bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal stability of sputtered Cu films with nanoscale growth twins
Authors: Anderoglu O. , Misra A. , Wang H. , Zhang X.
Published: May 2008

We have investigated the thermal stability of sputter-deposited Cu thin films with a high density of nanoscale growth twins by using high-vacuum annealing up to 800 °C for 1 h. Average twin lamella thickness gradually increased from approximately 4 nm for as-deposited films to slightly less than 20 nm after annealing at 800 °C. The average columnar grain size, on the other hand, rapidly increased from approximately 50 to 500 nm. In spite of an order of magnitude increase in grain size, the annealed films retained a high hardness of 2.2 GPa, reduced from 3.5 GPa in the as-deposited state. The high hardness of the annealed films is interpreted in terms of the thermally stable nanotwinned structures. This study shows that nanostructures with a layered arrangement of low-angle coherent twin boundaries may exhibit better thermal stability than monolithic nanocrystals with high-angle grain boundaries.
Optical modeling of a folded organic solar cell
Authors: Andersson Viktor , Tvingstedt Kristofer , Inganäs Olle
Published: May 2008

The optical behavior of a reflective tandem solar cell (V cell) is modeled by means of finite element method (FEM) simulations. The absorption of solar light in the active material as well as in both electrode layers is calculated. The FEM solves the electromagnetic wave equation on the entire defined geometry, resulting in the consideration of interference effects, as well as effects of refraction and reflection. Both single cells and tandem cells are modeled and confirmed to be in accordance with reflectance measurements. Energy dissipation in the active layers is studied as a function of layer thickness and folding angle, and the simulations clearly display the advantage of the light trapping feature of folded cells. This is especially prominent in cells with thinner active layers, where folding induces absorption in the active layer equivalent to that of much thicker cells.
Two-dimensional electron gas density in Al1−xInxN/AlN/GaN heterostructures (0.03≤x≤0.23)
Authors: Gonschorek M. , Carlin J.-F. , Feltin E. , Py M. A. , Grandjean N. , Darakchieva V. , Monemar B. , Lorenz M. , Ramm G.
Published: May 2008

Compared to the AlGaN alloy, which can only be grown under tensile strain on GaN, the AlInN alloy is predicted by Vegard’s law to be lattice-matched (LM) on fully relaxed GaN templates for an indium content of ∼17.5%, i.e., it can be grown either tensely or compressively on GaN. The effect of strain on the polarization induced sheet charge density at the Al1−xInxN/AlN/GaN heterointerfaces is carefully investigated for 6 and 14 nm thick AlInN barriers including a 1 nm thick AlN interlayer. The barrier indium content ranges at 0.03≤x≤0.23 for 6 nm thick barriers and 0.07≤x≤0.21 for 14 nm thick barriers. It is found that the two-dimensional electron gas (2DEG) density varies between (3.5±0.1)×1013cm−2 and (2.2±0.1)×1013 cm−2 for 14 nm thick barriers. Finally, a 2DEG density up to (1.7±0.1)×1013 cm−2 is obtained for a nearly LM AlInN barrier with ∼14.5% indium on GaN as thin as 6 nm.
Density and concentration fluctuations in SiO2–GeO2 optical fiber glass investigated by small angle x-ray scattering
Authors: Le Parc R. , Champagnon B. , Levelut C. , Martinez V. , David L. , Faivre A. , Flammer I. , Hazemann J. L. , Simon J. P.
Published: May 2008

Density and concentration fluctuations have been investigated in a 3mol% GeO2 doped silica glass as a function of the fictive temperature (the temperature at which the structure of the supercooled liquid has been frozen-in to form the glass) by small angle x-ray scattering measurements. The fluctuations increase in a way is quite similar to that observed for pure silica glass as a result of density fluctuation fictive temperature dependence. Fluctuations have also been studied in glasses containing different amounts of GeO2 up to 21mol% GeO2. The fluctuations are shown to increase very strongly with germanium amount as a result of strong concentration fluctuation increase. This observation is in agreement with already observed excess losses in light scattering measurements.
Organic light emitting diodes using a Ga:ZnO anode
Authors: Berry J. J. , Ginley D. S. , Burrows P. E.
Published: May 2008

