Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K4: Active Living Matter |
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Sponsoring Units: DBIO GSOFT GSNP Chair: Julien Tailleur, Université Paris-Diderot Room: 263 |
Wednesday, March 15, 2017 8:00AM - 8:12AM |
K4.00001: Swimming pattern of \textit{Pseudomonas putida} -- navigating with stops and reversals Marius Hintsche, Veronika Waljor, Zahra Alirezaeizanjani, Matthias Theves, Carsten Beta Bacterial swimming strategies depend on factors such as the chemical and physical environment, as well as the flagellation pattern of a species. For some bacteria the motility pattern and the underlying flagellar dynamics are well known, such as the classical run-and-tumble behavior of \textit{E. coli}. Here we study the swimming motility and chemotactic behavior of the polar, multi-flagellated soil dwelling bacterium \textit{Pseudomonas putida}. Compared to \textit{E. coli}, its motility pattern is more diverse. In addition to different speed levels, \textit{P. putida} exhibits two types of reorientation events, stops and reversals, the occurrence of which is modulated according to the growth conditions. We also analyzed the swimming pattern in the presence of chemical gradients. Using benzoate as a chemoattractant, we measured key motility parameters in order to characterize \textit{P. putida's} chemotaxis strategy and to quantify the directional bias in its random walk. Our results indicate a change in the reversal frequency depending on changes in the chemoattractant concentration consistent with the classical scenario of temporal sensing. [Preview Abstract] |
Wednesday, March 15, 2017 8:12AM - 8:24AM |
K4.00002: Singly-flagellated bacteria chemotax efficiently by unbiased motor regulation Vernita Gordon, Qiuxian Cai, Qi Ouyang, Chunxiong Luo, Zhaojun Li \textit{Pseudomonas aeruginosa} is a widespread bacterial pathogen that can chemotax. Genes that allow \textit{P. aeruginosa} to chemotax are homologous with genes in the paradigmatic model organism for chemotaxis, \textit{Escherichia coli}. However, \textit{P. aeruginosa} is singly-flagellated and \textit{E. coli} has multiple flagella. Therefore, the regulation of counter-clockwise/clockwise flagellar motor bias that allows \textit{E. coli} to efficiently chemotax by runs and tumbles would lead to inefficient chemotaxis by \textit{P. aeruginosa}, as half of a randomly-oriented population would respond to a chemoattractant gradient in the wrong sense. How \textit{P. aeruginosa} regulates flagellar rotation to achieve chemotaxis is not known. Here, we analyze the swimming trajectories of single cells in microfluidic channels and the rotations of cells tethered by their flagella to the surface of a flow cell with variable environment. We show that \textit{P. aeruginosa} chemotaxes by symmetrically increasing the durations of both counterclockwise and clockwise flagellar rotations when swimming up the chemoattractant gradient, and symmetrically decreasing rotation durations when swimming down the chemoattractant gradient. Unlike the case for \textit{E. coli}, the counter-clockwise/clockwise bias stays constant for \textit{P. aeruginosa}. Using analytical modeling and simulation, we show that, given \textit{P. aeruginosa}'s physiological constraints on motility, their symmetric regulation of motor switching optimizes chemotaxis. [Preview Abstract] |
Wednesday, March 15, 2017 8:24AM - 8:36AM |
K4.00003: Effects of number and configuration of flagella on motility of \textit{Helicobacter} species. Maira A. Constantino, Sinan Sharba, Zeli Shen, James G. Fox, Freddy Haesebrouck, Sara Linden, Rama Bansil \textit{Helicobacters} are ulcer-causing bacteria that colonize the viscoelastic gastric mucus layer of mammals. Previous studies have shown that motility and colonization are affected by helical body shape, number and configuration of flagella. In a recent study\footnote{Sci. Adv. 2, e1601661 (2016)}, using fast time-resolution and high-magnification 2-D phase-contrast microscopy to image individual helical and rod-shaped \textit{H. pylori} we measured the rotation rate of the cell body and flagella and found that helical shape produces less than 15\% changes in swimming speeds as compared to the rod-shaped cell. Motility of \textit{H. pylori} was strongly influenced by its multiple unipolar flagella. Here we compare rotational and translational speeds of \textit{H. cetorum} and \textit{H. suis} which have bipolar flagella, with \textit{H. cetorum} having single bipolar flagella and \textit{H. suis} having multiple flagella. Preliminary results show that \textit{H. suis} bacteria swim slower but rotate at the same rate as \textit{H. pylori} and present two swimming modes. It can swim as a pusher, with one active rotating bundle and one inactive bundle, wrapped around the body or with both bundles active. Similar work on \textit{H. cetorum} is ongoing and will also be presented. [Preview Abstract] |
