Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y50: Microbiological Physics |
Hide Abstracts |
Sponsoring Units: DBIO Chair: Vernita Gordon, Univ of Texas, Austin Room: LACC 511B |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y50.00001: Alignment and motion of uniflagellated bacteria in an electric field George Araujo, Jay Tang Most species of bacteria carry negative net charge. When motile bacteria are subject to an electric field, three physical factors are responsible for their motion: electrophoresis, electroosmosis, and swimming. Using Pseudomonas Aeruginosa, a uniflagellated bacterium as a model system, we record the speeds and trajectories of motile cells filled in a rectangular capillary whose ends are connected to a power supply. It has been observed that cells can be aligned by the electric field depending on its intensity. The alignment by the electric field makes it more convenient to study changes in rotation direction of the flagellar motor, as well as changes in the direction of travel for the cell body. Overall we aim to get a better comprehension of bacterial motion by aligning their trajectories using an external field. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y50.00002: Verticalization transition in Vibrio cholerae biofilms Farzan Beroz, Jing Yan, Yigal Meir, Howard Stone, Bonnie Bassler, Ned Wingreen Biofilms are groups of bacteria that adhere to and grow on surfaces. Recent advances in imaging technology allowed entire Vibrio cholerae biofilms to be observed at single-cell resolution in real time, revealing a growth program consisting of several architectural transitions. These early observations showed that cells are not arranged randomly within a colony, but instead grow from a branched, two-dimensional layer of founder cells into a three-dimensional structure with a vertically-aligned core. Here, we elucidate the physical mechanism of this transition using a combination of agent-based and continuum modeling. We find that the competing effects of cell growth and cell verticalization give rise to an exotic mechanical state in which the pressure becomes constant throughout the entire growing core of the colony. This “dynamical isobaricity” sets the velocity of surface expansion and thereby regulates how cells access the third dimension. In particular, our theory predicts that a longer average cell length yields more rapidly expanding, flatter colonies. We experimentally observed such changes in colony development by using chemicals that perturb cell length. |
Friday, March 9, 2018 11:39AM - 11:51AM |
Y50.00003: Study of early stages of HIV-1 budding Kevin Tsai, Ali Nematbakhsh, Mark Alber, Roya Zandi Formation of immature HIV shells at the cell membrane as the virus buds is unique in that the assembly and budding occur at the same time. The initiation of the budding process is characterized by the interaction between viral protein subunits and the cell membrane. While the important components of the formation and budding process of immature HIV were identified through various experimental observation, little is known about the underlying mechanisms such as the protein aggregation, protein-membrane interaction and formation of the spherical virion. We investigate these mechanisms using computational mechanical stress model. Using triangular discretization of the membrane coupled with the appropriate energy potentials, the protein-membrane related interactions are represented using molecular dynamics simulations. Simulation results show that the protein rigidity and preferred dihedral angle between protein subunits, characterized by Foppl von Karman number, have crucial impact on the dynamics of budding of HIV immature particles. |
Friday, March 9, 2018 11:51AM - 12:03PM |
Y50.00004: Capillary flow and mechanical buckling in a growing annular bacterial colony Tieyan Si, Zidong Ma, Jay Tang A growing bacterial colony is a dense suspension of an increasing number of cells. Starting by inoculating Pseudomonas aeruginosa over an annular area on an agar plate, we observe the growth and spread of the bacterial population, and model the process by consideration of physical effects that account for numerous features observed, such as edge accumulation, periodic protrusions, formation of bacterial droplets, and their growth, coalesce and surfing over the pre-wet agar surface. We conclude that fluid dynamics and elasto-mechanics together govern the bacterial colony pattern evolution. This study offers a clear example that physical effects account for large scale patterns that develop in growing bacterial colonies. |
Friday, March 9, 2018 12:03PM - 12:15PM |
