202 research outputs found
Nonadditivity of Polymeric and Charged Surface Interactions: Consequences for Doped Lamellar Phases
We explore theoretically the modifications to the interactions between
charged surfaces across an ionic solution caused by the presence of dielectric
polymers. Although the chains are neutral, the polymer physics and the
electrostatics are coupled; the intra-surface electric fields polarise any low
permittivity species (e.g., polymer) dissolved in a high permittivity solvent
(e.g., water). This coupling enhances the polymer depletion from the surfaces
and increases the screening of electrostatic interactions with respect to a
model which treats polymeric and electrostatic effects as independent. As a
result, the range of the ionic contribution to the osmotic interaction between
surfaces is decreased, while that of the polymeric contribution is increased.
These changes modify the total interaction in a nonadditive manner. Building on
the results for parallel surfaces, we investigate the effect of this coupling
on the phase behaviour of polymer-doped smectics.Comment: 29 pages, 11 figures, v2: minor corrections added, published version
available at http://dx.doi.org/10.1021/la050173
Sheared bioconvection in a horizontal tube
The recent interest in using microorganisms for biofuels is motivation enough
to study bioconvection and cell dispersion in tubes subject to imposed flow. To
optimize light and nutrient uptake, many microorganisms swim in directions
biased by environmental cues (e.g. phototaxis in algae and chemotaxis in
bacteria). Such taxes inevitably lead to accumulations of cells, which, as many
microorganisms have a density different to the fluid, can induce hydrodynamic
instabilites. The large-scale fluid flow and spectacular patterns that arise
are termed bioconvection. However, the extent to which bioconvection is
affected or suppressed by an imposed fluid flow, and how bioconvection
influences the mean flow profile and cell transport are open questions. This
experimental study is the first to address these issues by quantifying the
patterns due to suspensions of the gravitactic and gyrotactic green
biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow.
With no flow, the dependence of the dominant pattern wavelength at pattern
onset on cell concentration is established for three different tube diameters.
For small imposed flows, the vertical plumes of cells are observed merely to
bow in the direction of flow. For sufficiently high flow rates, the plumes
progressively fragment into piecewise linear diagonal plumes, unexpectedly
inclined at constant angles and translating at fixed speeds. The pattern
wavelength generally grows with flow rate, with transitions at critical rates
that depend on concentration. Even at high imposed flow rates, bioconvection is
not wholly suppressed and perturbs the flow field.Comment: 19 pages, 9 figures, published version available at
http://iopscience.iop.org/1478-3975/7/4/04600
The Antiviral Action of Interferon Is Potentiated by Removal of the Conserved IRTAM Domain of the IFNAR1 Chain of the Interferon α/β Receptor: Effects on JAK-STAT Activation and Receptor Down-regulation
The first cloned chain (IFNAR1) of the human interferon-α (IFNα) receptor acts as a species-specific transducer for type I IFN action when transfected into heterologous mouse cells. Stably transfected mouse L929 cell lines expressing truncation mutants of the intracellular domain of the human IFNAR1 chain were tested for biological responses to human IFNα. Deletion of the intracellular domain resulted in a complete loss of sensitivity to the biological activity of human IFN but markedly increased IFNAR1 cell surface expression, demonstrating that the intracellular domain is required for biological function and contains a domain that negatively regulates its cell surface expression. Removal of the conserved membrane distal 16-amino-acid IRTAM (InterferonReceptorTyrosineActivationMotif) sequence: (1) increased sensitivity to IFNα's antiviral activity, (2) increased the rapid IFNα-dependent formation of STAT-containing DNA-binding complexes, (3) prolonged tyrosine phosphorylation kinetics of the JAK-STAT pathway, and (4) blocked the IFN-dependent down-regulation of the IFNAR1 chain. These results indicate that the IRTAM negatively regulates signaling events required for the induction of IFN's biological actions via regulating receptor down-regulation
Microbial mutualism at a distance: The role of geometry in diffusive exchanges