Wednesday, March 15, 2017 8:36AM - 8:48AM |
K4.00004: Interface-Controlled Motility of Photoactive Microalgae in Confinement Tanya Ostapenko, Christian T. Kreis, Oliver Baeumchen The natural habitats of many biological microorganisms include complex interfaces and varying environmental conditions. For flagellated microalgae swimming in an aqueous medium, we showed that the curvature of the compartment wall governs their motility in geometric confinement [1]. This curvature-guided motility results in long detention times towards the interface, which we determined from the analysis of individual cell trajectories. For puller-type microswimmers, the precise nature of their flagella-wall interactions are important. We discovered a way to control these interactions for photoactive microalgae by manipulating the adhesiveness of their flagella in light [2]. Here, we report on the swimming dynamics of single photoactive microalgae in two-dimensional microfluidic chambers under different light conditions. We find that their motility can be switched reversibly in confinement, which could be exploited for use in biological optical traps and wastewater decontamination. [1] T. Ostapenko, et al. (arXiv:1608.00363), [2] C. Kreis, et al. (in review, 2016). [Preview Abstract] |
Wednesday, March 15, 2017 8:48AM - 9:00AM |
K4.00005: Mechanisms of algal and microplastic particle motion in the feeding current of Pseudodiaptamus pelagicus Aletha Spang, Jennifer Kreft Pearce Plastic pollution and degradation are major problems for the health of marine food webs, due to the accumulation of microplastics in zooplankton biomass and magnification in successive trophic levels. As the amount of plastic pollution in marine ecosystems increases, calanoid copepods have been observed ingesting microplastic particles trapped in their feeding currents, resulting in reduced amounts of nutrients available per energy expended (Desforges et al. 2015). In this study, the copepod Pseudodiaptamus pelagicus will be filmed feeding on similarly sized unicellular algae and polystyrene beads. Particles will be tracked and analyzed for retention times, average speeds, and motion patterns. This technique will investigate the specific mechanics of particle motion close to the mouthparts of the copepod, and whether significant differences exist between food and non-food particles. [Preview Abstract] |
Wednesday, March 15, 2017 9:00AM - 9:12AM |
K4.00006: The Interplay between the C-Terminal Tails of Tubulins and the Motility Parameters of Kinesin-1 Mitra Shojania Feizabadi The distribution of beta tubulin isoforms can be different from one cell to another. The distinctions among tubulin isoforms are mainly related to the existing differences in their Carboxy-terminal (C-terminal) tails. In this work, we examined the effects of C-terminal tails on the functions of one of the molecular motors, Kinesin-1. The results that will be presented here include the quantification of the motility parameters of a single Kinesin-1 molecule motor as well as multiple motors along bovine brain vs MCF7 microtubules. These two types of microtubules carry different compositional structures in terms of beta tubulin isoforms. We will then compare the results with the values of the similar parameters obtained from the motility of this motor along these two types of microtubules when they were treated with subtilisin. Our findings show that the nature of the microtubule track, along with the specifications of the C-terminal tails, significantly contribute to the functionality of a Kinesin-1 molecular motor. [Preview Abstract] |
Wednesday, March 15, 2017 9:12AM - 9:24AM |
K4.00007: Combing bacterial turbulence. Andrey Sokolov, Daiki Nishiguchi, Igor Aronson Living systems represented by ensembles of motile organisms demonstrate a transition from a chaotic motion to a highly ordered state. Examples of such living systems include suspensions of bacteria, schools of fish, flocks of birds and even crowds of people. In spite of significant differences in interacting mechanisms and motion scales, ordered living systems have many similarities: short-range alignment of organism, turbulent-like motion, emergence of large-scale flows and dynamic vortices. In this work, we rectify a turbulent dynamics in suspensions of swimming bacteria Bacillus subtilis by imposing periodical constraints on bacterial motion. Bacteria, swimming between periodically placed microscopic vertical pillars, may self-organize in a stable lattice of vortices. We demonstrate the emergence of a strong anti-ferromagnetic order of bacterial vortices in a rectangular lattice of pillars. Hydrodynamic interaction between vortices increases the stability of an emerged pattern. The highest stability of vortices in the anti-ferromagnetic lattice and the fastest vortices speed were observed in structures with the periods comparable with a correlation length of bacterial unconstrained motion. [Preview Abstract] |
Wednesday, March 15, 2017 9:24AM - 9:36AM |
