Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D11: Physics of Bacteria I |
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Sponsoring Units: DBP Chair: Yuhai Tu, IBM Room: A107-A109 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D11.00001: Type IV pili mediated ``walking'' motility of bacteria Maxsim Gibiansky, Jacinta Conrad, Vernita Gordon, Dominick Motto, Fan Jin, Joshua Shrout, Gerard Wong We develop image recognition and particle tracking algorithms to identify and track large numbers of surface-associated bacteria, up to $\sim$800 cells for up to 6 hours. To characterize the pili-dependent motility mechanisms, we image wild type (WT) and flagella-deficient (pilA) knockout strains of P. aeruginosa. In the pilA strain, we observe two motility mechanisms: a novel ``walking'' mechanism, characterized by bacteria orienting themselves normal to the surface, and a ``crawling'' mechanism, characterized by the bacteria lying flat on the surface. We find that ``crawling'' bacteria move along their long axis and maintain their orientation over time, whereas ``walking'' bacteria change direction rapidly, allowing them to sample microenvironments more efficiently. We also observe both ``walking'' and ``crawling'' in the WT strain, suggesting that flagella do not interfere with these mechanisms of Type-IV pili based motility. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D11.00002: The role of adhesins in bacteria motility modification Jacinta Conrad, Maxsim Gibiansky, Fan Jin, Vernita Gordon, Dominick Motto, Joshua Shrout, Matthew Parsek, Gerard Wong Bacterial biofilms are multicellular communities responsible for a broad range of infections. To investigate the early-stage formation of biofilms, we have developed high-throughput techniques to quantify the motility of surface-associated bacteria. We translate microscopy movies of bacteria into a searchable database of trajectories using tracking algorithms adapted from colloidal physics. By analyzing the motion of both wild-type (WT) and isogenic knockout mutants, we have previously characterized fundamental motility mechanisms in {\it P. aeruginosa}. Here, we develop biometric routines to recognize signatures of adhesion and trapping. We find that newly attached bacteria move faster than previously adherent bacteria, and are more likely to be oriented out-of-plane. Motility appendages influence the bacterium's ability to become trapped: WT bacteria exhibit two types of trapped trajectories, whereas flagella-deficient bacteria rarely become trapped. These results suggest that flagella play a key role in adhesion. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D11.00003: Possible increase in microorganism motility due to motion-dependent nutrient uptake Carlos A. Condat, Mario E. Di Salvo, Gustavo J. Sibona Since they have to beat Brownian forces or reach evanescent nutrient patches, some microorganisms inhabiting marine habitats must swim at very high speeds. As a consequence, they must allocate a considerable part of their energetic budget to motion. It is therefore tempting to assume that they enhance their nutrient uptake by increasing their swimming speed. Enhanced nutrient absorption would itself help the microorganism sustain higher translational speeds. We have formulated a model that uses the concept of internal energy depot to investigate this assumption, postulating that the nutrient absorption rate is an increasing function of the microorganism speed. Using realistic values of the parameters, we show that the resulting increment in speed can be substantial, even in the case of small marine bacteria. We find the stationary solutions to the equations of motion and analyze their stability and the influence of thermal noise using various forms for the energy-transfer function. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:42PM |
