Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H10: Physics of Bacteria II |
Hide Abstracts |
Sponsoring Units: DBP Chair: Eshel Ben-Jacob, Tel Aviv University Room: A106 |
Tuesday, March 16, 2010 8:00AM - 8:12AM |
H10.00001: Cell shape acquisition and maintenance in rodlike bacteria Sven van Teeffelen, Ned Wingreen, Zemer Gitai The shape of rodlike bacteria such as \textit{Escherichia coli }is mainly governed by the expansion and reorganization of the peptidoglycan cell wall. ~The cell wall is a huge, mostly single-layered molecule of stiff glycan strands that typically run perpendicular to the long axis and are crosslinked by short peptides. The wall resists the excess pressure from inside the cell. Although much is known about the enzymes that synthesize the wall, the mechanisms by which the cell maintains a constant rod diameter and uniform glycan strand orientation during growth remain unknown. Here we present quantitative results on the structure and dynamics of two essential proteins, which are believed to play an important role in cell wall synthesis. In particular, we have focused on the filament-forming protein MreB, an actin homolog that forms a long helical bundle along the inner membrane of the cell, and penicillin-binding protein 2, an essential protein for peptide bond formation in the periplasm. Based on their interplay we discuss the possibility of MreB serving as a guide and ruler for cell wall synthesis. [Preview Abstract] |
Tuesday, March 16, 2010 8:12AM - 8:24AM |
H10.00002: Cell-wall dynamics in growing bacteria Leon Furchtgott, Ned Wingreen, Kerwyn Casey Huang Bacterial cells come in a large variety of shapes, and cell shape plays an important role in the regulation of many biological functions. Cell shape in bacterial cells is dictated by a cell wall composed of peptidoglycan, a polymer made up of long, stiff glycan strands and flexible peptide crosslinks. Although much is understood about the structural properties of peptidoglycan, little is known about the dynamics of cell wall organization in bacterial cells. In particular, during cell growth, how does the bacterial cell wall continuously expand and reorganize while maintaining cell shape? In order to investigate this question quantitatively, we model the cell wall of the Gram-negative bacterium \textit{Escherichia coli} using a simple elastic model, in which glycan and peptide subunits are treated as springs with different spring constants and relaxed lengths. We consider the peptidoglycan network as a single-layered network of these springs under tension due to an internal osmotic pressure. Within this model, we simulate possible hypotheses for cell growth as different combinations of addition of new springs and breakage of old springs. [Preview Abstract] |
Tuesday, March 16, 2010 8:24AM - 8:36AM |
H10.00003: Computational assessment of the stiffness of the Gram-negative bacterial cell wall Sandhya Sinha, Yao Zhao, K.C. Huang The bacterial cytoplasm exists in a state of constant metabolic activity, leading to a turgor pressure across the membrane that measures an atmosphere or more. For most bacteria, the peptidoglycan cell wall bears this stress and is also a primary determinant of the cell's shape. In this work, we investigate how the elastic properties of Gram-negative cell walls emerge from the molecular organization of the peptidoglycan network by studying the structure of a mechanical model of the cell wall under the computational application of several types of strain. Experimental evidence has suggested that the Young's modulus of the cell wall increases nonlinearly with the turgor pressure. We have conducted simulations to determine what intrinsic physical characteristics of the molecular components of the cell wall, including bending, tension, and anisotropy, are necessary and sufficient for recapitulating the nonlinear rise in stiffness. Furthermore, we have modeled the effect of missing springs on the elastic response of the cell-wall network to bridge the gap between molecular organization and a continuum model of cell-wall elasticity. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 8:48AM |
H10.00004: The role of spatial asymmetries in the development of the bacterium \textit{Caulobacter crescentus} Carolina Tropini, Erin Chen, Stephen Sciochetti, Austin Newton, Michael Laub, Kerwyn Casey Huang \textit{Caulobacter} is a model organism for cell cycle regulation and development. Upon division it differentiates into a sessile stalked cell and a motile swarmer cell. Throughout the cell cycle, the localization of several key proteins is highly regulated. We address the importance of spatial localization in signal transduction and development. Flagellar pole development is controlled by the response regulator DivK, whose phosphorylation state is controlled by the kinase DivJ and the phosphatase PleC. PleC localizes to the swarmer pole, while DivJ localizes at the stalked pole. We have constructed strains with a variety of PleC and DivJ localization patterns. Our results indicate that localization is not absolutely necessary in this system, rather localized proteins enhance the robustness to fluctuations. We further investigate the importance of spatial asymmetries in the regulation of the master cell-cycle-regulator CtrA. In its phosphorylated form, CtrA binds to the replication origin in \textit{Caulobacter} in a highly cooperative fashion, and prevents DNA replication. The CtrA distribution is tightly controlled not only by localized phosphorylation and dephosphorylation but also synthesis and degradation. We find that physiological degradation rates exert only a small perturbation on the distribution generated by asymmetric phosphorylation. [Preview Abstract] |