We report the application of gallium doped zinc oxide (GZO) films as anodes in organic light emitting diodes (OLEDs). Pulsed laser deposited GZO films of differing Ga composition are examined. Bilayer OLEDs using GZO and indium tin oxide (ITO) anodes are compared. Relative to ITO, the GZO anodes have a slightly better sheet resistance and transparency in the visible spectral region. Device data suggest that GZO effectively injects holes into an aromatic triamine hole transporting layer. Indium-free anodes such as GZO are expected to improve OLED stability while lowering the cost per unit area, which is crucial for OLED based lighting applications.
Mode excitation Monte Carlo simulations of mesoscopically large membranes
Authors: Farago Oded
Published: May 2008

Solvent-free coarse grained models represent one of the most promising approaches for molecular simulations of mesoscopically large membranes. In these models, the size of the simulated membrane is limited by the slow relaxation time of the longest bending mode. Here, we present a Monte Carlo algorithm with update moves in which all the lipids are simultaneously displaced. These collective moves result in fast excitation and relaxation of the long wavelength thermal fluctuations. We apply the method to simulations of a bilayer membrane with linear size of ∼50nm and show reduction in the relaxation time by two orders of magnitudes when compared to conventional Monte Carlo.
On Burnett coefficients in periodic media in low contrast regime
Authors: Conca C. , San Martín J. , Smaranda L. , Vanninathan M.
Published: May 2008

In this work, we consider low contrast periodic media and we study the dependence of the effective or homogenized tensor and the dispersion tensor in terms of the microstructure. We treat both one-dimensional structures and some laminated structures in higher dimension. Interesting properties of the sign of these coefficients are found. Surprisingly, these depend on the microstructure only through the local proportion parameter, and in some cases, they do not depend on the microstructure at all.
The range of ambient radius for an active bubble in sonoluminescence and sonochemical reactions
Authors: Yasui Kyuichi , Tuziuti Toru , Lee Judy , Kozuka Teruyuki , Towata Atsuya , Iida Yasuo
Published: May 2008

Numerical simulations of nonequilibrium chemical reactions inside an air bubble in liquid water irradiated by ultrasound have been performed for various ambient bubble radii. The intensity of sonoluminescence (SL) has also been calculated taking into account electron-atom bremsstrahlung, radiative attachment of electrons to neutral molecules, radiative recombination of electrons and ions, chemiluminescence of OH, molecular emission from nitrogen, etc. The lower bound of ambient radius for an active bubble in SL and sonochemical reactions nearly coincides with the Blake threshold for transient cavitation. The upper bound is in the same order of magnitude as that of the linear resonance radius. In actual experiments, however, the distribution of ambient radius for active bubbles may be narrow at around the threshold ambient radius for the shape instability. The threshold peak temperature inside an air bubble for nitrogen burning is higher than that for oxidant formation. The threshold peak temperatures depend on ultrasonic frequency and acoustic amplitude because chemical reactions inside a bubble are in nonequilibrium. The dominant emission mechanism in SL is electron-atom bremsstrahlung except at a lower bubble temperature than 2000K, for which molecular emissions may be dominant.
Γ
-point lattice free energy estimates from O(1) force calculations
Authors: Voss Johannes , Vegge Tejs
Published: May 2008

We present a new method for estimating the vibrational free energy of crystal (and molecular) structures employing only a single force calculation, for a particularly displaced configuration, in addition to the calculation of the ground state configuration. This displacement vector is the sum of the phonon eigenvectors obtained from a fast—relative to, e.g., density-functional theory (DFT)—Hessian calculation using interatomic potentials. These potentials are based here on effective charges obtained from a DFT calculation of the ground state electronic charge density but could also be based on other, e.g., empiric approaches.
Laser-induced self-organization in silicon-germanium thin films
Authors: Weizman M. , Nickel N. H. , Sieber I. , Yan B.
Published: May 2008

