Bacterial quorum sensing and interference by naturally occurring biomimics

Bacteria are able to coordinate gene expression as a community through the secretion and detection of signalling molecules so that the members of the community can simultaneously express specific behaviours. This mechanism of regulation of behaviour appears to be a key trait for adaptation to specific environments and has been shown to regulate a variety of important phenotypes, from virulence factor production to biofilm formation to symbiosis related behaviours such as bioluminescence. The ability to communicate and communally regulate gene expression is hypothesised to have evolved as a way for organisms to delay expression of phenotypes until numerical supremacy is reached. For example, in the case of infection, if an invading microorganism were to express virulence factors too early, the host may be able to mount a successful defence and repel the invaders. There is growing evidence that bacterial quorum sensing (QS) systems are involved in cross-kingdom signalling with eukaryotic organisms and that eukaryotes are capable of actively responding to bacteria in their environment by detecting and acting upon the presence of these signalling molecules. Likewise, eukaryotes produce compounds that can interfere with QS systems in bacteria by acting as agonists or antagonists. An exciting new field of study, biomimetics, takes inspiration from nature’s models and attempts to design solutions to human problems, and biomimics of QS systems may be one such solution. This article presents the acylated homoserine lactone and autoinducer 2 QS systems in bacteria, the means of intercepting or interfering with bacterial QS systems evolved by eukaryotes, and the rational design of synthetic antagonists. Figure Natural products, furanones, from the red alga Delisea pulchra inhibit the quorum sensing regulated production of violacein by Chromobacterium violaceum “The proof of evolution lies in those adaptations that arise from improbable foundations”—Stephen Jay Gould     

Sliding plate rheometry of planar oriented concentrated fiber suspension

The rheology of concentrated planar fiber suspensions is investigated. A new experimental technique for fiber suspensions based on a sliding plate rheometer incorporating a shear stress transducer is developed. It is shown that this instrument works well for the tested material systems. The rheological behavior in steady shear is subsequently investigated. The results can be largely explained by a combination of frictional and hydrodynamic interaction. Despite this evidence of friction no yield stress could be detected for the investigated shear rates. It was also found that the fiber aspect ratio did not influence the steady shear viscosity.     

Study of geometry effects in torsional rheometry of fibre suspensions

This work concerns the problem of measuring the viscosity of a suspension of fibres that are not short compared to the dimensions of the measurement device. We have examined various geometry effects in shear using parallel-plate and cone-and-plate configurations. Steady state viscosity, transient viscosity and first normal stress difference have been studied. Silicone oil with milled glass fibres at 8 vol.% was used as a model fibre suspension. The conventional parallel-plate geometry exhibits a significant gap dependence of the apparent viscous response. Too small gaps constrain the fibre rotations, thus lowering the initial stress peak and delaying the development of steady state. Too large gaps lead to a loss of liquid at the sample perimeter, which reduces the apparent steady state viscosity. The steady state response, however, seems to be correct for gaps in the range 1–2 mm when the maximum fibre length is 1.5 mm. The cone-and-plate geometry is less sensitive to large gaps. Too small a gap at the (truncated) cone apex leads to anomalous response due to bridging of the gap by fibres. The use of unusually large plates (100 mm diameter) and different cone angles and truncations was explored. It was shown that cone-and-plate configurations of large diameter and large truncation can effectively decrease the geometry disturbances both due to fibre bridging at the cone apex and loss of liquid at the edge.     

Structure of a potassium-ion-exchanged nepheline hydrate I crystal

A nepheline hydrate I crystal, ion-exchanged using KCl(aq) at 80°C, was found to be orthorhombic, space group Pnm2₁, a = 8.113(3), b = 15.223(2), c = 5.1817(7)Å and showed no superlattice X-ray reflections. Structure analysis by means of Fourier and least-squares methods led to the composition K_1.1Na_1.9Al₃Si₃O₁₂ · H₂O (Z = 2, D_c = 2.40 g cm^?3) and the agreement factors: R = 0.032 and R_w = 0.040. Species assigned to the observed extra framework sites were K(1), K(2), and W(1) in the 8-ring tunnels along c plus Na(1) and Na(2) in the smaller 6-ring voids forming connections in the b direction. The atoms Na(1) and Na(2) coordinated framework oxygens exclusively and were but little affected by the ion-exchange process; K(1) was found near the center of an 8-ring and had five O atoms and two water molecules as closest neighbors, while the weakly occupied K(2) site was near a 6-ring and was found to have a coordination consisting of at least five oxygens and one water. The 10% vacancy of Na(2) is compensated for by an equal amount of K(2), which does not enter the Na(2) site for sterical reasons.     

A general gel layer model for the transport of colloids and macroions in dilute solution

A general boundary element methodology for studying the dilute solution transport of rigid macroions that contain gel layers on their outer surfaces is developed and applied to several model systems. The methodology can be applied to particles of arbitrary size, shape, charge distribution, and gel layer geometry. Account is also taken of the steady state distortion of the ion atmosphere from equilibrium, which makes it applicable to the transport of highly charged structures. The coupled field equations (Poisson, ion-transport, low-Reynolds-number Navier-Stokes, and Brinkman) are solved numerically and from this, transport properties (diffusion constants, electrophoretic mobilities, excess viscosities) can be computed. In the present work, the methodology is first applied to a gel sphere model over a wide range of particle charge and the resulting transport properties are found to be in excellent agreement with independent theory under those conditions where independent theory is available. It is then applied to several prolate spheroidal models of a particular silica sol sample in an attempt to identify possible solution structures. A single model, that is able to account simultaneously for all of the transport behavior, which does not undergo significant conformational change with salt concentration, could not be found. A model with a thin (相似文献   

Substructure of a rubidium-ion-exchanged form of the silicate nepheline hydrate I

X-Ray structure analysis of a nepheline hydrate I crystal, Rb⁺-exchanged at 80°C, was performed making use only of the main diffractions. The resulting substructure was found to be orthorhombic with a = 8.0802(8), b = 15.259(2), c = 5.1584(5) Å, V = 636.0ų, space group Pnm2₁. Fourier and least-squares techniques gave the residuals R = 0.048 and R_w = 0.058, and a tentative formula of RbNa₂Al₃Si₃O₁₂ · H₂O (Z = 2, D_c = 2.65 g cm^?3). Tetrahedral distances were consistent with Al,Si alternation in the framework. Of the channel species, Na(1) and Na(2) were found not to be exchangeable at the current temperature. These sodium atoms are located in the small cages, formed by 6-rings of O atoms which connect the 8-ring channels parallel to c into two-dimensional pore systems. In the larger tunnels the replacement was complete and these contain a Rb⁺ ion and probably a water molecule in symmetry-related positions. According to this model, Rb⁺ coordinates four oxygens of an 8-ring and two water molecules, with RbO distances in the range 2.81–3.36 Å. Additional O atoms are found at greater distances.