Y50.00005: Mono-to-multilayer transition in growing bacterial micro-colonies Zhihong You, Anupam Sengupta, Daniel Pearce, Luca Giomi Mono-to-multilayer transition is ubiquitous in cellular systems, such as the delamination of epithelia, and the formation of fruiting bodies in myxo-bacteria upon starvation. Using experiments, numerical and analytical modelling, we study the mono-to-multilayer transition in growing bacterial micro-colonies. We demonstrate that such a transition is governed by two competing effects: compression from neighbouring cells, which tends to extrude the cell from the mono-layer, and adhesion to the substrate, which tends to maintain the cells in contact with the agarose. In quasi-one-dimensional colonies (i.e. a single row of bacteria prevented from lateral motion), the mono-to-multilayer transition is completely deterministic and can be analytically described by means of a simple bead model. In two-dimensional colonies, on the other hand, the transition is a stochastic process and must be described in probabilistic terms. The dependency of the extrusion probability on various local quantities are obtained from both experiments and computer simulations, with a good agreement between the two methods. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y50.00006: High-Speed “4D” Computational Microscopy of Bacterial Surface Motility Jaime De Anda, Ernest Lee, Calvin Lee, Rachel Bennett, Xiang Ji, Soheil Soltani, Mark Harrison, Amy Baker, Yun Luo, Thomas Chou, George O'Toole, Andrea Armani, Ramin Golestanian, Gerard Wong Bacterial surface appendage-driven motility modes play key roles in early-stage biofilm community development. Examples include type IV pili-driven “twitching” and flagellum-driven “spinning” and “swarming” motilities controlled by molecular motors. Analysis of surface motility behavior is complicated by its inherently 3D nature, the speed of which is too fast for confocal microscopy to capture. Here, we combine electromagnetic field computation and statistical image analysis to generate 3D movies close to a surface at 5 ms time resolution using conventional inverted microscopes. Application of this technique on Pseudomonas aeruginosa, accompanied with hydrodynamic calculations, revealed that these tiny organisms act like spinning tops in the low Reynolds number regime. They undergo complex flagellum-driven dynamical behavior, including precession and nutation. We also observe an unexpected taxonomy of surface motility mechanisms, like horizontal bacteria that follow helicoidal trajectories and exhibit superdiffusive movements parallel to the surface. We further apply this technique to discern the effects on motility by each set of flagellum stators, MotAB and MotCD, in P. aeruginosa. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y50.00007: Exploring the responses of individual bacteria to micropillar arrays Pooja Chopra, Yu Zeng, David Quint, Ajay Gopinathan, Bin Liu Mechanosensing is an emergent field of interests in bacterial studies due to its close relevance to their adaption to natural habitats and many pathogenic processes. Here, we studied the mechanosensing of bacteria to an array of micro-pillars fabricated through photolithography. Using a 3D tracking microscope, we extended the tracking of motile Escherichia coli and captured their individual interactions with multiple pillars. We analyzed the run-and-tumble statistics of E. coli as they interacted with these micro-pillars, which show no significant differences from the non-pillar cases. We also obtained the spatial dependency of the run-and-tumble statistics by mapping the individual bacterial movements in an extended lattice of micro-pillars into a single lattice unit cell. Through this analysis, we have shown that the bacterial movement is highly confined to the proximity near the solid surfaces, suggesting a passive response of E. coli to their mechanical interactions with the micro-pillars. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y50.00008: Mechanical Principles of Biofilm Formation Jing Yan, Howard Stone, Ned Wingreen, Bonnie Bassler
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Friday, March 9, 2018 12:51PM - 1:03PM |
Y50.00009: Active Dispersal and Rolling Motility in Bacterial Aggregates Yu Zeng, Bin Liu Microorganism dispersal is fundamental to their abundance in nature and plays a crucial role in pathogen transmissions. In general, active dispersals are attributed to the movements of self-powered planktonic cells, and sessile cells can only be transported passively through flow entrainment for their lack of powering organelles. Here, we report an active means of dispersal employed by aggregates of motile and sessile cells. The sessile cells of bacterium Caulobacter crescentus can form spherical rosette colonies, within which a small proportion can grow active flagella and enable whole-rosette motility. We show that these rosettes disperse actively both in bulk water and near the solid-liquid interface. In particular, the proximity of a self-powered rosette to the solid surface promotes a rolling movement, leading to its persistent transportation along the solid boundary. The active dispersal of these rosettes demonstrates an exclusive mode of colonial transportation that is based on the division of labor. Such aggregates are potentially adaptive for more efficient colonial transport and may provide guidance for biomimetic engineering of microsystems. |
Friday, March 9, 2018 1:03PM - 1:15PM |