The exchange of diffusive metabolites is known to control the spatial patterns formed by microbial populations, as revealed by recent studies in the laboratory. However, the matrices used, such as agarose pads, lack the structured geometry of many natural microbial habitats, including in the soil or on the surfaces of plants or animals. Here we address the important question of how such geometry may control diffusive exchanges and microbial interaction. We model mathematically mutualistic interactions within a minimal unit of structure: two growing reservoirs linked by a diffusive channel through which metabolites are exchanged. The model is applied to study a synthetic mutualism, experimentally parametrized on a model algal-bacterial co-culture. Analytical and numerical solutions of the model predict conditions for the successful establishment of remote mutualisms, and how this depends, often counterintuitively, on diffusion geometry. We connect our findings to understanding complex behavior in synthetic and naturally occurring microbial communities.Gates Cambridge Trust
The Winton Foundation for the Physics of Sustainability
The Royal Society
The Schlumberger Chair Fun
Migration of chemotactic bacteria in soft agar: role of gel concentration
We study the migration of chemotactic wild-type Escherichia coli populations
in semisolid (soft) agar in the concentration range C = 0.15-0.5% (w/v). For C
< 0.35%, expanding bacterial colonies display characteristic chemotactic rings.
At C = 0.35%, however, bacteria migrate as broad circular bands rather than
sharp rings. These are growth/diffusion waves arising because of suppression of
chemotaxis by the agar and have not been previously reported experimentally to
our knowledge. For C = 0.4-0.5%, expanding colonies do not span the depth of
the agar and develop pronounced front instabilities. The migration front speed
is weakly dependent on agar concentration at C < 0.25%, but decreases sharply
above this value. We discuss these observations in terms of an extended
Keller-Segel model for which we derived novel transport parameter expressions
accounting for perturbations of the chemotactic response by collisions with the
agar. The model makes it possible to fit the observed front speed decay in the
range C = 0.15-0.35%, and its solutions qualitatively reproduce the observed
transition from chemotactic to growth/diffusion bands. We discuss the
implications of our results for the study of bacteria in porous media and for
the design of improved bacteriological chemotaxis assays.Comment: 28 pages, 5 figures. Published online at
http://www.sciencedirect.com/science/article/pii/S000634951100721
Transcriptional dysregulation of Interferome in experimental and human Multiple Sclerosis
Recent evidence indicates that single multiple sclerosis (MS) susceptibility genes involved in interferon (IFN) signaling display altered transcript levels in peripheral blood of untreated MS subjects, suggesting that responsiveness to endogenous IFN is dysregulated during neuroinflammation. To prove this hypothesis we exploited the systematic collection of IFN regulated genes (IRG) provided by the Interferome database and mapped Interferome changes in experimental and human MS. Indeed, central nervous system tissue and encephalitogenic CD4 T cells during experimental autoimmune encephalomyelitis were characterized by massive changes in Interferome transcription. Further, the analysis of almost 500 human blood transcriptomes showed that (i) several IRG changed expression at distinct MS stages with a core of 21 transcripts concordantly dysregulated in all MS forms compared with healthy subjects; (ii) 100 differentially expressed IRG were validated in independent case-control cohorts; and (iii) 53 out of 100 dysregulated IRG were targeted by IFN-beta treatment in vivo. Finally, ex vivo and in vitro experiments established that IFN-beta administration modulated expression of two IRG, ARRB1 and CHP1, in immune cells. Our study confirms the impairment of Interferome in experimental and human MS, and describes IRG signatures at distinct disease stages which can represent novel therapeutic targets in MS
A Disease-Mediated Trophic Cascade in the Serengeti and its Implications for Ecosystem C
The removal of rinderpest had cascading effects on herbivore populations, fire, tree density, and even ecosystem carbon in the Serengeti ecosystem of East Africa
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