Genome Sequence of a Lancefield Group C Streptococcus zooepidemicus Strain Causing Epidemic Nephritis: New Information about an Old Disease

Outbreaks of disease attributable to human error or natural causes can provide unique opportunities to gain new information about host-pathogen interactions and new leads for pathogenesis research. Poststreptococcal glomerulonephritis (PSGN), a sequela of infection with pathogenic streptococci, is a common cause of preventable kidney disease worldwide. Although PSGN usually occurs after infection with group A streptococci, organisms of Lancefield group C and G also can be responsible. Despite decades of study, the molecular pathogenesis of PSGN is poorly understood. As a first step toward gaining new information about PSGN pathogenesis, we sequenced the genome of Streptococcus equi subsp. zooepidemicus strain MGCS10565, a group C organism that caused a very large and unusually severe epidemic of nephritis in Brazil. The genome is a circular chromosome of 2,024,171 bp. The genome shares extensive gene content, including many virulence factors, with genetically related group A streptococci, but unexpectedly lacks prophages. The genome contains many apparently foreign genes interspersed around the chromosome, consistent with the presence of a full array of genes required for natural competence. An inordinately large family of genes encodes secreted extracellular collagen-like proteins with multiple integrin-binding motifs. The absence of a gene related to speB rules out the long-held belief that streptococcal pyrogenic exotoxin B or antibodies reacting with it singularly cause PSGN. Many proteins previously implicated in GAS PSGN, such as streptokinase, are either highly divergent in strain MGCS10565 or are not more closely related between these species than to orthologs present in other streptococci that do not commonly cause PSGN. Our analysis provides a comparative genomics framework for renewed appraisal of molecular events underlying APSGN pathogenesis

INFLUENCE DU PROCESSUS D'ELABORATION SUR LES DEFAUTS CRISTALLOGRAPHIQUES DANS LES COUCHES DE GaAs EPITAXIEES PAR JETS MOLECULAIRES

Nous présentons une étude sur Les défauts cristallographiques présents dans les couches d'AsGa épitaxiées par la technique des jets moléculaires. La principale technique de caractérisation utilisée est l'attaque chimique sélective, assistée de techniques complémentaires telles que microscopie électronique à transmission et microsonde électronique. Cette étude a été réalisée en vue de mettre en évidence l'effet des dégâts mécaniques, de la contamination et des défauts liés aux impuretés sur la présence de dislocations, de fautes d'empilement et de défauts macroscopiques affectant la morphologie de surface de la couche. En effet pour chaque défaut considéré, une étude est faite pour clarifier son origine et l'influence du choix du substrat, de sa préparation et des conditions de croissance, est prise en compte. Cela est réalisé par le biais d'épitaxies simultanées réalisées sur substrats différents et de l'analyse d'un grand nombre de couches. Enfin, sur la base des tendances observées, des moyens pour réduire la densité des défauts sont proposés.A study of crystallographic defects in MBE GaAs Layers is presented. The main technique used for this purpose was selective chemical etching assisted by others techniques such as transmission electron microscopy and electron microprobe when necessary. This study has been done in order to show the effect of mechanical damages, contamination and impurity related defects on the presence of dislocations, stacking faults and macroscopic defects affecting the surface morphology of the layer. For each defect considered, an attempt is made to clarify its origin and the influence of substrate choice, substrate preparation and growth conditions is taken into account. This is done in the light of the experimental results obtained on a large number of layers and on some particular results obtained by simultaneous epitaxy on different substrates. Finally, on the basis of the trends observed, means for defect density reduction are proposed

OPTICAL CHARACTERIZATION OF Nd3+ DOPED CaF2 LAYERS GROWN BY MOLECULAR BEAM EPITAXY

Nd3+ doped CaF2 monocrystalline films have been grown by molecular beam epitaxy using CaF2 and NdF3 evaporation on CaF2 substrates. The effect of Nd3+ concentration is studied by means of the photoluminescence signal of Nd3+ ions. The results can be compared favourably with those obtained on CaF2:Nd bulk crystals. For a given Nd3+ concentration a lower quantity of Nd3+ aggregate centers is observed. Nd3+ ions can therefore be efficiently incorporated as isolated Nd3+-F- centers up to a concentration of 6 wt % Nd3+ without emission quenching of their associated line at 1045.7 nm

Low phonon energy Nd:LaF₃ channel waveguide lasers fabricated by molecular beam epitaxy

We report the first fabrication and laser operation of channel waveguides based on LaF3 planar thin films grown by molecular beam epitaxy. To our knowledge, this is the lowest phonon energy dielectric material to have shown guided-wave laser operation to date. A full characterization, in terms of spectroscopy, laser results, and propagation losses, is given for the planar thin films upon which the channel waveguides are based. Two channel-fabrication methods are then described, the first involves ion milling and the second takes the novel approach of using a photo-definable polymer overlay. Laser operation in Nd-doped samples is demonstrated at 1.06, 1.05, and 1.3 µm, and the potential for mid-infrared laser sources based on such guides is discussed

ELIMINATION OF TWINNING IN MOLECULAR BEAM EPITAXY OF GaAs/Si and GaAs/INSULATOR

Nous présentons une étude de la nucléation de GaAs sur des surfaces (100) de Si et de (Ca, Sr) F2 montrant qu'une croissance 2D peut être obtenue pour des dépôts très fins ( 15 A°) réalisés à basse température (25-100°C). La stoechiométrie du dépôt est obtenue en augmentant la température jusqu'à 300-400°C ce qui conduit, en même temps, à la cristallisation de la couche. Le diagramme de diffraction électronique met en évidence l'absence de micromacles et l'obtention d'une surface plane au niveau atomique. La procédure proposée ouvre la voie à l'utilisation de techniques basse température pour l'élaboration de structures co-intégrées associant GaAs et Si.We present a study of GaAs nucleation on Si and (Ca, Sr)F2(100) surfaces which shows that a 2D growth can be achieved for very thin (15 A°) deposits obtained at low temperature (25-100°C). Raising the temperature to 300-400°C leads to a stoichiometric and monocrystalline layer. Electron diffraction shows a diagram without twin spots and a surface devoid of noticeable roughness. The proposed procedure paves the way for the subsequent use of low temperature techniques aimed at the fabrication of co-integrated structures associating GaAs and Si