We report on the formation of self-organized structures in thin films of silicon-germanium (Si1−xGex) with 0.3<x<0.7 after exposing the films to laser irradiation. Amorphous SiGe samples that are exposed to a single laser pulse exhibit a ripple structure that changes to a hillock structure when the samples are irradiated with additional laser pulses. The topographic structure is coupled to a periodic compositional variation of the SiGe alloy. The periodicity length of the structure after a single laser pulse is in the range of 0.3–1.1 μm, depending on Ge content, layer thickness, and laser fluence, and rapidly grows with increasing number of laser pulses. In situ conductivity measurements during solidification support the theoretical instability analysis that we have done, based on the Mullins–Sekerka theory, to elucidate the nature of this phenomenon. Moreover, as theoretically predicted, the self-organization phenomenon can be turned off by increasing the solidification velocity.
Brightness enhancement method for a high-intensity positron beam produced by an electron accelerator
Authors: Oshima Nagayasu , Suzuki Ryoichi , Ohdaira Toshiyuki , Kinomura Atsushi , Narumi Takamitsu , Uedono Akira , Fujinami Masanori
Published: May 2008

A method for enhancing the brightness of an intense slow positron beam produced by an electron linear accelerator (LINAC) in order to obtain an intense positron microbeam was developed. The developed brightness enhancement system is simple and consists only of a few beam optics and a transmission remoderator. The slow positron beam produced by the LINAC is magnetically guided from the positron source to an experimental room. The beam is extracted from the magnetic field and is focused by a lens on the remoderator to enhance its brightness. The brightness-enhanced beam is extracted from the remoderator and focused on a sample by a lens. The beam size at the sample was 90 μm, which was two orders of magnitude smaller than that in the magnetic transport system that was about 10 mm. The efficiency of the transmission remoderator was about 5%. Adiabatic rules in the magnetic transport and the paraxial-ray equation were used to estimate the beam size that could be produced using this method.
Tunnel spectroscopy in ac-driven quantum dot nanoresonators
Authors: Villavicencio J. , Maldonado I. , Sánchez R. , Cota E. , Platero G.
Published: May 2008

Electronic transport in a triple quantum dot shuttle device in the presence of an ac field is analyzed within a fully quantum mechanical framework. A generalized density matrix formalism is used to describe the time evolution for electronic state occupations in a dissipative phonon bath. In the presence of an ac gate voltage, the electronic states are dressed by photons and the interplay between photon and vibrational sidebands produces current characteristics that obey selection rules. Varying the ac parameters allows to tune the tunneling current features. In particular, we show that coherent destruction of tunneling can be achieved in our device.
Ultrafast electron emission from metallic nanotip arrays induced by near infrared femtosecond laser pulses
Authors: Tsujino S. , Beaud P. , Kirk E. , Vogel T. , Sehr H. , Gobrecht J. , Wrulich A.
Published: May 2008

The authors explore the impact of femtosecond light pulses on the field-emission properties of single-gate molybdenum field-emitter arrays with nanometer scale tip apex. Despite the small fraction of the emission area, we observed a single-photon photoelectric current from the emitter tips on top of the dc field-emission current under the irradiation of 50fs laser pulses at a wavelength of 800nm with an external quantum efficiency up to ∼2×10−7 and the emitter tip quantum efficiency of ∼10−2. The result indicates that metallic field-emitter arrays are promising for applications that require high-brightness short electron beams.
Electrical properties of A∕B-site substituted Ni-deficient La(Ni0.6Fe0.3)O3 perovskites with A=Ag+, Pb2+, Nd3+ and B=Mn3+, Ga3+
Authors: Chou Chang-Yi , Kaurav N. , Kuo Yung-Kang , Kuo Dong-Hau
Published: May 2008

We report the structural and electron transport properties of a series of compounds where A∕B-site substituted Ni-deficient La(Ni0.6Fe0.3)O3 compounds with A=Ag+, Pb2+, Nd3+ and B=Mn3+, Ga3+ have been synthesized by a chemical route. Electrical resistivity was used as an indicator to explore the response of La(Ni0.6Fe0.3)O3 for different A∕B-site substitutions. The rationalized explanation for the variations of resistivity with substitutions was based on the change in bond length and modification of band structure due to substitutional disorder. A variable range hopping conduction approach is employed to explain the temperature variation of resistivity, and the result suggest that disorder-induced localization of charge carriers dominates the electrical transport in these compounds.
Controlled synthesis and characterization of Ag2S films with varied microstructures and its role as asymmetric barrier layer in trilayer junctions with dissimilar electrodes
Authors: Lekshmi I. C. , Berera G. , Afsar Y. , Miao G. X. , Nagahama T. , Santos T. , Moodera J. S.
Published: May 2008

