123 research outputs found
Phase separation and rotor self-assembly in active particle suspensions
Adding a non-adsorbing polymer to passive colloids induces an attraction
between the particles via the `depletion' mechanism. High enough polymer
concentrations lead to phase separation. We combine experiments, theory and
simulations to demonstrate that using active colloids (such as motile bacteria)
dramatically changes the physics of such mixtures. First, significantly
stronger inter-particle attraction is needed to cause phase separation.
Secondly, the finite size aggregates formed at lower inter-particle attraction
show unidirectional rotation. These micro-rotors demonstrate the self assembly
of functional structures using active particles. The angular speed of the
rotating clusters scales approximately as the inverse of their size, which may
be understood theoretically by assuming that the torques exerted by the
outermost bacteria in a cluster add up randomly. Our simulations suggest that
both the suppression of phase separation and the self assembly of rotors are
generic features of aggregating swimmers, and should therefore occur in a
variety of biological and synthetic active particle systems.Comment: Main text: 6 pages, 5 figures. Supplementary information: 5 pages, 4
figures. Supplementary movies available from
httP://www.pnas.org/lookup/suppl/doi:10.1073/pnas.1116334109/-/DCSupplementa
When are active Brownian particles and run-and-tumble particles equivalent? Consequences for motility-induced phase separation
Active Brownian particles (ABPs, such as self-phoretic colloids) swim at
fixed speed along a body-axis that rotates by slow angular
diffusion. Run-and-tumble particles (RTPs, such as motile bacteria) swim with
constant \u until a random tumble event suddenly decorrelates the
orientation. We show that when the motility parameters depend on density
but not on , the coarse-grained fluctuating hydrodynamics of
interacting ABPs and RTPs can be mapped onto each other and are thus strictly
equivalent. In both cases, a steeply enough decreasing causes phase
separation in dimensions , even when no attractive forces act between
the particles. This points to a generic role for motility-induced phase
separation in active matter. However, we show that the ABP/RTP equivalence does
not automatically extend to the more general case of \u-dependent motilities
Differential Dynamic Microscopy of Bacterial Motility
We demonstrate 'differential dynamic microscopy' (DDM) for the fast, high
throughput characterization of the dynamics of active particles. Specifically,
we characterize the swimming speed distribution and the fraction of motile
cells in suspensions of Escherichia coli bacteria. By averaging over ~10^4
cells, our results are highly accurate compared to conventional tracking. The
diffusivity of non-motile cells is enhanced by an amount proportional to the
concentration of motile cells.Comment: 4 pages, 4 figures. In this updated version we have added simulations
to support our interpretation, and changed the model for the swimming speed
probability distribution from log-normal to a Schulz distribution. Neither
modification significantly changes our conclusion
A new method for post-translationally labeling proteins in live cells for fluorescence imaging and tracking
This work was funded by the Raymond and Beverley Sackler Institute for Biological, Physical, and Engineering sciences [to L.R.]; the National Institute of Health [Grant nos. GM118528 and CA209992 to M. H. and L. R.]; the Medical Research Council [Grant no. MR/K001485 to U.S.L. and J. M. E.]; a Leverhulme Trust Visiting Professorship [to L. R.]; and Royal Society of Edinburgh [Caledonian Scholarship to O.K.M.].We present a novel method to fluorescently label proteins, post-translationally, within live Saccharomycescerevisiae. The premise underlying this work is that fluorescent protein (FP) tags are less disruptive to normal processing and function when they are attached post-translationally, because target proteins are allowed to fold properly and reach their final subcellular location before being labeled. We accomplish this post-translational labeling by expressing the target protein fused to a short peptide tag (SpyTag), which is then covalently labeled in situ by controlled expression of an open isopeptide domain (SpyoIPD, a more stable derivative of the SpyCatcher protein) fused to an FP. The formation of a covalent bond between SpyTag and SpyoIPD attaches the FP to the target protein. We demonstrate the general applicability of this strategy by labeling several yeast proteins. Importantly, we show that labeling the membrane protein Pma1 in this manner avoids the mislocalization and growth impairment that occur when Pma1 is genetically fused to an FP. We also demonstrate that this strategy enables a novel approach to spatiotemporal tracking in single cells and we develop a Bayesian analysis to determine the protein’s turnover time from such data.PostprintPeer reviewe
Filling an Emulsion Drop with Motile Bacteria
We have measured the spatial distribution of motile Escherichia coli inside
spherical water droplets emulsified in oil. At low cell concentrations, the
cell density peaks at the water-oil interface; at increasing concentration, the
bulk of each droplet fills up uniformly while the surface peak remains.
