our experiments show that the swimming speed regulation of \textit{Paramecium caudatum} to magnetically simulated gravity is a true physiological response. In addition, they reveal a maximum propulsion force for paramecia. This advance establishes a general technique for applying continuously variable forces to cells or cell populations suitable for exploring their force transduction mechanisms. [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B29.00004: Precise adaptation in chemotaxis through ``assistance neighborhoods" Robert Endres, Ned Wingreen The chemotaxis network in Escherichia coli is remarkable for its sensitivity to small relative changes in the concentrations of multiple chemical signals over a broad range of ambient concentrations. Key to this sensitivity is an adaptation system that relies on methylation and demethylation/deamidation of specific modification sites of the chemoreceptors by the enzymes CheR and CheB, respectively. These enzymes can access 5-7 receptors once tethered to a particular receptor. Based on these ``assistance neighborhoods'', we present a model for precise adaptation of mixed clusters of two-state chemoreceptors. In agreement with experiment the response of adapted cells to addition/removal of attractant scales with the free-energy change at fixed ligand affinity. Our model further predicts two possible limits of precise adaptation: either the response to further addition of attractant stops through saturation of the receptors, or receptors fully methylate before they saturate and therefore stop adapting. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B29.00005: Cellular individuality in the gradient sensing response of \textit{Dictyostelium} Azadeh Samadani, Jerome Mettetal, Alexander van Oudenaarden It is generally assumed that single cells in an isogenic population exhibit the same behavior. However, it is becoming increasingly clear that even in a genetically identical population, gene expression levels can vary significantly from cell-to-cell. As a result of this variation, a physiological response such as gradient sensing might also display a significant variability from cell-to-cell. Although it is known that most cell populations are heterogeneous, the response of a typical cell or the average response of a population is often reported. However, quantitative information of cellular variability may contain important information on the intracellular signaling events. Here we explore the chemotactic response in single \textit{Dictyostelium} cells in response to repeated spatio-temporal pulses of chemoattractant. We find that the response of a single cell is reproducible from pulse-to-pulse. In contrast, a large variability in the chemotactic response is observed from cell-to-cell even when different cells in the population are exposed to the same pulse. We propose a simple model, which allows for the broken symmetry of the chemotactic response and reproduces the cellular variability within the population fairly well. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B29.00006: How does Dicty find its way? Erin Rericha, Carole Parent, Wolfgang Losert As a cell chemotaxes, moves towards a chemical signal,~it transduces external chemical signals into mechanical motion.~~ Efficient chemotaxis is crucial for many biological processes~from wound healing to the spread of cancer.~~ We present our experimental investigations on \textit{Dictyostelium discodeum} a model organism for chemotaxis.~ We expose cells to three types of external signals: a shallow background gradient of the signaling molecule cyclic-AMP~a localized signal composed of cyclic-AMP attached to beads~and a mechanical stimulus caused by pushing beads against the exterior of the cell.~ For each stimulus we ask: what is the stability of the gradient sensing pathway and how does it influence the mechanical response. We find that the cell speed increases with increasing concentration of cyclic-AMP. In addition, Dictyostelium cells relay the signal by releasing internally manufactured cyclic-AMP out of the back of the cell. A collection of cells moves in streams, where cells follow closely behind one another. We find that the cells moving in a stream move slower towards a source of attractant than cells that are chemically treated such that they do not stream. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 1:03PM |
B29.00007: Dictyostelium discoideum chemotaxis: threshold for directed motion Invited Speaker: The chemotactic response of Dictyostelium discoideum cells to stationary, linear gradients of cyclic adenosine 3$'$,5$'$-monophosphate (cAMP) was studied using microfluidic devices. In shallow gradients of less than 10$^{-3}$ nM/$\mu$m, the cells showed no directional response and exhibited a constant basal motility. In steeper gradients, cells moved up the gradient on average. The chemotactic speed and the motility increased with increasing steepness up to a plateau at around 10$^{-1}$ nM/$\mu$m. In very steep gradients, above 10 nM/$\mu$m, the cells lost directionality and the motility returned to the sub-threshold level. In the regime of optimal response the difference in receptor occupancy at the front and back of the cell is estimated to be only about 100 molecules. The work is supported by the Biocomplexity Program of the NSF and the Max Planck Society. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B29.00008: Impact of Helicobacter Pylori on Mucus Rheology Jonathan Celli, Sarah Keates, Ciaran Kelly, Bradley Turner , Rama Bansil, Shyamsunder Erramilli It is well known that the viscoelastic properties of gastric mucin are crucial to the protection of the lining of the stomach against its own acidic secretions and other agents. Helicobacter Pylori, a rod shaped, gram-negative bacteria that dwells in the mucus layer of approximately 50{\%} of the world's population is a class I carcinogen and is associated with gastric ulcers and severe gastritis. The structural damage to the mucus layer caused by H. Pylori is an important aspect of infection with this bacteria. We are examining the impact of H. Pylori on mucin and mucus rheology quantitatively using a combination of dynamic light scattering and multiple particle tracking experiments. Video microscopy data will also be presented on the motility of this bacteria in mucin at different pH and in other viscoelastic gels. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B29.00009: The effect of viscosity on the contraction of the stalk of \textit{Vorticella Convallaria} Deependra Kantha, David Van Winkle The contraction of the stalk of \textit{Vorticella Convallaria} behaves as one of the most powerful single cell biological engines.Contractions in three different viscous mediums were recorded by a PhantomV5 camera (Vision Research) on a bright field microscope with 20X objective. The contractions were recorded as cines (image sequences) with the image resolution of 256 pixels X 128 pixels at 7000 pictures per second. The maximum variation in maximum velocity of one organism was 5.2 cm/s and the minimum variation for a different organism was 0.4 cm/s for the same viscous medium. It occurred at 2-3ms after the start of contraction. The force of contraction and the force constant were calculated as a function of the time to see the effect of viscosity on the contraction of the stalk. This experiment needs further investigation to see the net effect of viscosity on the contraction of the stalk. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B29.00010: Optimal Foraging Strategy: Angle Matters Udo Erdmann, Sebastian G\"oller, Igor M. Sokolov, Lutz Schimansky-Geier We report a theory to describe the motion of zooplankton. In contrast to move just randomly like a classical Brownian particle, zooplankters like Daphnia or Copepods pick their turning angle from a distribution which is far from being Gaussian or equally distributed. This leads to different behavior in the motion compared to normal diffusion. The question which can be asked here is: Is there an evolutionary reason to forage for food in the aforementioned manner? The talk is planned to give an answer into that direction. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B29.00011: Nonlinear electromagnetic responses of active molecular motors in live cells and organelles Dharmakirthi Nawarathna, Jeffrey Gardner, Gustavo Cardenas, David Warmflash, John Miller, William Widger, James Claycomb The response of biological cells to an oscillatory electric field contains both linear and nonlinear (eg. induced harmonic) components. At low frequencies (about 10Hz), harmonic generation by budding yeast cells is observed. These induced harmonics are sensitive to sodium metavanadate, an inhibitor, and glucose, a substrate, respectively, of P-type ATPase membrane pumps. At higher frequencies, two peaks, around 3kHz and 12kHz, are observed in the frequency-dependent harmonic responses. These are sensitive to potassium cyanide, a respiratory inhibitor that blocks cytochrome c oxidase, an enzyme of the mitochondrial respiratory chain. We have also measured the response of uncoupled mitochondria extracted from bovine heart cells, for which a second harmonic sensitive to pericidin A and carboxin is detected at applied frequencies of 3-4kHz. Finally, in coupled mouse mitochondria, an ADP sensitive peak (12-15kHz) is observed, likely due to the F0 domain of ATP synthase, which acts as a molecular turbine. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B29.00012: Mechanism of formin-associated actin filament elongation Dimitrios Vavylonis, David R. Kovar, Ben O'Shaughnessy, Thomas D. Pollard Formins control the kinetics of actin filament polymerization by remaining processively attached at the growing filament end. The FH2 formin domain associates with the barbed end while FH1 interacts with profilin and is thought to mediate profilin-actin polymerization. We combined TIR microscopy (Kovar et al., Cell, in press) with theoretical modeling to study the dependence of the rate of formin-associated actin filament elongation on profilin and actin concentrations. We assume a transfer mechanism of profilin-actin from FH1 to the barbed end, gated by FH2 as described by the gating parameter p measuring the accessibility of the barbed end. The model explains the main experimental trends and rationalizes (i) how filaments associated with formin mDia1 elongate more rapidly than formin-free filaments due to the the large number of profilin binding sites and p~1, and (ii) how filaments associated with formins with p<1 elongate slowly in the absence of profilin but more rapidly in the presence of profilin. High profilin concentrations suppress elongation and in the model this is attributed largely to the saturation of FH1 by profilin. Consistent with our ADP-actin experiments, the proposed mechanism does not require ATP hydrolysis though we cannot exclude the possibility that formin translocation accelerates ATP hydrolysis for ATP-actin. [Preview Abstract] |
Monday, March 13, 2006 2:03PM - 2:15PM |
B29.00013: Spreading of neutrophils: from activation to migration Helim Aranda-Espinoza, Kheya Sengupta, Lee Smith, Paul Janmey, Daniel Hammer Reflection Interference Contrast Microscopy was used to study the dynamics of neutrophil spreading. The images were analyzed to identify simultaneously the changes in the overall cell adhesion and the zones of close contact with the substrate. We show that for neutrophils, cell spreading is anisotropic and directional from the very initial stage. The curve describing the spreading area of the cell as a function of time can be fitted piecewise as power laws. Different spreading regimes are identified and related to the adhesion state and/or dynamical state of the cell. All cells follow a slow spreading, fast spreading and finally area saturation. After saturation the adhesion area of the cell fluctuates as the neutrophil migrates. Close contacts occur at opposite sides of the cell where the uropod and lamellipodium develop. The apparition of close contacts seems to indicate a change in adhesion regime. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700