28 research outputs found
Enhanced diffusion due to active swimmers at a solid surface
We consider two systems of active swimmers moving close to a solid surface,
one being a living population of wild-type \textit{E. coli} and the other being
an assembly of self-propelled Au-Pt rods. In both situations, we have
identified two different types of motion at the surface and evaluated the
fraction of the population that displayed ballistic trajectories (active
swimmers) with respect to those showing random-like behavior. We studied the
effect of this complex swimming activity on the diffusivity of passive tracers
also present at the surface. We found that the tracer diffusivity is enhanced
with respect to standard Brownian motion and increases linearly with the
activity of the fluid, defined as the product of the fraction of active
swimmers and their mean velocity. This result can be understood in terms of
series of elementary encounters between the active swimmers and the tracers.Comment: 4 pages, 2 figures in color, Physical Review Letters (in production
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
Reversible and Irreversible Adhesion of Motile Escherichia coli Cells Analyzed by Total Internal Reflection Aqueous Fluorescence Microscopy
The initial events in bacterial adhesion are often explained as resulting from electrostatic and van der Waals forces between the cell and the surface, as described by DLVO theory (developed by Derjaguin, Landau, Verwey, and Overbeek). Such a theory predicts that negatively charged bacteria will experience greater attraction toward a negatively charged surface as the ionic strength of the medium is increased. In the present study we observed both smooth-swimming and nonmotile Escherichia coli bacteria close to plain, positively, and hydrophobically coated quartz surfaces in high- and low-ionic-strength media by using total internal reflection aqueous fluorescence microscopy. We found that reversibly adhering cells (cells which continue to swim along the surface for extended periods) are too distant from the surface for this behavior to be explained by DLVO-type forces. However, cells which had become immobilized on the surface did seem to be affected by electrostatic interactions. We propose that the “force” holding swimming cells near the surface is actually the result of a hydrodynamic effect, causing the cells to swim at an angle along the glass, and that DLVO-type forces are responsible only for the observed immobilization of irreversibly adhering cells. We explain our observations within the context of a conceptual model in which bacteria that are interacting with the surface may be thought of as occupying one of three compartments: bulk fluid, near-surface bulk, and near-surface constrained. A cell in these compartments feels either no effect of the surface, only the hydrodynamic effect of the surface, or both the hydrodynamic and the physicochemical effects of the surface, respectively