In this study, we examine the possibility of electrode-barrier interactions in modifying the electrical characteristics and current switching behavior of a trilayer junction with silver sulfide as the barrier layer. A series of Al–Ag2S–Ag crossbar junction is fabricated by thermal evaporation technique varying the thickness (30–110Å) of the sulfide layer. Current-voltage characteristics of the junctions are studied as a function of barrier layer thickness, which can suggest any role that electrode-barrier interaction may have in tuning their electrical behavior. To fully understand the performance of the barrier layer, structure and physical properties of Ag2S films are independently investigated. The microstructure of Ag2S films strongly depends on the deposition conditions that, in turn, affect their electrical and optical characteristics. The polarization of the lattice prevalent in Ag2S is shown to affect the charge carrier conduction in their films and dominates their electrical behavior and that of the junctions.
Nanoporous alumina enhanced surface plasmon resonance sensors
Authors: Koutsioubas Alexandros G. , Spiliopoulos Nikolaos , Anastassopoulos Dimitris , Vradis Alexandros A. , Priftis George D.
Published: May 2008

The signal enhancement of an easy to fabricate, nanoporous alumina assisted surface plasmon resonance (SPR) sensor is investigated. It is theoretically shown that the presence of a thin (under 200nm) porous alumina layer on top of an aluminum film supporting the surface plasmons, may significantly increase (over one order of magnitude) the sensitivity of the SPR method in the case where the adsorption of relatively small molecules is probed. The comparative experimental investigation of self-assembled monolayer formation on planar metal films and porous alumina layers verifies the theoretical predictions. Based on these results, we discuss the extended applicability of this setup in biosensor and other related applications.
Efficient suppression of charge trapping in ZnO-based transparent thin film transistors with novel Al2O3∕HfO2∕Al2O3 structure
Authors: Chang Seongpil , Song Yong-Won , Lee Sanggyu , Lee Sang Yeol , Ju Byeong-Kwon
Published: May 2008

Charge trapping is dramatically suppressed in ZnO transparent thin film transistors (TFTs) employing a multilayered gate insulator with HfO2 layer sandwiched by Al2O3 layers. In spite of its high dielectric constant, HfO2 has critical drawbacks including huge charge trap density in interfaces. We suggest and demonstrate an elegant solution to minimize the charge trapping introducing Al2O3 buffer layers. The operation of Al2O3∕HfO2∕Al2O3 multilayered gate-insulator structure in the ZnO transparent TFT is evaluated to ensure the voltage difference in the hysteresis loop as low as 0.2V, and the immunization to the threshold voltage shift induced by repeated sweeps of gate voltage.
Intersubband transitions in ZnO multiple quantum wells
Authors: Belmoubarik M. , Ohtani K. , Ohno H.
Published: May 2008

Intersubband transitions in ZnO∕MgZnO multiple quantum wells (MQWs) are investigated by a photocurrent spectroscopy. Photocurrent peaks are observed in the energy range from 300to400meV and shifted to higher energy by reducing the ZnO well thickness. Polarization-resolved photocurrent spectra show that these peaks are observed when the polarization of incident lights is TM mode, following the intersubband selection rule. Calculation indicates that the photocurrent peaks are the intersubband transition from the first to the third subband in ZnO∕MgZnO MQWs.
Spectral tuning of liquid microdroplets standing on a superhydrophobic surface using electrowetting
Authors: Kiraz A. , Karadağ Y. , Coskun A. F.
Published: May 2008