K4.00008: How Bacterial Population Soliton Waves Can Defeat a Funnel Ring Robert Austin, Ryan Morris, Average Phan, Matthew Black, Ke-Chih Lin, Julia Bos We have constructed using microfabrication a circular corral for bacteria made of rings of concentric funnels which channel motile bacteria outwards via non-hydrodynamic interactions with the funnel walls. Although initially bacteria do move rapidly outwards with the funnels, they are able with increasing cell density on the perimeter to defeat the physical constraints of the funnel by launching collective, soliton like waves of bacteria inwards against the funnel ring. We present the basic data and some non-linear modeling which can explain the basic way that bacterial population solitons propagate across a funnel landscape. There are three surprising aspects to the experiments: (1) The bifurcation of the population into motile bacteria which are pumped by the funnels and bacteria which are non-motile (i.e., not pumped); (2) The launching of a collective wave which rapidly circles the device and radiates inwards {\em against} the pumping action of the funnel; (3) the subsequent loss of motility by all the bacteria after this burst of very high motility. [Preview Abstract] |
Wednesday, March 15, 2017 9:36AM - 9:48AM |
K4.00009: Bioconvection as a Consequence of Bio-Stratification in Bacterial Populations Daniel Shoup, Benjamin Strickland, Kentaro Hoeger, Tristan Ursell The collective motion of bacterial populations in solution can generate convective currents that significantly alter fluid motion and material transport. ~Known as bioconvection, this process is highly influenced by stimuli such as nutrients and toxins that can attract or repel bacteria via chemotaxis. ~Despite its prevalence in natural environments, ranging from the ocean floor to fluid in the human gut, this dynamic process and the physical and biological factors that influence it remain largely unexplored. ~To close this gap, we measure and analyze spontaneous bioconvection arising from the collective movement of dense populations of bacteria, such as \textit{Escherichia coli} and \textit{Bacillus subtilis}. ~By combining microscopy and image analysis, we find that modulations of the fluid volume geometry, erasure of the air-liquid interface, chemical perturbations like nutrients or antibiotics all alter the development of these dense bacterial masses and in turn the bio-convective currents and corresponding transport phenomena they generate. ~Our work suggests biophysical principles of material and organismal transport that apply to a broad range of systems where organisms can sense gradients and move within their environments. [Preview Abstract] |
Wednesday, March 15, 2017 9:48AM - 10:00AM |
K4.00010: Compression and release dynamics of an active matter system of \textit{Euglena gracilis} Amy Lam, Alan C H Tsang, Nicholas Ouellette, Ingmar Riedel-Kruse Active matter, defined as ensembles of self-propelled particles, encompasses a large variety of systems at all scales, from nanoparticles to bird flocks. Though various models and simulations have been created to describe the dynamics of these systems, experimental verification has been difficult to obtain. This is frequently due to the complex interaction rules which govern the particle behavior, in turn making systematic varying of parameters impossible. Here, we propose a model for predicting the system evolution of compression and release of an active system based on experiments and simulations. In particular, we consider ensembles of the unicellular, photo-responsive algae, \textit{Euglena gracilis}, under light stimulation. By varying the spatiotemporal light patterns, we are able to finely adjust cell densities and achieve arbitrary non-homogeneous distributions, including compression into high-density aggregates of varying geometries. We observe the formation of depletion zones after the release of the confining stimulus and investigate the effects of the density distribution and particle rotational noise on the depletion. These results provide implications for defining state parameters which determine system evolution. [Preview Abstract] |
Wednesday, March 15, 2017 10:00AM - 10:12AM |
K4.00011: Topological hindrance for molecular transport in cylindrical geometries Emanuel Reithmann, Patrick Wilke, Erwin Frey Active molecular transport in biological systems often occurs along cylindrical structures such as microtubules in eukaryotic cells. Based on a driven lattice gas model, we study the effect of a cylindrical geometry on collective motion for multiple species of active particles with distinct directions of motion on the cylinder. We demonstrate that the transport properties of the system strongly differ from transport in one dimension or with a single species only. Our results show that the number of accessible states depends in an intricate way on the particle density due to a complex connectivity of the state space. To quantify this additional topological hindrance, we set up an effective hydrodynamic theory that allows us to predict central observables like the macroscopic particle flux. Further, we develop analytic methods to characterize the phase behavior. These include an exact solution for a new cramming-phase as well as a renormalization approach for low densities. [Preview Abstract] |
