109 research outputs found
Shearing or Compressing a Soft Glass in 2D: Time-concentration superposition
We report surface shear rheological measurements on dense insoluble
monolayers of micron sized colloidal spheres at the oil/water interface and of
the protein -lactoglobulin at the air/water surface. As expected, the
elastic modulus shows a changing character in the response, from a viscous
liquid towards an elastic solid as the concentration is increased, and a change
from elastic to viscous as the shear frequency is increased. Surprisingly,
above a critical packing fraction, the complex elastic modulus curves measured
at different concentrations can be superposed to form a master curve, by
rescaling the frequency and the magnitude of the modulus. This provides a
powerful tool for the extrapolation of the material response function outside
the experimentally accessible frequency range. The results are discussed in
relation to recent experiments on bulk systems, and indicate that these two
dimensional monolayers should be regarded as being close to a soft glass state.Comment: to appear in PR
Positive selection at high temperature reduces gene transcription in the bacteriophage ϕX174
<p>Abstract</p> <p>Background</p> <p>Gene regulation plays a central role in the adaptation of organisms to their environments. There are many molecular components to gene regulation, and it is often difficult to determine both the genetic basis of adaptation and the evolutionary forces that influence regulation. In multiple evolution experiments with the bacteriophage ϕX174, adaptive substitutions in <it>cis</it>-acting regulatory sequences sweep through the phage population as the result of strong positive selection at high temperatures that are non-permissive for laboratory-adapted phage. For one <it>cis</it>-regulatory region, we investigate the individual effects of four adaptive substitutions on transcript levels and fitness for phage growing on three hosts at two temperatures.</p> <p>Results</p> <p>The effect of the four individual substitutions on transcript levels is to down-regulate gene expression, regardless of temperature or host. To ascertain the conditions under which these substitutions are adaptive, fitness was measured by a variety of methods for several bacterial hosts growing at two temperatures, the control temperature of 37°C and the selective temperature of 42°C. Time to lysis and doublings per hour indicate that the four substitutions individually improve fitness over the ancestral strain at high temperature independent of the bacterial host in which the fitness was measured. Competition assays between the ancestral strain and either of two mutant strains indicate that both mutants out-compete the ancestor at high temperature, but the relative frequencies of each phage remain the same at the control temperature.</p> <p>Conclusions</p> <p>Our results strongly suggest that gene transcription plays an important role in influencing fitness in the bacteriophage ϕX174, and different point mutations in a single <it>cis</it>-regulatory region provided the genetic basis for this role in adaptation to high temperature. We speculate that the adaptive nature of these substitutions is due to the physiology of the host at high temperature or the need to maintain particular ratios of phage proteins during capsid assembly. Our investigation of regulatory evolution contributes to interpreting genome-level assessments of regulatory variation, as well as to understanding the molecular basis of adaptation.</p
Far-field approximation for hydrodynamic interactions in parallel-wall geometry
A complete analysis is presented for the far-field creeping flow produced by
a multipolar force distribution in a fluid confined between two parallel planar
walls. We show that at distances larger than several wall separations the flow
field assumes the Hele-Shaw form, i.e., it is parallel to the walls and varies
quadratically in the transverse direction. The associated pressure field is a
two-dimensional harmonic function that is characterized by the same multipolar
number m as the original force multipole. Using these results we derive
asymptotic expressions for the Green's matrix that represents Stokes flow in
the wall-bounded fluid in terms of a multipolar spherical basis. This Green's
matrix plays a central role in our recently proposed algorithm [Physica A xx,
{\bf xxx} (2005)] for evaluating many-body hydrodynamic interactions in a
suspension of spherical particles in the parallel-wall geometry. Implementation
of our asymptotic expressions in this algorithm increases its efficiency
substantially because the numerically expensive evaluation of the exact matrix
elements is needed only for the neighboring particles. Our asymptotic analysis
will also be useful in developing hydrodynamic algorithms for wall-bounded
periodic systems and implementing acceleration methods by using corresponding
results for the two-dimensional scalar potential.Comment: 28 pages 5 figure
Direct measurements of the effects of salt and surfactant on interaction forces between colloidal particles at water-oil interfaces
The forces between colloidal particles at a decane-water interface, in the
presence of low concentrations of a monovalent salt (NaCl) and of the
surfactant sodium dodecylsulfate (SDS) in the aqueous subphase, have been
studied using laser tweezers. In the absence of electrolyte and surfactant,
particle interactions exhibit a long-range repulsion, yet the variation of the
interaction for different particle pairs is found to be considerable. Averaging
over several particle pairs was hence found to be necessary to obtain reliable
assessment of the effects of salt and surfactant. It has previously been
suggested that the repulsion is consistent with electrostatic interactions
between a small number of dissociated charges in the oil phase, leading to a
decay with distance to the power -4 and an absence of any effect of electrolyte
concentration. However, the present work demonstrates that increasing the
electrolyte concentration does yield, on average, a reduction of the magnitude
of the interaction force with electrolyte concentration. This implies that
charges on the water side also contribute significantly to the electrostatic
interactions. An increase in the concentration of SDS leads to a similar
decrease of the interaction force. Moreover the repulsion at fixed SDS
concentrations decreases over longer times. Finally, measurements of three-body
interactions provide insight into the anisotropic nature of the interactions.