Tuesday, March 16, 2010 8:48AM - 9:00AM |
H10.00005: Bacteria in Confined Spaces Connie Wilking, David Weitz Bacterial cells can display differentiation between several developmental pathways, from planktonic to matrix-producing, depending upon the colony conditions. We study the confinement of bacteria in hydrogels as well as in liquid-liquid double emulsion droplets and observe the growth and morphology of these colonies as a function of time and environment. Our results can give insight into the behavior of bacterial colonies in confined spaces that can have applications in the areas of food science, cosmetics, and medicine. [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H10.00006: Microbial Nanowire Electronic Structure Probed by Scanning Tunneling Microscopy Joshua P. Veazey, Sanela Lampa-Pastirk, Gemma Reguera, Stuart H. Tessmer Complex molecules produced by living organisms provide laboratories for interesting physical properties. The study of such interesting physics, likewise, gives new insight into intriguing biological processes. We have studied the pilus nanowires expressed by the bacterium, \textit{Geobacter sulfurreducens}, using high resolution scanning tunneling microscopy (STM). \textit{G. sulfurreducens} is a metal reducing bacterium that has evolved electrically conductive pili to efficiently transfer electrons across large distances.\footnote{G. Reguera, K.D. McCarthy, T. Mehta, J.S. Nicoll, M.T. Tuominen, and D.R. Lovley, Nature \textbf{435}, 1098 (2005)} Here we employ the electronic sensitivity of STM to resolve the molecular substructure and the local electronic density of states (LDOS) along the nanowire, in an effort to elucidate the mechanism of conduction. We observe LDOS dependent upon the location of the tip above the nanowire. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H10.00007: Viability of adhered bacterial cells: tracking MinD protein oscillations Matt Barrett, Keegan Colville, Chris Schultz-Nielsen, Manfred Jericho, John Dutcher To study bacterial cells using atomic force microscopy, it is necessary to immobilize the cells on a substrate. Because bacterial cells and common substrates such as glass and mica have a net negative charge, positively charged polymers such as poly-L-lysine (PLL) and polyethyleneimine (PEI) are commonly used as adhesion layers. However, the use of adhesion polymers could stress the cell and even render it inviable. Viable \textit{E. coli} cells use oscillations of Min proteins along the axis of the rod-shaped cells to ensure accurate cell division. By tagging MinD proteins with GFP, oscillations can be observed using fluorescence microscopy. For a healthy cell in an ideal environment, the oscillation period is measured to be $\sim $40 s. Prior experiments have shown that PLL increases the oscillation period significantly (up to 80{\%}). In the present study, we have used epifluorescence and total internal reflection fluorescence (TIRF) to track MinD protein oscillations in \textit{E. coli} bacteria adhered to a variety of positively charged polymers on mica as a function of polymer surface coverage. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H10.00008: Curved microchannels and bacterial streamers Roberto Rusconi, Sigolene Lecuyer, Laura Guglielmini, Howard Stone Bacterial biofilms are commonly identified as microbial communities attached to a surface and encased in a self-secreted extracellular matrix. Due to their increased resistance to antimicrobial agents, biofilms have an enormous impact on health and medicine (e.g., wound healing, implant-associated infections, disease transmission). On the other hand, they constitute a major component of the stream ecosystem by increasing transport of nutrients and retention of suspended particles. In this talk, we present an experimental study of bacterial biofilm development in a microfluidic device. In particular, we show the formation of filamentous structures, or streamers, in curved channels and how these suspended biofilms are linked to the underlying hydrodynamics. [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 9:48AM |
H10.00009: Physical properties of native bacterial biofilm cells measured by atomic force microscopy Katherine Aidala, Catherine Volle, Megan Ferguson, Eileen Spain, Megan Nunez Atomic force microscopy offers a way to probe physical properties of bacteria that are adhered to a surface. We study early stage biofilms that natively adhere to a glass surface, without artificial fixation methods. We present images and force curves from five different bacteria, consisting of two gram positive and three gram negative strains, as well as both smooth and rough gram negative strains. The linear portion of the approach curve reveals the gram positive strains are stiffer than the gram negative strains. The non-linear portion of the approach curve, determined by the initial interaction between the tip and cell, differentiates the smooth and rough strains. Fixation of free-swimming planktonic cells by NHS and EDC dramatically changed the measured properties. These results can be understood from the structure of the cells. [Preview Abstract] |