D11.00004: Behavior of the Flagellar Rotary Motor Near Zero Load Invited Speaker: Many bacteria are propelled by the rotation of helical flagellar filaments that extend out into the external medium. Each filament is driven at its base by a reversible rotary motor embedded in the cell wall. The motor runs far from thermal equilibrium near zero load, so studies of the motor in this regime allow one to gain more insights into the kinetics of motor function. Previously, the load regime was limited to high to medium loads. Here, we describe an assay that allows systematic study of the motor near zero load [1]. Sixty-nanometer-diameter gold spheres were attached to hooks of cells lacking flagellar filaments, and light scattered from a sphere was monitored at the image plane of a microscope through a small pinhole. Resurrection experiments were carried out near zero load [2]. Paralyzed motors of cells carrying a motA point mutation were resurrected at 23\r{ }C by expression of wild-type MotA, and speeds jumped from zero to a maximum value of about 300 Hz in one step. The temperature and solvent-isotope dependence of the speed near zero load were also studied and showed a high activation enthalpy comparable to that observed previously in electrorotation experiments. The assay has been modified so that both the speed and the direction of rotation can be monitored near zero load. Switching properties of the flagellar motor near zero load were investigated and showed a near linear dependence of the switching rate on motor torque [3]. \\[4pt] [1]. J. Yuan and H.C. Berg (2009) Following the behavior of the flagellar rotary motor near zero load. \textit{Exp. Mech.} DOI: 10.1007/s11340-009-9290-1. \\[0pt] [2]. J. Yuan and H.C. Berg (2008) Resurrection of the flagellar rotary motor near zero load. \textit{Proc. Natl. Acad. Sci. USA} \textbf{105}:1182-1185. \\[0pt] [3]. J. Yuan, K.A. Fahrner and H.C. Berg (2009) Switching of the bacterial flagellar motor near zero load. \textit{J. Mol. Biol.} \textbf{390}:394-400. [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 4:18PM |
D11.00005: Tug of war: The dynamics of flagellar motor with multiple stators Invited Speaker: In a single flagellar motor, there are multiple stator units that drive the rotation of the flagellar filaments. Here, we introduce a ``tug-of-war'' model for the flagellar motor where each individual stator can generate either positive or negative torque depending on its relative mechano-chemical state with respect to the rotor. The key ingredient of our model is that the instantaneous chemical switching (stepping) rate of a stator depends on the torque it generates: stators that generate negative torque switch faster. We show that the experimentally observed torque-speed relationship can be explained by the moving time and waiting time of the motor and their different dependences on the load. The model reveals the dependence of the motor dynamics on the number of stators. In particular, we show that the maximum speed of the motor is independent of the number of stators, which agrees with recent resurrection experiments at near zero loads [Yuan {\&} Berg, PNAS 105, 1182-1185, 2008]. The model is also used to study stepping statistics in single flagellar motor and understand different noise sources for rotational speed fluctuation. Finally, our model can be generalized to study other motor system with multiple power generating units. Part of the work is published in [Meacci {\&} Tu, PNAS 106, 3746-3751, 2009]. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D11.00006: The multifunctional roles of Type IV pili in bacterial surface motility Fan Jin, Jacinta Conrad, Vernita Gordon, Maxsim Gibiansky, Gerard Wong In \textit{Pseudomonas aeruginosa}, a commonly-studied model for biofilm formation, type IV pili mediate two different mechanisms of surface motility: a ``crawling'' mechanism, in which the bacterium moves parallel to the surface with high directional persistence; and a ``walking'' mechanism, in which a vertically-oriented bacterium moves rapidly but with low directional persistence. To characterize these mechanisms with high spatial and temporal resolution, we develop a novel tracking method called `two focus tracking' which allows us to quantitatively estimate the tilt angle between a single bacterium and the surface. By analyzing the motion of $\Delta $\textit{fliM}, a flagella-deficient isogenic knockout mutant of \textit{P. aeruginosa}, we demonstrate that a single bacterium can switch between the ``walking'' and ``crawling'' mechanisms. To elucidate the role of pili in crawling, we show that the velocity profiles of single trajectories can be decomposed into alternating ``pulses'' and ``square waves.'' The direction of alternation ``pulses and ``square waves'' are significantly correlated. We show how these observations can be explained by cooperative deployment of multiple pili. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 4:42PM |