The unique time-dependent and anisotropic interactions between particles at the
oil-water interface allow tailoring of the aggregation kinetics and structure
of the suspension structure.Comment: Submitted to Langmui
O manuscrito e o iconográfico em cartões-postais belicosos: da apologia cavalheiresca à contestação da Grande Guerra (1914-1918) na França
O trabalho analisa mensagens transmitidas por cartões-postais produzidos e circulados na França no contexto da Primeira Guerra Mundial (1914-1918), apresentando temática associada ao conflito. O objetivo é contrapor as mensagens iconográficas e textuais neles impressas à quelas que foram manuscritas por seus remetentes, de modo a evidenciar formas de expressão e percepções do conflito, conforme empregadas por civis e militares, em diferentes momentos de seu desenvolvimento.The paper analyzes messages conveyed by postcards produced and circulated in France during the First World War (1914-1918), with themes referring to the conflict. The intention is to compare the iconographic and textual messages printed with handwritten messages, to display forms of expression and perception of war, used by civilian and military on different occasions
The influence of cultivation methods on Shewanella oneidensis physiology and proteome expression
High-throughput analyses that are central to microbial systems biology and ecophysiology research benefit from highly homogeneous and physiologically well-defined cell cultures. While attention has focused on the technical variation associated with high-throughput technologies, biological variation introduced as a function of cell cultivation methods has been largely overlooked. This study evaluated the impact of cultivation methods, controlled batch or continuous culture in bioreactors versus shake flasks, on the reproducibility of global proteome measurements in Shewanellaoneidensis MR-1. Variability in dissolved oxygen concentration and consumption rate, metabolite profiles, and proteome was greater in shake flask than controlled batch or chemostat cultures. Proteins indicative of suboxic and anaerobic growth (e.g., fumarate reductase and decaheme c-type cytochromes) were more abundant in cells from shake flasks compared to bioreactor cultures, a finding consistent with data demonstrating that “aerobic” flask cultures were O2 deficient due to poor mass transfer kinetics. The work described herein establishes the necessity of controlled cultivation for ensuring highly reproducible and homogenous microbial cultures. By decreasing cell to cell variability, higher quality samples will allow for the interpretive accuracy necessary for drawing conclusions relevant to microbial systems biology research
Proteomic Analysis of Growth Phase-Dependent Expression of Legionella pneumophila Proteins Which Involves Regulation of Bacterial Virulence Traits
Legionella pneumophila, which is a causative pathogen of Legionnaires' disease, expresses its virulent traits in response to growth conditions. In particular, it is known to become virulent at a post-exponential phase in vitro culture. In this study, we performed a proteomic analysis of differences in expression between the exponential phase and post-exponential phase to identify candidates associated with L. pneumophila virulence using 2-Dimentional Fluorescence Difference Gel Electrophoresis (2D-DIGE) combined with Matrix-Assisted Laser Desorption/Ionization–Mass Spectrometry (MALDI-TOF-MS). Of 68 identified proteins that significantly differed in expression between the two growth phases, 64 were up-regulated at a post-exponential phase. The up-regulated proteins included enzymes related to glycolysis, ketone body biogenesis and poly-3-hydroxybutyrate (PHB) biogenesis, suggesting that L. pneumophila may utilize sugars and lipids as energy sources, when amino acids become scarce. Proteins related to motility (flagella components and twitching motility-associated proteins) were also up-regulated, predicting that they enhance infectivity of the bacteria in host cells under certain conditions. Furthermore, 9 up-regulated proteins of unknown function were found. Two of them were identified as novel bacterial factors associated with hemolysis of sheep red blood cells (SRBCs). Another 2 were found to be translocated into macrophages via the Icm/Dot type IV secretion apparatus as effector candidates in a reporter assay with Bordetella pertussis adenylate cyclase. The study will be helpful for virulent analysis of L. pneumophila from the viewpoint of physiological or metabolic modulation dependent on growth phase
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