Using electrowetting, we demonstrate reversible spectral tuning of the whispering gallery modes of glycerol/water microdroplets standing on a superhydrophobic surface by up to 4.7nm at 400V. Our results can inspire electrically tunable optical switches and filters based on microdroplets on a superhydrophobic surface. The sensitivity of the observed spectral drift to the contact angle can also be used to measure the contact angles of microdroplets on a superhydrophobic surface.
Computer simulation of ferroelectric domain structures in epitaxial BiFeO3 thin films
Authors: Zhang J. X. , Li Y. L. , Choudhury S. , Chen L. Q. , Chu Y. H. , Zavaliche F. , Cruz M. P. , Ramesh R. , Jia Q. X.
Published: May 2008

Ferroelectric domain structures of (001)c, (101)c, and (111)c oriented epitaxial BiFeO3 thin films were studied by using the phase-field approach. Long-range elastic and electrostatic interactions were taken into account. The effects of various types of substrate constraint on the domain morphologies were systematically analyzed. It is demonstrated that domain structures of BiFeO3 thin films could be controlled by selecting proper film orientations and substrate constraint. The dependence of the {110}c-type domain wall orientation on substrate constraint for the (001)c oriented BiFeO3 thin film was also discussed.
Stoichiometric growth of high Curie temperature heavily alloyed GaMnAs
Authors: Mack S. , Myers R. C. , Heron J. T. , Gossard A. C. , Awschalom D. D.
Published: May 2008

Heavily alloyed, 100nm Ga1−xMnxAs (x>0.1) films are obtained via low-temperature molecular beam epitaxy by utilizing a combinatorial technique which allows systematic control of excess arsenic during growth. Reproducible electronic, magnetic, and structural properties are optimized in a narrow range of stoichiometric growth conditions. In contrast to a prediction of the Zener model of hole-mediated ferromagnetism, the Curie temperature of the stoichiometric material is independent of x (for x>0.1), while substitutional Mn content is proportional to x within a large window of growth conditions.
Two-dimensional tungsten photonic crystals as selective thermal emitters
Authors: Celanovic Ivan , Jovanovic Natalija , Kassakian John
Published: May 2008

This paper presents theory, design, fabrication, and optical characterization of two-dimensional (2D) tungsten (W) photonic crystals (PhC) as selective thermal emitters. We use the photonic band gap of a 2D W PhC, radiating out of a plane of periodicity, to design a selective infrared thermal radiation source that exhibits close to blackbody emittance near the the band gap wavelength and relatively sharp cutoff for wavelengths above the band gap. In addition, we present simple design rules and detailed simulation results for several representative geometries. Microfabrication steps are also presented. Finally, we present detailed experimental results of the optical characterization of three fabricated prototypes that exhibit good agreement with simulation results.
Dynamic amorphization and recrystallization of metals in kinetic spray process
Authors: Xiong Yuming , Kang Kicheol , Bae Gyuyeol , Yoon Sanghoon , Lee Changhee
Published: May 2008

We reported dynamic amorphization and recrystallization processes of metals upon impact of micron-scaled particles at a high strain rate (109s−1) combining adiabatic heating with rapid cooling (1010Ks−1) in a kinetic spray process. At the interface of the particle/substrate, an amorphous zone with a thickness of about 3nm was observed after individual particle impact. It is consistent with the mechanism of amorphous shear lamella and adiabatic shear instability characteristics in kinetic spray process. At the interface of coating/substrate, a rapid phase transition from unstable amorphous to crystalline helps the formation of ductile joints of coatings.
Silicon nanowire tunneling field-effect transistors
Authors: Björk M. T. , Knoch J. , Schmid H. , Riel H. , Riess W.
Published: May 2008

We demonstrate the implementation of tunneling field-effect transistors (TFETs) based on silicon nanowires (NWs) that were grown using the vapor-liquid-solid growth method. The Si NWs contain p-i-n+ segments that were achieved by in situ doping using phosphine and diborane as the n- and p-type dopant source, respectively. Electrical measurements of the TFETs show a band-to-band tunneling branch in the transfer characteristics. Furthermore, an increase in the on-state current and a decrease in the inverse subthreshold slope upon reducing the gate oxide thickness are measured. This matches theoretical calculations using a Wenzel Kramer Brillouin approximation with nanowire diameter and oxide thickness as input parameters.
Giant magnetocaloric effect in the Ising antiferromagnet DySb
Authors: Hu W. J. , Du J. , Li B. , Zhang Q. , Zhang Z. D.
Published: May 2008

