Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A6: Self-organization in Bacteria Colonies and SuspensionsFocus
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Sponsoring Units: DBIO GSNP Chair: Hugues Chate, CEA-Saclav, France and Beijing Computational Research Center Room: 265 |
Monday, March 13, 2017 8:00AM - 8:36AM |
A6.00001: Topological defects and collective dynamics in colonies of filamentous bacteria Invited Speaker: Hepeng Zhang Active liquid crystals are non-equilibrium fluids consisting of self-propelled elongated units. These systems are driven at the scale of individual nematogen and exhibit novel defect dynamics. Here, I will report experimental data of topological defects and collective dynamics in colonies of filamentous bacteria. Our experiments show that elongated cells form an active nematic phase characterized by dynamic creation and annihilation of topological defects. Spatial correlations of orientation and velocity fields are measured at various defects densities. Results show that the motile defects not only dictate the global structure of the director field, but also act as local sources of motion. [Preview Abstract] |
Monday, March 13, 2017 8:36AM - 8:48AM |
A6.00002: Quantifying Spatiotemporal Patterns in the Expansion of Twitching Bacterial Colonies Erin Shelton, Lori Burrows, John Dutcher Type IV pili (T4P) are very thin (5-8 nm diameter) protein filaments that can be extended and retracted by certain classes of Gram-negative bacteria including P. aeruginosa [1]. These bacteria use T4P to move across viscous interfaces, referred to as twitching motility. Twitching can occur for isolated cells or in a collective manner [2]. The advancing front of the colony has finger-like protrusions consisting of aligned bacteria with between 5 to 30 cells across each finger. Although the average motion of the cells is radially outward, cells within rafts often reverse direction. Using a custom-built, temperature and humidity controlled environmental chamber, we have studied the motion of fingers at high spatial and temporal resolution. We have developed a bacterial segmentation and tracking technique to identify the trajectories of individual bacteria within the densely packed fingers, and we have used this technique to characterize the distance, displacement, orientation and direction reversals of the bacteria in the fingers. [1] Burrows, L.L. (2012) Annu. Rev. Microbiol. 66: 493--520. [2] Semmler, A.B., Whitchurch, C.B., Mattick, J.S. (1999) Microbiology 145: 2863-2873. [Preview Abstract] |
Monday, March 13, 2017 8:48AM - 9:00AM |
A6.00003: Fingering instabilities in bacterial community phototaxis Ritwika VPS, Rosanna Man Wah Chau, Kerwyn Casey Huang, Ajay Gopinathan Synechocystis sp PCC 6803 is a phototactic cyanobacterium that moves directionally in response to a light source. During phototaxis, these bacterial communities show emergent spatial organisation resulting in the formation of finger-like projections at the propagating front. In this study, we propose an analytical model that elucidates the underlying physical mechanisms which give rise to these spatial patterns. We describe the migrating front during phototaxis as a one-dimensional curve by considering the effects of phototactic bias, diffusion and surface tension. By considering the propagating front as composed of perturbations to a flat solution and using linear stability analysis, we predict a critical bias above which the finger-like projections appear as instabilities. We also predict the wavelengths of the fastest growing mode and the critical mode above which the instabilities disappear. We validate our predictions through comparisons to experimental data obtained by analysing images of phototaxis in Synechocystis communities. Our model also predicts the observed loss of instabilities in taxd1 mutants (cells with inactive TaxD1, an important photoreceptor in finger formation), by considering diffusion in mutually perpendicular directions and a lower, negative bias. [Preview Abstract] |
Monday, March 13, 2017 9:00AM - 9:12AM |
A6.00004: Multiscale Characterization of Bacterial Swarming Illuminates Principles Governing Directed Surface Motility Ben Strickland, Kentaro Hoeger, Tristan Ursell In many systems, individual characteristics interact, leading to the spontaneous emergence of order and complexity. In biological settings like microbes, such collective behaviors can imbue a variety of benefits to constituent individuals, including increased spatial range, improved access to nutrients, and enhanced resistance to antibiotic threats. To untangle the biophysical underpinnings of collective motility, we use passive tracers and a curated genetic library of \textit{Bacillus subtilis}, including motile, non-motile, biofilm-deficient, and non-chemotactic mutants. We characterize and connect individual behavior on the microscopic scale to macroscopic colony morphology and motility of dendritic swarming. We analyze the persistence and dynamics of coordinated movement on length scales up to 4 orders of magnitude larger than that of individual cells, revealing rapid and directed responses of microbial groups to external stimuli, such as avoidance dynamics across chemical gradients. Our observations uncover the biophysical interplay between individual motility, surface wetness, phenotypic diversity, and external physical forces that robustly precipitate coordinated group behavior in microbes, and suggest general principles that govern the transition from individual to group behavior. [Preview Abstract] |