Simulations and theory show that the bulk density results from a `traffic' of
cells leaving the surface layer, increasingly due to cell-cell scattering as
the surface coverage rises above . Our findings show similarities
with the physics of a rarefied gas in a spherical cavity with attractive walls.Comment: 5 pages, 4 figures, Supporting Information (5 pages, 5 figures
The Role of the Mucus Barrier in Digestion
Mucus forms a protective layer across a variety of epithelial surfaces. In the gastrointestinal (GI) tract, the barrier has to permit the uptake of nutrients, while excluding potential hazards, such as pathogenic bacteria. In this short review article, we look at recent literature on the structure, location, and properties of the mammalian intestinal secreted mucins and the mucus layer they form over a wide range of length scales. In particular, we look at the structure of the gel-forming glycoprotein MUC2, the primary intestinal secreted mucin, and the influence this has on the properties of the mucus layer. We show that, even at the level of the protein backbone, MUC2 is highly heterogeneous and that this is reflected in the networks it forms. It is evident that a combination of charge and pore size determines what can diffuse through the layer to the underlying gut epithelium. This information is important for the targeted delivery of bioactive molecules, including nutrients and pharmaceuticals, and for understanding how GI health is maintained
Oscillatory surface rheotaxis of swimming E. coli bacteria
Bacterial contamination of biological conducts, catheters or water resources
is a major threat to public health and can be amplified by the ability of
bacteria to swim upstream. The mechanisms of this rheotaxis, the reorientation
with respect to flow gradients, often in complex and confined environments, are
still poorly understood. Here, we follow individual E. coli bacteria swimming
at surfaces under shear flow with two complementary experimental assays, based
on 3D Lagrangian tracking and fluorescent flagellar labelling and we develop a
theoretical model for their rheotactic motion. Three transitions are identified
with increasing shear rate: Above a first critical shear rate, bacteria shift
to swimming upstream. After a second threshold, we report the discovery of an
oscillatory rheotaxis. Beyond a third transition, we further observe
coexistence of rheotaxis along the positive and negative vorticity directions.
A full theoretical analysis explains these regimes and predicts the
corresponding critical shear rates. The predicted transitions as well as the
oscillation dynamics are in good agreement with experimental observations. Our
results shed new light on bacterial transport and reveal new strategies for
contamination prevention.Comment: 12 pages, 5 figure
Characterization of transcription within sdr region of Staphylococcus aureus
Staphylococcus aureus is an opportunistic pathogen responsible for various infections in humans and animals. It causes localized and systemic infections, such as abscesses, impetigo, cellulitis, sepsis, endocarditis, bone infections, and meningitis. S. aureus virulence factors responsible for the initial contact with host cells (MSCRAMMs—microbial surface components recognizing adhesive matrix molecules) include three Sdr proteins. The presence of particular sdr genes is correlated with putative tissue specificity. The transcriptional organization of the sdr region remains unclear. We tested expression of the sdrC, sdrD, or sdrE genes in various in vitro conditions, as well as after contact with human blood. In this work, we present data suggesting a separation of the sdr region into three transcriptional units, based on their differential reactions to the environment. Differential reaction of the sdrD transcript to environmental conditions and blood suggests dissimilar functions of the sdr genes. SdrE has been previously proposed to play role in bone infections, whilst our results can indicate that sdrD plays a role in the interactions between the pathogen and human immune system, serum or specifically reacts to nutrients/other factors present in human blood
Painting with light-powered bacteria
External control of the swimming speed of `active particles' can be used to
self assemble designer structures in situ on the micrometer to millimeter
scale. We demonstrate such reconfigurable templated active self assembly in a
fluid environment using light powered strains of Escherichia coli. The physics
and biology controlling the sharpness and formation speed of patterns is
investigated using a bespoke fast-responding strain.Comment: 19 pages, 11 figure
IGD Motifs, Which Are Required for Migration Stimulatory Activity of Fibronectin Type I Modules, Do Not Mediate Binding in Matrix Assembly
Picomolar concentrations of proteins comprising only the N-terminal 70-kDa region (70K) of fibronectin (FN) stimulate cell migration into collagen gels. The Ile-Gly-Asp (IGD) motifs in four of the nine FN type 1 (FNI) modules in 70K are important for such migratory stimulating activity. The 70K region mediates binding of nanomolar concentrations of intact FN to cell-surface sites where FN is assembled. Using baculovirus, we expressed wildtype 70K and 70K with Ile-to-Ala mutations in 3FNI and 5FNI; 7FNI and 9FNI; or 3FNI, 5FNI, 7FNI, and 9FNI. Wildtype 70K and 70K with Ile-to-Ala mutations were equally active in binding to assembly sites of FN-null fibroblasts. This finding indicates that IGD motifs do not mediate the interaction between 70K and the cell-surface that is important for FN assembly. Further, FN fragment N-3FNIII, which does not stimulate migration, binds to assembly sites on FN-null fibroblast. The Ile-to-Ala mutations had effects on the structure of FNI modules as evidenced by decreases in abilities of 70K with Ile-to-Ala mutations to bind to monoclonal antibody 5C3, which recognizes an epitope in 9FNI, or to bind to FUD, a polypeptide based on the F1 adhesin of Streptococcus pyogenes that interacts with 70K by the β-zipper mechanism. These results suggest that the picomolar interactions of 70K with cells that stimulate cell migration require different conformations of FNI modules than the nanomolar interactions required for assembly
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