The magnetic phase transitions and the magnetocaloric effect in the Ising antiferromagnet DySb have been studied. A field-induced sign change of the magnetocaloric effect has been observed which is related to a first-order field-induced metamagnetic transition from the antiferromagnetic to the ferromagnetic states at/below the Néel temperature TN, while the negative field-induced entropy change is found to be associated with the first-order magnetic transition from the paramagnetic to the ferromagnetic states above TN. The large magnetic-entropy change (−20.6J∕kgK at 11K for a field change of 7T), together with small hysteresis, suggests that DySb could be a potential material for magnetic refrigeration in the low-temperature range.
EB1 promotes Aurora-B kinase activity through blocking its inactivation by protein phosphatase 2A
Authors: Sun Lei , Gao Jinmin , Dong Xin , Liu Min , Li Dengwen , Shi Xingjuan , Dong Jin-Tang , Lu Xianyu , Liu Chunyong , Zhou Jun
Published: May 2008

EB1 (end-binding protein 1) is a key player in the regulation of microtubule dynamics. In concert with its binding partners, adenomatous polyposis coli and p150
glued
, EB1 plays a crucial role in a variety of microtubule-based cellular processes. In this study we have identified in a yeast two-hybrid screen the mitotic kinase and chromosome passenger protein Aurora-B as a binding partner of EB1. GST pull-down and immunoprecipitation experiments reveal a specific interaction between Aurora-B and EB1 both in cells and
in vitro
. Immunofluorescence microscopy shows that these two proteins colocalize on the central spindle in anaphase and in the midbody during cytokinesis. Kinase assays using both immunoprecipitated and purified Aurora-B demonstrate that EB1 is not a substrate of Aurora-B. Rather, EB1 positively regulates Aurora-B kinase activity. EB1 overexpression remarkably enhances Aurora-B activity and knockdown of its expression impairs Aurora-B activity. Our data further show that EB1 is able to protect Aurora-B from dephosphorylation/inactivation by protein phosphatase 2A (PP2A) by blocking PP2A binding to Aurora-B. These findings establish Aurora-B as an EB1-interacting protein and suggest that EB1 stimulates Aurora-B activity through antagonizing its dephosphorylation/inactivation by PP2A.
Electrical microstimulation thresholds for behavioral detection and saccades in monkey frontal eye fields
Authors: Murphey Dona K. , Maunsell John H. R.
Published: May 2008

The frontal eye field (FEF) is involved in the transformation of visual signals into saccadic eye movements. Although it is often considered an oculomotor structure, several lines of evidence suggest that the FEF also contributes to visual perception and attention. To better understand the range of behaviors to which the FEF can contribute, we tested whether monkeys could detect activation of their FEF by electrical microstimulation with currents below those that cause eye movements. We found that stimulation of FEF neurons could almost always be detected at levels below those needed to generate saccades and that the electrical current needed for detection was highly correlated with that needed to generate a saccade. This relationship between detection and saccade thresholds can be explained if FEF neurons represent preparation to make particular saccades and subjects can be aware of such preparations without acting on them when the representation is not strong.
Atomic interactions of neonicotinoid agonists with AChBP: Molecular recognition of the distinctive electronegative pharmacophore
Authors: Talley Todd T. , Harel Michal , Hibbs Ryan E. , Radić Zoran , Tomizawa Motohiro , Casida John E. , Taylor Palmer
Published: May 2008