Monday, March 13, 2017 9:12AM - 9:24AM |
A6.00005: Motility-induced bacterial pattern formation in multi-species bacterial colonies Agnese Curatolo, Yongfeng Zhao, Nan Zhou, Adrian Daerr, Jiangdong Huang, Julien Tailleur The ability to form patterns is a feature shared by a large variety of systems: from hydrodynamics (e.g. thermal convection) to biological processes (e.g. morphogenesis), the emergence of repeated ordered structures can have very complicated and different origins. Sometimes, however, simple and general underlying principles can be found. In this talk I will present a generic mechanism by which two types of bacteria can migrate and self-organize spatially, using a mutual control of their motilities. Depending on whether each species enhances or lowers the motility of the other species, initially mixed colonies grow in a variety of patterns leading to co-localization or demixing of the two species. The rich phenomenology described by our model and the robustness of the underlying pattern-formation mechanism suggests that it could be generically encountered in Nature. Moreover, it could also be used to promote the mixing or demixing of active particles in a controlled way. [Preview Abstract] |
Monday, March 13, 2017 9:24AM - 9:36AM |
A6.00006: Collective Motion in Bacterial Populations with Mixed Phenotypic Behaviors Kentaro Hoeger, Ben Strickland, Daniel Shoup, Tristan Ursell The motion of large, densely packed groups of organisms is often qualitatively distinct from the motion of individuals, yet hinges on individual properties and behaviors. Collective motion of bacteria depends strongly on the phenotypic behaviors of individual cells, the physical interactions between cells, and the geometry of their environment, often with multiple phenotypes coexisting in a population. Thus, to characterize how these selectively important interactions affect group traits, such as cell dispersal, spatial segregation of phenotypes, and material transport in groups, we use a library of \textit{Bacillus subtilis} mutants that modulate chemotaxis, motility, and biofilm formation. By mixing phenotypes and observing bacterial behaviors and motion at single cell resolution, we probe collective motion as a function of phenotypic mixture and environmental geometry. Our work demonstrates that collective microbial motion exhibits a transition, from `turbulence' to semiballistic burrowing, as phenotypic composition varies. This work illuminates the role that individual cell behaviors play in the emergence of collective motion, and may signal qualitatively distinct regimes of material transport in bacterial populations. [Preview Abstract] |
Monday, March 13, 2017 9:36AM - 9:48AM |
A6.00007: Effect of added surfactant on bacterial swarming Jordan Bell, Jay Tang In a matter of hours, a microliter droplet of bacteria can grow into a swarming colony that spreads over several square centimeters of an agar gel surface. A bacterial swarm is an active fluid whose advance is aided by a rapid increase in total cell number and flagellated motion, but limited by water availability and surface tension. Here we report two experiments designed to observe the influence of added surfactant on the swarming dynamics of Pseudomonas aeruginosa on the gel surface. 1. When the agar was infused with surfactant, we found notable enhancement in swarming. 2. When a microliter drop of surfactant was deposited at a distance away from a growing swarm front, we noted accelerated advance of the swarm front towards the surfactant spot. Both observations contradict a recent model (Fauvart et al., Soft Matter, 2012), relying on Marangoni flow to explain the swarm motility. We propose that a significant decrease in surface tension caused by the added surfactants suffices to facilitate swarming, rather than the surface tension gradients responsible for Marangoni flow. [Preview Abstract] |
Monday, March 13, 2017 9:48AM - 10:00AM |