Wednesday, March 15, 2017 10:12AM - 10:24AM |
K4.00012: Applying Magneto-rheology to Reduce Blood Viscosity and Suppress Turbulence to Prevent Heart Attacks R Tao Heart attacks are the leading causes of death in USA. Research indicates one common thread, high blood viscosity, linking all cardiovascular diseases. Turbulence in blood circulation makes different regions of the vasculature vulnerable to development of atherosclerotic plaque. Turbulence is also responsible for systolic ejection murmurs and places heavier workload on heart, a possible trigger of heart attacks. Presently, neither medicine nor method is available to suppress turbulence. The only method to reduce the blood viscosity is to take medicine, such as aspirin. However, using medicine to reduce the blood viscosity does not help suppressing turbulence. In fact, the turbulence gets worse as the Reynolds number goes up with the viscosity reduction by the medicine. Here we report our new discovery: application of a strong magnetic field to blood along its flow direction, red blood cells are polarized in the magnetic field and aggregated into short chains along the flow direction. The blood viscosity becomes anisotropic: Along the flow direction the viscosity is significantly reduced, but in the directions perpendicular to the flow the viscosity is considerably increased. In this way, the blood flow becomes laminar, turbulence is suppressed, the blood circulation is greatly improved, and the risk for heart attacks is reduced. While these effects are not permanent, they last for about 24 hours after one magnetic therapy treatment. [Preview Abstract] |
Wednesday, March 15, 2017 10:24AM - 10:36AM |
K4.00013: Fluid-Structure Model of Lymphatic Valve and Vessel Ki Wolf, Matthew Ballard, Zhanna Nepiyushchikh, Mohammad Razavi, Brandon Dixon, Alexander Alexeev The lymphatic system is a part of the circulatory system that performs a range of important functions such as transportation of interstitial fluid, fatty acid, and immune cells. The lymphatic vessels are composed of contractile walls to pump lymph against adverse pressure gradient and lymphatic valves that prevent back flow. Despite the importance of lymphatic system, the contribution of mechanical and geometric changes of lymphatic valves and vessels in pathologies of lymphatic dysfunction, such as lymphedema, is not well understood. We developed a coupled fluid-solid computational model to simultaneously simulate a lymphatic vessel, valve, and flow. A lattice Boltzmann model is used to represent the fluid component, while lattice spring model is used for the solid component of the lymphatic vessel, whose mechanical properties are derived experimentally. Behaviors such as lymph flow pattern and lymphatic valve performance against backflow and adverse pressure gradient under varied parameters of lymphatic valve and vessel geometry and mechanical properties are investigated to provide a better insight into the dynamics of lymphatic vessels, valves, and system and give insight into how they might fail in disease. [Preview Abstract] |
Wednesday, March 15, 2017 10:36AM - 10:48AM |
K4.00014: Strain-weakening rheology of marine sponges and its evolutionary implication Emily Kraus, Paul Janmey, Alison Sweeney, Anne van Oosten Animal cells respond to mechanical stimuli as sensitively as they do to chemical stimuli. Further, cell proliferation is dependent on the viscoelasticity of the polymeric extracellular matrix (ECM) in which they are embedded. Biophysicists are therefore motivated to understand the biomechanics of the ECM itself. To date, this work has focused on the more familiar Bilateria, animals, including humans, with bilateral symmetry. The ECM of this group of animals is now understood to exhibit non-linear rheology that is typically strain- and compression-stiffening, and shear moduli that are frequency-dependent. These complex properties have been attributed to the semi-flexible nature of the underlying polymers. In contrast, we show that marine sponges are markedly strain-weakening under physiologically relevant conditions. Since sponges are a much earlier evolutionary branch than Bilateria, we interrogate the evolutionary potential and biochemical underpinnings of this novel complex rheology in filamentous networks, and cells’ ability to respond. Further, their life history strategy is uniquely dependent on flow and correlated shear stress, making them a model organism to study self-assembly algorithms organized around flow. [Preview Abstract] |
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