144 research outputs found
Swimming in circles: Motion of bacteria near solid boundaries
Near a solid boundary, E. coli swims in clockwise circular motion. We provide
a hydrodynamic model for this behavior. We show that circular trajectories are
natural consequences of force-free and torque-free swimming, and the
hydrodynamic interactions with the boundary, which also leads to a hydrodynamic
trapping of the cells close to the surface. We compare the results of the model
with experimental data and obtain reasonable agreement. In particular, we show
that the radius of curvature of the trajectory increases with the length of the
bacterium body.Comment: Also available at http://people.deas.harvard.edu/~lauga
Curved Tails in Polymerization-Based Bacterial Motility
The curved actin ``comet-tail'' of the bacterium Listeria monocytogenes is a
visually striking signature of actin polymerization-based motility. Similar
actin tails are associated with Shigella flexneri, spotted-fever Rickettsiae,
the Vaccinia virus, and vesicles and microspheres in related in vitro systems.
We show that the torque required to produce the curvature in the tail can arise
from randomly placed actin filaments pushing the bacterium or particle. We find
that the curvature magnitude determines the number of actively pushing
filaments, independent of viscosity and of the molecular details of force
generation. The variation of the curvature with time can be used to infer the
dynamics of actin filaments at the bacterial surface.Comment: 8 pages, 2 figures, Latex2
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Overview of mathematical approaches used to model bacterial chemotaxis II: bacterial populations
We review the application of mathematical modeling to understanding the behavior of populations of chemotactic bacteria. The application of continuum mathematical models, in particular generalized Keller–Segel models, is discussed along with attempts to incorporate the microscale (individual) behavior on the macroscale, modeling the interaction between different species of bacteria, the interaction of bacteria with their environment, and methods used to obtain experimentally verified parameter values. We allude briefly to the role of modeling pattern formation in understanding collective behavior within bacterial populations. Various aspects of each model are discussed and areas for possible future research are postulated
Quantitative Modeling of Escherichia coli Chemotactic Motion in Environments Varying in Space and Time
Escherichia coli chemotactic motion in spatiotemporally varying environments is studied by using a computational model based on a coarse-grained description of the intracellular signaling pathway dynamics. We find that the cell's chemotaxis drift velocity vd is a constant in an exponential attractant concentration gradient [L]∝exp(Gx). vd depends linearly on the exponential gradient G before it saturates when G is larger than a critical value GC. We find that GC is determined by the intracellular adaptation rate kR with a simple scaling law: . The linear dependence of vd on G = d(ln[L])/dx directly demonstrates E. coli's ability in sensing the derivative of the logarithmic attractant concentration. The existence of the limiting gradient GC and its scaling with kR are explained by the underlying intracellular adaptation dynamics and the flagellar motor response characteristics. For individual cells, we find that the overall average run length in an exponential gradient is longer than that in a homogeneous environment, which is caused by the constant kinase activity shift (decrease). The forward runs (up the gradient) are longer than the backward runs, as expected; and depending on the exact gradient, the (shorter) backward runs can be comparable to runs in a spatially homogeneous environment, consistent with previous experiments. In (spatial) ligand gradients that also vary in time, the chemotaxis motion is damped as the frequency ω of the time-varying spatial gradient becomes faster than a critical value ωc, which is controlled by the cell's chemotaxis adaptation rate kR. Finally, our model, with no adjustable parameters, agrees quantitatively with the classical capillary assay experiments where the attractant concentration changes both in space and time. Our model can thus be used to study E. coli chemotaxis behavior in arbitrary spatiotemporally varying environments. Further experiments are suggested to test some of the model predictions
Measuring the Intrapersonal Component of Psychological Empowerment: Confirmatory Factor Analysis of the Sociopolitical Control Scale
The Sociopolitical Control Scale (SPCS) is a widely used measure of the intrapersonal component of psychological empowerment. Confirmatory factor analyses (CFA) were conducted with data from two samples to test the hypothesized structure of the SPCS, the potential effects of method bias on the measure's psychometric properties, and whether a revised version of the scale (SPCS‐R) yielded improved model fit. Sample 1 included 316 randomly selected community residents of the Midwestern United States. Sample 2 included 750 community residents of the Northeastern U.S. Results indicated that method bias from the use of negatively worded items had a significant effect on the factor structure of the SPCS. CFA of the SPCS‐R, in which negatively worded items were rephrased so that all statements were positively worded, supported the measure's hypothesized two‐factor structure (i.e., leadership competence and policy control). Subscales of the SPCS‐R were found reliable and related in expected ways with measures of community involvement. Implications of the study for empowerment‐based research and practice are described, and strategies to further develop the SPCS are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/117223/1/ajcp9070.pd
Cell morphology governs directional control in swimming bacteria
The ability to rapidly detect and track nutrient gradients is key to the ecological success of motile bacteria in aquatic systems. Consequently, bacteria have evolved a number of chemotactic strategies that consist of sequences of straight runs and reorientations. Theoretically, both phases are affected by fluid drag and Brownian motion, which are themselves governed by cell geometry. Here, we experimentally explore the effect of cell length on control of swimming direction. We subjected Escherichia coli to an antibiotic to obtain motile cells of different lengths, and characterized their swimming patterns in a homogeneous medium. As cells elongated, angles between runs became smaller, forcing a change from a run-and-tumble to a run-and-stop/reverse pattern. Our results show that changes in the motility pattern of microorganisms can be induced by simple morphological variation, and raise the possibility that changes in swimming pattern may be triggered by both morphological plasticity and selection on morphology
Novel Methods for Analysing Bacterial Tracks Reveal Persistence in Rhodobacter sphaeroides
Tracking bacteria using video microscopy is a powerful experimental approach to probe their motile behaviour. The
trajectories obtained contain much information relating to the complex patterns of bacterial motility. However, methods for
the quantitative analysis of such data are limited. Most swimming bacteria move in approximately straight lines,
interspersed with random reorientation phases. It is therefore necessary to segment observed tracks into swimming and
reorientation phases to extract useful statistics. We present novel robust analysis tools to discern these two phases in tracks.
Our methods comprise a simple and effective protocol for removing spurious tracks from tracking datasets, followed by
analysis based on a two-state hidden Markov model, taking advantage of the availability of mutant strains that exhibit
swimming-only or reorientating-only motion to generate an empirical prior distribution. Using simulated tracks with varying
levels of added noise, we validate our methods and compare them with an existing heuristic method. To our knowledge this
is the first example of a systematic assessment of analysis methods in this field. The new methods are substantially more
robust to noise and introduce less systematic bias than the heuristic method. We apply our methods to tracks obtained
from the bacterial species Rhodobacter sphaeroides and Escherichia coli. Our results demonstrate that R. sphaeroides exhibits
persistence over the course of a tumbling event, which is a novel result with important implications in the study of this and
similar species
The Relation Between Students’ Perceptions of Instructional Quality and Bullying Victimization
“If You Can Dream It, You Can Achieve It.” Parent Memorable Messages as Indicators of College Student Success
This study investigated various aspects of parents’ memorable messages about college as they relate to indicators of college student success. Findings revealed that parents’ memorable messages about college focused on working (and playing) hard, the necessity of attending college, providing encouragement and support, and general advice based on parents’ own experiences. Although these message themes were not uniquely predictive of college student success, the students’ perceptions of message and sender characteristics emerged as significant predictors of cognitive learning indicators, learner empowerment, college motivation, and satisfaction with college. Theoretical and practical implications for findings are discussed
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