A metallic phase in quantum Hall systems

The electronic eigenstates of a quantum Hall (QH) system are chiral states. Strong inter-Landau-band mixings among these states can occur when the bandwidth is comparable to the spacing of two adjacent Landau bands. We show that mixing of localized states with opposite chirality can delocalize electronic states. Based on numerical results, we propose the existence of a metallic phase between two adjacent QH phases and between a QH phase and the insulating phase. This result is consistent with non-scaling behaviors observed in recent experiments on quantum-Hall-liquid-to-insulator transition.Comment: 5 pages, 3 figures. Will be published in Phys. Rev. Let

A Putative P-Type ATPase Required for Virulence and Resistance to Haem Toxicity in Listeria monocytogenes

Regulation of iron homeostasis in many pathogens is principally mediated by the ferric uptake regulator, Fur. Since acquisition of iron from the host is essential for the intracellular pathogen Listeria monocytogenes, we predicted the existence of Fur-regulated systems that support infection. We examined the contribution of nine Fur-regulated loci to the pathogenicity of L. monocytogenes in a murine model of infection. While mutating the majority of the genes failed to affect virulence, three mutants exhibited a significantly compromised virulence potential. Most striking was the role of the membrane protein we designate FrvA (Fur regulated virulence factor A; encoded by frvA [lmo0641]), which is absolutely required for the systemic phase of infection in mice and also for virulence in an alternative infection model, the Wax Moth Galleria mellonella. Further analysis of the ΔfrvA mutant revealed poor growth in iron deficient media and inhibition of growth by micromolar concentrations of haem or haemoglobin, a phenotype which may contribute to the attenuated growth of this mutant during infection. Uptake studies indicated that the ΔfrvA mutant is unaffected in the uptake of ferric citrate but demonstrates a significant increase in uptake of haem and haemin. The data suggest a potential role for FrvA as a haem exporter that functions, at least in part, to protect the cell against the potential toxicity of free haem

Identification of N-acyl-l-homoserine lactones produced by non-pigmented Chromobacterium aquaticum CC-SEYA-1T and pigmented Chromobacterium subtsugae PRAA4-1T

Many members of the genus Chromobacterium produce violacein, a characteristic purple pigment which is induced by small diffusible N-acyl homoserine lactones (AHL) quorum-sensing molecules. In this study, the production of AHL of the non-pigmented C. aquaticum CC-SEYA-1T and the pigmented C. subtsugae PRAA4-1T were determined by using a CV026 biosensor assay. The profile of AHL was identified from the extracts of stationary phase cultures using gas chromatography–mass spectroscopy (GC–MS) and thin layer chromatography (TLC). CV026 biosensor assay revealed that both the non-pigmented C. aquaticum CC-SEYA-1T and the pigmented C. subtsugae PRAA4-1T produced AHL molecules, which were identified, respectively, as N-octanoyl homoserine lactone (OHL) [also known as C-8 homoserine lactone (C8-HSL)] and N-hexanoyl homoserine lactone (HHL) [also known as C-6 homoserine lactone (C6-HSL)]. The pigment produced by C. subtsugae PRAA4-1T was similar to that of Chromobacterium violaceum ATCC12472T but no characteristic visible spectral peaks of the pigment were observed in the extracts of C. aquaticum CC-SEYA-1T. In addition, C. aquaticum CC-SEYA-1T and C. subtsugae PRAA4-1T showed hemolytic activities

Activation of heme biosynthesis by a small molecule that is toxic to fermenting Staphylococcus aureus

Staphylococcus aureus is a significant infectious threat to global public health. Acquisition or synthesis of heme is required for S. aureus to capture energy through respiration, but an excess of this critical cofactor is toxic to bacteria. S. aureus employs the heme sensor system (HssRS) to overcome heme toxicity; however, the mechanism of heme sensing is not defined. Here, we describe the identification of a small molecule activator of HssRS that induces endogenous heme biosynthesis by perturbing central metabolism. This molecule is toxic to fermenting S. aureus, including clinically relevant small colony variants. The utility of targeting fermenting bacteria is exemplified by the fact that this compound prevents the emergence of antibiotic resistance, enhances phagocyte killing, and reduces S. aureus pathogenesis. Not only is this small molecule a powerful tool for studying bacterial heme biosynthesis and central metabolism; it also establishes targeting of fermentation as a viable antibacterial strategy

Methodology for Designing a Sodium-Cooled Fast Reactor with Inherent Safety

International audienceSodium-cooled fast reactors (SFRs) remain the favoritecandidate in France for a Generation IV (Gen IV)reactor fleet to be deployed within this century. Comparedwith earlier generations (Phénix, Superphénix, andEuropean fast reactor), Gen IV SFRs require attractiveeconomics together with enhanced safety and nonproliferationcriteria. An innovative approach named MathematicalEstimation of Transients for Reactor designOrientation (METRO) has been developed with the objectiveof taking into account both SFR core economicperformance and SFR transient incident behavior at anearly stage of the core design process. Loss-of-flow, lossof-heat-sink, and overpower transients are evaluated. Simplifiedmodeling of transients has been developed andbenchmarked against reference calculations with satisfactoryresults. The METRO approach to assessing theefficiency of design orientations is described in the followingand applied to a carbide-fueled reactor core

DESIGN OPTIMIZATION OF THE HOLOS-QUAD MICRO-REACTOR CONCEPT

The Holos-Quad micro-reactor concept is proposed by HolosGen LLC to generate 22 MWt with a lifetime of approximately 8 effective full power years (EFPYs) for civilian applications. The design is based on a very innovative high-temperature gas-cooled reactor concept using four Subcritical Power Modules (SPMs) that fit into one commercial 40-foot transport ISO container. A rigorous design approach was employed in order to ensure that all input parameters were fully investigated and the best solutions possible were considered. The first step of this approach consisted of properly defining the Holos-Quad design problem by identifying the design objectives, the operational constraints, and the input parameters. In the second step, a sensitivity analysis was performed to enable a preliminary investigation of the input parameters to identify the correlations among input and output variables. Finally, the design optimization was performed in the third step, employing a genetic algorithm to effectively explore the highly constrained input parameters and find global optimal solutions. The multi-objective optimization identified various core solutions that would be optimum solutions for different types of applications. For applications where economics matters less and the ease of transportation matters more, a core weight of ~15 tons could achieve a lifetime of ~3.5 EFPYs. For applications where economics matters more and the ease of transportation matters less, a lifetime of 8.3 EFPYs could be achieved with a total core weight of ~26.7 tons