Acetylcholine-binding proteins (AChBPs) from mollusks are suitable structural and functional surrogates of the nicotinic acetylcholine receptors when combined with transmembrane spans of the nicotinic receptor. These proteins assemble as a pentamer with identical ACh binding sites at the subunit interfaces and show ligand specificities resembling those of the nicotinic receptor for agonists and antagonists. A subset of ligands, termed the neonicotinoids, exhibit specificity for insect nicotinic receptors and selective toxicity as insecticides. AChBPs are of neither mammalian nor insect origin and exhibit a distinctive pattern of selectivity for the neonicotinoid ligands. We define here the binding orientation and determinants of differential molecular recognition for the neonicotinoids and classical nicotinoids by estimates of kinetic and equilibrium binding parameters and crystallographic analysis. Neonicotinoid complex formation is rapid and accompanied by quenching of the AChBP tryptophan fluorescence. Comparisons of the neonicotinoids imidacloprid and thiacloprid in the binding site from
Aplysia californica
AChBP at 2.48 and 1.94 Å in resolution reveal a single conformation of the bound ligands with four of the five sites occupied in the pentameric crystal structure. The neonicotinoid electronegative pharmacophore is nestled in an inverted direction compared with the nicotinoid cationic functionality at the subunit interfacial binding pocket. Characteristic of several agonists, loop C largely envelops the ligand, positioning aromatic side chains to interact optimally with conjugated and hydrophobic regions of the neonicotinoid. This template defines the association of interacting amino acids and their energetic contributions to the distinctive interactions of neonicotinoids.
Filter Performance of N99 and N95 Facepiece Respirators Against Viruses and Ultrafine Particles
Authors: Eninger Robert M. , Honda Takeshi , Adhikari Atin , Heinonen-Tanski Helvi , Reponen Tiina , Grinshpun Sergey A.
Published: May 2008

The performance of three filtering facepiece respirators (two models of N99 and one N95) challenged with an inert aerosol (NaCl) and three virus aerosols (enterobacteriophages MS2 and T4 and Bacillus subtilis phage)—all with significant ultrafine components—was examined using a manikin-based protocol with respirators sealed on manikins. Three inhalation flow rates, 30, 85, and 150 l min−1, were tested. The filter penetration and the quality factor were determined. Between-respirator and within-respirator comparisons of penetration values were performed. At the most penetrating particle size (MPPS), >3% of MS2 virions penetrated through filters of both N99 models at an inhalation flow rate of 85 l min−1. Inhalation airflow had a significant effect upon particle penetration through the tested respirator filters. The filter quality factor was found suitable for making relative performance comparisons. The MPPS for challenge aerosols was <0.1 μm in electrical mobility diameter for all tested respirators. Mean particle penetration (by count) was significantly increased when the size fraction of <0.1 μm was included as compared to particles >0.1 μm. The filtration performance of the N95 respirator approached that of the two models of N99 over the range of particle sizes tested (∼0.02 to 0.5 μm). Filter penetration of the tested biological aerosols did not exceed that of inert NaCl aerosol. The results suggest that inert NaCl aerosols may generally be appropriate for modeling filter penetration of similarly sized virions.
Microarray analysis of the transcriptional responses of Clostridium difficile to environmental and antibiotic stress
Authors: Emerson Jenny E. , Stabler Richard A. , Wren Brendan W. , Fairweather Neil F.
Published: May 2008

Clostridium difficile is a spore-forming anaerobic bacterium that is an emerging nosocomial threat; incidence of infection in hospitals is increasing, both in frequency and severity, resulting in considerable morbidity and mortality. In order to adapt to the intestinal environment, C. difficile must react to the many stresses involved with colonization, including exposure to antibiotics, which represents the most frequent precipitating agent of C. difficile infection. The responses of C. difficile to environmental shocks (heat, pH and oxidative shock) and to growth in the presence of subinhibitory concentrations of antibiotics (amoxicillin, clindamycin and metronidazole) were investigated using the C. difficile 630 microarray developed by the Bacterial Microarray Group at St George's, University of London, UK ( μG@S). Significantly regulated genes and operons were identified that are unique to or common between different stresses. The transcriptional profiles of C. difficile 630 are similar after growth in the presence of amoxicillin and clindamycin: both increased transcription of ribosomal protein genes and altered transcription of genes encoding surface-associated proteins. In contrast, metronidazole treatment resulted in minor changes in transcription patterns. The general stress response is observed after heat shock and acid shock. Heat shock also affected transcription of several biochemical pathways. Exposure to atmospheric oxygen induced a large number of electron transporters. This study provides a starting point for detailed analyses of numerous genes whose expression is affected by stress and may therefore be involved in adaptation to the host environment.
Isolation of Clostridium difficile from food animals in Slovenia
Authors: Pirs Tina , Ocepek Matjaz , Rupnik Maja
Published: May 2008

