Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J7: Biofilms and Multicellularity |
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Sponsoring Units: GSNP Chair: Sigolene Lecuyer, Harvard University Room: Portland Ballroom 254 |
Tuesday, March 16, 2010 11:15AM - 11:51AM |
J7.00001: Catch bonds enable bacterial and cell adhesion under flow Invited Speaker: How well does our intuition serve us when trying to prevent bacterial infections? If receptor-ligand complexes are being pulled apart by tensile forces, conventional wisdom implied that the lifetime of the complex would be shortened. As supported by many recent biophysical studies, force indeed accelerates the probability of a ligand slipping out of the binding pocket. In contrast to these so-called \textit{slip bonds}, recent data show that some receptor-ligand complexes can form catch bonds whose lifetime increases under tensile force. Some bacteria and cells take advantage of catch bonds to adhere to surfaces under fluid flow conditions. We will explore where such \textit{catch bonds} can be found, how they work and how cells exploit them for a multitude of tasks.\\[4pt] In collaboration with Viola Vogel, Laboratory of Biologically Oriented Materials, Department of Materials, ETH Zurich, CH, 8049, Switzerland. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:27PM |
J7.00002: Bacterial Swarming: social behaviour or hydrodynamics? Invited Speaker: Bacterial swarming of colonies is typically described as a social phenomenon between bacteria, whereby groups of bacteria collectively move atop solid surfaces. This multicellular behavior, during which the organized bacterial populations are embedded in an extracellular slime layer, is connected to important features such as biofilm formation and virulence. Despite the possible intricate quorum sensing mechanisms that regulate swarming, several physico-chemical phenomena may play a role in the dynamics of swarming and biofilm formation. Especially the striking fingering patterns formed by some swarmer colonies on relatively soft sub phases have attracted the attention as they could be the signatures of an instability. Recently, a parallel has been drawn between the swarming patterns and the spreading of viscous drops under the influence of a surfactant, which lead to similar patterns [1]. Starting from the observation that several of the molecules, essential in swarming systems, are strong biosurfactants, the possibility of flows driven by gradients in surface tension, has been proposed. This Marangoni flows are known to lead to these characteristic patterns. For {\it Rhizobium etli} not only the pattern formation, but also the experimentally observed spreading speed has been shown to be consistent with the one expected for Marangoni flows for the surface pressures, thickness, and viscosities that have been observed [2]. We will present an experimental study of swarming colonies of the bacteria {\it Pseudomonas aeruginosa}, the pattern formation, the surfactant gradients and height profiles in comparison with predictions of a thin film hydrodynamic model.\\[4pt] [1] Matar O.K. and Troian S., Phys. Fluids {\bf 11} : 3232 (1999)\\[0pt] [2] Daniels, R et al., PNAS, {\bf 103} (40): 14965-14970 (2006) [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 1:03PM |
J7.00003: Swimming in Turbulent Waters: a New Mechanism for Phytoplankton Patchiness in the Ocean Invited Speaker: Marine phytoplankton are responsible for nearly half of the world's oxygen production and comprise the base of the Oceans' food web. The spatial distribution of these unicellular organisms is observed to be heterogeneous at nearly all scales; the largest accumulations extend hundreds of kilometers, while the smallest span only a few millimeters. This endemic patchiness has profound consequences on trophic dynamics and biogeochemical cycles in the Ocean; a mechanistic understanding of the underlying processes is a primary research goal in oceanography. Many phytoplankton species, particularly those responsible for harmful algal blooms, are motile, propelling themselves through water at low Reynolds numbers. Yet, motility is rarely taken into account when considering the mechanisms that drive patchiness. In this study we find two simple ingredients are sufficient to generate striking heterogeneity in the distribution of motile phytoplankton: asymmetric cell morphology and hydrodynamic shear, both of which are ubiquitous in the ocean. For example, cells can be asymmetric due to uneven distribution of organelles or flagella, while shear in the Ocean occurs at all scales, from large-scale currents to small-scale turbulence. We propose a new mechanism - gyrotactic trapping - whereby motile asymmetric cells in shear preferentially accumulate in specific regions of the flow. We present experimental and theoretical evidence demonstrating that gyrotactic trapping can produce patchiness over a wide range of scales, from kilometer-scale thin phytoplankton layers to patchiness at the Kolmogorov scale, both of which are routinely observed by oceanographers. We find that the intensity, time scale, and location of the resultant accumulations are a function of two dimensionless numbers, containing properties of both the cells and the flow. These findings demonstrate that cell motility can shape principal features of the marine environment and provide oceanographers with quantitative tools to predict phytoplankton distributions in the Ocean. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:39PM |
J7.00004: Bacterial Biofilms as Complex Communities Invited Speaker: Many microbial populations form surface-associated multicellular communities known as biofilms. These multicellular communities are encased in a self-produced extracellular matrix composed of polysaccharides and proteins. Division of labor is a key feature of these communities and different cells serve distinct functions. We have found that in biofilms of the bacterium \textit{Bacillus subtilis}, different cell types including matrix-producing and sporulating cells coexist and localize to distinct regions within the structured community. We were interested in understanding how these different cell types arise. Using fluorescence reporters under the control of promoters that are specific for distinct cell types we were able to follow the dynamics of differentiation throughout biofilm development. We found that a series of extracellular signals leads to differentiation of distinct cell types during biofilm formation. In addition, we found that extracellular matrix functions as a differentiation signal for timely sporulation within a biofilm and mutants unable to produce matrix were delayed in sporulation. Our results indicate that within a biofilm, cell-cell signaling is directional in that one cell type produces a signal that is sensed by another distinct cell type. Furthermore, once differentiated, cells become resistant to the action of other signaling molecules making it possible to maintain distinct cell populations over prolonged periods. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 2:15PM |
J7.00005: Collective behavior of biological cells Invited Speaker: Interacting biological cells can exhibit collective behaviors that our knowledge at the scale of the single cell cannot fully explain. These highly non-linear systems strongly depend on the boundary conditions on which microfabrication techniques offer a good control. A first example is given by chemotactic \textit{Escherichia coli} bacteria swimming in liquid medium. These cells communicate by expressing soluble chemoattractants which gives rise to spatial concentration heterogeneities that can be evidenced particularly well in micro-geometries. In particular, when the bacteria are concentrated at the extremity of a microchannel, a concentration wave appears and propagates along the channel. We will discuss new results obtained on this system, in particular by studying closely the details of the trajectories of single bacteria. The situation with eukaryotic epithelial cells, although basically different, presents some formal similarities. By using microfabricated micro-stencils to trigger the collective migration, we observe long-range coordinated displacements well within the monolayer. In parallel, the edges of these migrating monolayers roughen drastically and exhibit a strong directional fingering where a ``leader cell'', that exhibits a clearly non-epithelial phenotype, leads the others. In these fingers, the velocity field, as well as the orientation and polarization of the cells align with the fingers but are described by different order parameters and kinetics. The exact role of the leader can be clarified by mapping the traction forces exerted by the cells using a microfabricated array of force sensors. [Preview Abstract] |
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