Tuesday, March 16, 2010 9:48AM - 10:00AM |
H10.00010: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 10:00AM - 10:12AM |
H10.00011: Growth mechanics of bacterial cell wall and morphology of bacteria Hongyuan Jiang, Sean Sun The peptidoglycan cell wall of bacteria is responsible for maintaining the cell shape and integrity. During the bacterial life cycle, the growth of the cell wall is affected by mechanical stress and osmotic pressure internal to the cell. We develop a theory to describe cell shape changes under the influence of mechanical forces. We find that the theory predicts a steady state size and shape for bacterial cells ranging from cocci to spirillum. Moreover, the theory suggest a mechanism by which bacterial cytoskeletal proteins such as MreB and crescentin can maintain the shape of the cell. The theory can also explain the several recent experiments on growing bacteria in micro-environments. [Preview Abstract] |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H10.00012: Intercellular interactions~in early biofilm formation probed with image analysis and~laser trapping Vernita Gordon, Jacinta Conrad, Maxsim Gibiansky, Fan Jin, Nyrene Haque, Dominick Motto, Margie Mathewson, Gabe Spalding, Matthew Parsek, Joshua Shrout, Gerard Wong Inter-bacterial interactions are essential to such fundamental phenomena as motility and biofilm development.~ Many of these interactions are mediated by quorum sensing to coordinate gene expression among groups of cells. Other influences include contact with a common surface and proximity of neighboring cells.~ All such mechanisms depend strongly on the spatial structure of the system.~ We investigate the early stages of biofilm formation.~These cells show striking cooperative behavior: neighbor proximity and number correlate with the post-division~detachment likelihood of daughter cells. To better study the effects of such spatial structure, we develop a platform that uses laser trapping to control bacterial patterning.~ We place bacteria on a surface with micron-lengthscale precision and reproducibility. This platform allows systematic study of the effects of neighbor number, density, and orientation on intercellular interactions. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H10.00013: Genetic Control of Rheology in Bacterial Biofilms James Wilking, Michael Brenner, David Weitz Bacteria often form surface-associated colonies known as biofilms. Within these colonies, bacteria embed themselves in an extracellular matrix composed primarily of polysaccharides and proteins. The synthesis of each matrix component is under genetic control. As a result, the mechanical properties of the matrix are as well. By selectively removing primary components of the extracellular matrix through genetic mutations, we develop an understanding of the bulk mechanical properties of B. subtilus bacterial biofilms. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H10.00014: Atomic Force Microscope Investigations of Biofilms Treated with Gas Discharge Plasmas Kurt Vandervoort, Gregory Stough, Anna Zelaya , Graciela Brelles-Marino We present investigations of bacterial biofilms before and after treatment with gas discharge plasmas. Gas discharge plasmas represent a way to inactivate bacteria under conditions where conventional disinfection methods are often ineffective. These conditions involve bacteria in biofilm communities, where cooperative interactions between cells make organisms less susceptible to standard inactivation methods. In this study, biofilms formed by the opportunistic bacterium \textit{Pseudomonas aeruginosa} were imaged before and after plasma treatment using an atomic force microscope (AFM). Cell morphology and biofilm structure were investigated through AFM images obtained for various plasma exposure times. Also, structural properties of the biofilms were studied through force-distance curves by pressing the AFM tip into the film surface while monitoring the cantilever deflection. [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H10.00015: Relationship between model bacterial peptidoglycan network structures and AFM force-distance curves Aidan Brown, Robert Wickham, Ahmed Touhami, John Dutcher Recent atomic force microscopy (AFM) measurements have involved pulling on Gram-negative bacterial sacculi with the AFM tip as a means of distinguishing between different proposed structures of the peptidoglycan network. The goal of the present study is to provide the theoretical connection between a given network structure and its response to the pulling force. We model the glycan strands as hinged rods, and the peptide cross-links as wormlike chains. Using Monte Carlo simulation to equilibrate the three-dimensional network, subject to a fixed AFM tip-to-substrate distance, we can compute the force exerted by the network on the AFM tip. The effects of adhesion of the sacculi to the substrate and enzymatic action on the network are included. We have modeled both the layered and the scaffold model for the peptidoglycan network structure. We have compared our theoretical force-distance curves for each network structure with experimental curves to determine which structure provides the best agreement with experiment. [Preview Abstract] |
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