D11.00007: Studying bacterial quorum-sensing at the single cell level Pablo Delfino Perez, Leslie Pelakh, Jonathan Young, Elaine Johnson, Stephen Hagen Like many bacterial species, \textit{Vibrio fischeri} can detect its own population density through a quorum sensing (QS) mechanism. The bacterium releases a signal molecule (AI, autoinducer), which accumulates at high population density and triggers a genetic switch. In \textit{V.fischeri} this leads to bioluminescence. Little is known about how stochastic gene expression affects QS at the level of single cells. We are imaging the luminescence of individual \textit{V.fischeri }cells in a flow chamber and directly measuring the intercell variability in AI activation of the QS circuit. Our single-cell luminescence experiments allow us to track cells over time and characterize variations in their response to AI levels. We find heterogeneous response to the external signal: at a given AI concentration some cells may be strongly luminescent while others are virtually dark. The analysis of noise in the individual cell response can eventually lead to a better understanding of how cells use QS to gather information about their environment. [Preview Abstract] |
Monday, March 15, 2010 4:42PM - 4:54PM |
D11.00008: Shear-enhanced adhesion of Pseudomonas aeruginosa Sigolene Lecuyer, Roberto Rusconi, Yi Shen, Alison Forsyth, Howard Stone Bacterial adhesion is the first step in the development of surface-associated communities known as biofilms, which are the cause of many problems in medical devices and industrial water systems. However the underlying mechanisms of initial bacterial attachment are not fully understood. We have investigated the effects of hydrodynamics on the probability of adsorption and detachment of \textit{Pseudomonas aeruginosa} strain PA14 on model surfaces under flow, in straight microfluidic channels, and measured the distribution of bacteria residence time as a function of the shear rate. Our main discovery is a counter-intuitive enhanced adhesion as the shear stress is increased over a wide range of shear rates. In order to identify the origin of this phenomenon, we have performed experiments with several mutant strains. Our results show that shear-enhanced adhesion is not regulated by primary surface organelles, and that this process is not specific to a certain type of surface, but rather appears a general feature of the adhesive behavior of \textit{P. aeruginosa}. These results suggest that shear-induced adhesion could be a very widespread strategy in nature. [Preview Abstract] |
Monday, March 15, 2010 4:54PM - 5:06PM |
D11.00009: Chromosomal Loci Move Subdiffusively through a Viscoelastic Cytoplasm Andrew Spakowitz, Stephanie Weber, Julie Theriot Tracking of fluorescently labeled chromosomal loci in live bacterial cells reveals a robust scaling of the mean square displacement (MSD) as $\tau^{0.39}$. Brownian dynamics simulations show that this anomalous behavior cannot be fully accounted for by the classic Rouse or reptation models for polymer dynamics. Instead, the observed motion arises from the characteristic relaxation of the Rouse modes of the DNA polymer within the viscoelastic environment of the cytoplasm. To demonstrate these physical effects, we exploit our general analytical solution of the subdiffusive scaling for a monomer in a polymer embedded in a viscoelastic medium. The time-averaged and ensemble-averaged MSD of chromosomal loci exhibit ergodicity, and the velocity autocorrelation function is negative at short time lags. These observations are most consistent with fractional Brownian motion and rule out a continuous time random walk model. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D11.00010: E. coli chemotaxis and super-diffusion Jure Dobnikar, Franziska Matth\"aus, Marko Jagodic The bacteria E. coli actively propel by switching between clockwise and anti-clockwise rotation of the flagella attached to their cell membranes. This results in two modes of motion: tumbling and swimming. The switching between the two modes is coupled to the ligand sensing through the chemotactic signalling pathway inside the cell. We modelled the signalling pathway and performed numerical simulations of the chemotactic motion of a large number of E. coli bacteria under various external conditions. We have shown that under certain conditions the thermal noise in the level of receptor-bound CheR (an enzyme responsible for methylation of the receptor sites) leads to super-diffusive behaviour (L\'{e}vy walk) which is advantageous for the bacterial populations in environments with scarce food. Exerting external pressure we might observe evolution of the wild-type to the super-diffusive populations. [Preview Abstract] |
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