Y50.00010: Characterizing the Formation of Concentric Rings-Pattern by Proteus mirabilis Emrah Simsek, Minsu Kim Many organisms, including bacteria, can form patterns through intricate self-organization processes. Here, we study a concentric rings-pattern formed by Proteus mirabilis. When a small droplet of a Proteus mirabilis culture is inoculated on solid medium, cells grow on the surface for some time. This initial growth phase is followed by periodic cycles of surface-associated group motility (i.e., swarming) and cell multiplication (i.e., consolidation), which eventually leads to a macroscopic concentric rings-pattern. Yet, what regulates the formation of this pattern remains unclear. Our experimental results suggest that both initiation and maintenance of the formation of this pattern depend on cell density. For example, swarming begins only when cells reach a critical cell density, or its period can be tuned by manipulating cell density. In this talk, I will present our data to support this idea of density dependence and discuss its implications. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y50.00011: The Effect Of Slime Navigation On Spreading Of Protein In Swarming Groups Of
Myxococcus Xanthus Bacteria Alireza Ramezani, Ali Nematbakhsh, Aboutaleb Amiri, Roya Zandi, Mark Alber
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Friday, March 9, 2018 1:27PM - 1:39PM |
Y50.00012: Magnetic properties and effective temperature of magnetotactic bacteria Lucas Le Nagard, Solomon Barkley, Xiaohui Zhu, Dennis Bazylinski, Adam Hitchcock, Cecile Fradin Magnetotactic bacteria are motile prokaryotes that synthesize magnetosomes, which are magnetic single domain crystals surrounded by a lipid membrane. These organelles confer to the cells a permanent magnetic moment that makes them align passively in external magnetic fields and therefore behave as active micro-compasses that swim along magnetic field lines. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y50.00013: Bacterial Proteins Associated With Cell Shape Homeostasis Localize to Specific 3D Geometries Benjamin Bratton, Randy Morgenstein, Zemer Gitai, Joshua Shaevitz The bacterial kingdom exhibits a wide variety of cell shapes and sizes which are crucial for the lifestyle of each species. We have developed an image-processing framework that extracts precise 3D shapes from fluorescence microscopy data allowing us to calculate geometric parameters such as local curvature, surface area, and the relative enrichment of fluorescent signals. We use this to measure the geometric localization of proteins responsible for the characteristic shape of Gram-negative bacteria (straight rod Escherichia coli, curved rod Vibrio cholerae, and helical rod Helicobacter pylori). In E. coli, the bacterial actin MreB localizes away from positive Gaussian curvature and toward low curvature. Without the MreB modulator RodZ, MreB loses its curvature localization and cells lose their uniform rod-like shape. In cholera, CrvA localizes to areas of negative Gaussian curvature and slows the inner curve's growth, causing cells to curve. In the bacterial carcinogen H. pylori, we have begun to examine the localization of many cell shape determinants with various curvature preferences. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y50.00014: Emergence of collective motion in suspensions of swimming cells Maria Chiara Roffin, Anton Bukatin, Petr Denissenko, Vasily Kantsler We report a study on transition to collective dynamics in suspensions of mammalian sperm cells in microfluidic confinement. The concentration of motile spermatozoa in the region of interest is altered either in steps through manual dilution of the original sample or gradually by a microfluidic ratcheted device, designed to direct and concentrate motile cells. Velocity fields have been measured using PIV (Particle Image Velocimetry) from videos of both passive fluorescent beads and of the motile cells. The establishment of the collective motion is identified through calculation of a number of fluid parameters such as vorticity, enstrophy, velocity spatial and temporal correlations, and average velocities. The measured parameters exhibit a step change indicating transition from random and unorganised motion of individual cells to locally organised motion upon reaching a critical concentration of motile spermatozoa. The shortest length scale in the collective dynamics remains nearly constant throughout the range of concentrations, while the largest scale depends on the spermatozoa concentration and the geometrical confinement. |
Friday, March 9, 2018 2:03PM - 2:15PM |
Y50.00015: Large Scale Self-organization in Viscoelastic Bacterial Suspensions Song Liu, Yilin Wu Active matter systems such as cytoskeletal filaments driven by molecular motors and suspensions of motile bacteria have drawn much attention not only because of the direct biophysical significance, but also because they serve as model systems to study non-equilibrium physics. Microscopic energy input in these systems can lead to spontaneous emergence of rich collective behaviour. Here we will present our recent discovery of large scale bacterial self-organization mediated by cell-fluid interactions in a viscoelastic environment, and will explore the potential application of the findings in active matter engineering. |
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