A6.00008: Hydrodynamic Hunters Hossein Jashnsaz, Mohammed Al Juboori, Corey Weistuch, Nicholas Miller, Tyler Nguyen, Viktoria Meyerhoff, Bryan McCoy, Stephanie Perkins, Ross Wallgren, Bruce Ray, Konstantinos Tsekouras, Gregory Anderson, Steve Presse In order to pinpoint the location of mobile bacterial prey from diffuse chemical cues in 3D, bacterial predators would need to be exquisitely sensitive to those cues. In addition, bacterial predators would need to forecast their mobile prey's future position on the basis of previously detected chemical signals. While not implausible, this is a difficult search problem for a bacterium. Here we identify a novel, hydrodynamic, mechanism by which the model predator bacterium, \textit{Bdellovibrio bacteriovorus} (BV), locates its prey bacteria. We demonstrate that BV strongly interacts with its own, self-generated, hydrodynamic flow field, reducing the dimensionality of the predator's search space. This work illustrate how bacteria may use hydrodynamics to resolve a difficult search problem and provide a starting point to investigate hydrodynamic effects on bacterial interactions that go beyond the chemical sensing paradigm. [Preview Abstract] |
Monday, March 13, 2017 10:00AM - 10:12AM |
A6.00009: Phase separation dynamics explains \emph{Myxococcus xanthus} aggregation Guannan Liu, Fatmagul Bahar, Adam Patch, Shashi Thutupalli, David Yllanes, Roy Welch, M. Cristina Marchetti, Joshua Shaevitz The soil-dwelling bacteria \emph{Myxococcus xanthus} exhibits a wide range of self-organizing social behaviors during its developmental cycle. When nutrients are scarce, \emph{M. xanthus} cells aggregate into multicellular structures and eventually form massive clusters called fruiting bodies, where cells sporulate as a self-preservation mechanism. In light of recent advancements in active matter theory, we identify the aggregation process of \emph{M. xanthus} as a spinodal decomposition phase separation. We show that without long-range communication, local mechanical interactions are sufficient to drive the system out of equilibrium. \emph{M. xanthus} cells actively modulate their gliding motility and reversal rate to cross a boundary in the P\'{e}clet~Number-density phase plane to achieve phase separation. [Preview Abstract] |
Monday, March 13, 2017 10:12AM - 10:48AM |
A6.00010: Weak synchronization and large-scale collective oscillation in dense bacterial suspensions Invited Speaker: Yilin Wu Collective oscillatory behavior is ubiquitous in nature and it plays a vital role in many biological processes. Collective oscillations in biological multicellular systems often arise from coupling mediated by diffusive chemicals, by electrochemical mechanisms, or by biomechanical interaction between cells and their physical environment. In these examples, the phase of some oscillatory intracellular degree of freedom is synchronized. Here, in contrast, we discovered a unique 'weak synchronization' mechanism that does not require long-range coupling, nor even inherent oscillation of individual cells: We found that millions of motile cells in dense bacterial suspensions can self-organize into highly robust collective oscillatory motion, while individuals move in an erratic manner. Over large spatial scales we found that the phase of the oscillations is in fact organized into a centimeter scale traveling wave. We present a model of noisy self-propelled particles with strictly local interactions that accounts faithfully for our observations. These findings expand our knowledge of biological self-organization and reveal a new type of long-range order in active matter systems. The mechanism of collective oscillation uncovered here may inspire new strategies to control the self-organization of active matter and swarming robots. [Preview Abstract] |
Monday, March 13, 2017 10:48AM - 11:00AM |
A6.00011: Killing mediated spatial structure in \textit{V. Cholerae} biofilms David Yanni Most bacteria live in biofilms, which are implicated in $60-80\%$ of microbial infections in the body. The spatial structure of a biofilm confers advantages to its member-cells, such as antibiotic resistance, and is strongly affected by competition between strains and taxa. However, A complete picture of how competition affects the self-organized structure of these complex, far-from-equilibrium systems, is yet to emerge. To that end, we investigate phase separation dynamics driven by T6SS-facilitated bacterial warfare in a system composed of two strains of mutually antagonistic \textit{V. cholerae}. T6SS is a contact mediated killing mechanism present in $25\%$ of all gram negative bacteria, and has been shown by recent work to play a major role in the spatial assortment of biofilms. T6SS events induce lysis, causing variations in local mechanical pressure, and acting as thermalizing events. We study cells immobilized in biofilms at the air-solid interface, so our experimental system represents a different type active matter, wherein activity is due to cell death and reproduction, not mobility. Here, we show how that activity imposes a constraint of minimal curvature on strain-strain interfaces; an effective Laplace pressure is characterized which governs interfacial dynamics. [Preview Abstract] |
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