A total of 313 faecal samples from three pig farms and two cattle farms was collected, and Clostridium difficile was isolated from 133/257 piglet samples (51.8 %) and from 1/56 calf samples (1.8 %). Toxins were tested only in calf samples and were positive in 44.6 % (25/56). The only bovine isolate belonged to toxinotype XIa (A−B−CDT+). Porcine isolates belonged to toxinotype 0 (A+B+CDT−) and toxinotype V (A+B+CDT+). Of the two ribotypes usually found in toxinotype V, the strains isolated in this study showed a greater similarity to ribotype 066 than to ribotype 078.
Antibiotics involved in Clostridium difficile-associated disease increase colonization factor gene expression
Authors: Denève Cécile , Deloménie Claudine , Barc Marie-Claude , Collignon Anne , Janoir Claire
Published: May 2008

Clostridium difficile is the most common cause of antibiotic-associated diarrhoea. Antibiotics are presumed to disturb the normal intestinal microbiota, leading to depletion of the barrier effect and colonization by pathogenic bacteria. This first step of infection includes adherence to epithelial cells. We investigated the impact of various environmental conditions in vitro on the expression of genes encoding known, or putative, colonization factors: three adhesins, P47 (one of the two S-layer proteins), Cwp66 and Fbp68, and a protease, Cwp84. The conditions studied included hyperosmolarity, iron depletion and exposure to several antibiotics (ampicillin, clindamycin, ofloxacin, moxifloxacin and kanamycin). The analysis was performed on three toxigenic and three non-toxigenic C. difficile isolates using real-time PCR. To complete this work, the impact of ampicillin and clindamycin on the adherence of C. difficile to Caco-2/TC7 cells was analysed. Overall, for the six strains of C. difficile studied, exposure to subinhibitory concentrations (1/2 MIC) of clindamycin and ampicillin led to the increased expression of genes encoding colonization factors. This was correlated with the increased adherence of C. difficile to cultured cells under the same conditions. The levels of gene regulation observed among the six strains studied were highly variable, cwp84 being the most upregulated. In contrast, the expression of these genes was weakly, or not significantly, modified in the presence of ofloxacin, moxifloxacin or kanamycin. These results suggest that, in addition to the disruption of the normal intestinal microbiota and its barrier effect, the high propensity of antibiotics such as ampicillin and clindamycin to induce C. difficile infection could also be explained by their direct role in enhancing colonization by C. difficile.
Enhancement of the cytotoxic activity of Clostridium difficile toxin A by surface-associated antigens
Authors: Sánchez-Hurtado Karla , Poxton Ian R.
Published: May 2008

Cell-surface antigens of Clostridium difficile and LPS from Escherichia coli were investigated for modulating effects on the activity of C. difficile toxin A on Vero and Caco2 cells. The antigens of C. difficile tested comprised: (i) an EDTA extract, which contained several major and minor cell-surface proteins and the membrane-associated lipocarbohydrate (LC); (ii) a guanidine hydrochloride extract, which mainly contained the surface-layer proteins; (iii) an aqueous phenol-extracted, protein-free LC. On their own, none of the antigens had a detrimental effect on the cells, with the EDTA extract and LC having a marginally protective effect. When these antigens were added to suboptimal levels of toxin A, there was significant enhancement of its cytotoxicity by the EDTA and LC preparations on both cell types. LPS showed some enhancement of the effect of toxin on Vero cells at the lowest levels of toxin investigated. It was concluded that this effect seen in vitro may have a role to play in the colon during infection with C. difficile.
Systemic antibody response to Clostridium difficile in colonized patients with and without symptoms and matched controls
Authors: Sánchez-Hurtado Karla , Corretge Maria , Mutlu Esvet , McIlhagger Rowan , Starr John M. , Poxton Ian R.
Published: May 2008

It has been proposed that patients who develop Clostridium difficile-associated disease (CDAD) do so because they are unable to mount an adequate immune response. Serum was collected from three groups of elderly in-patients: (i) cases (n=21) of CDAD, being toxin A/B-positive; (ii) carriers (n=21) asymptomatic for CDAD (no diarrhoea) but at least toxin or culture positive; and (iii) controls (n=26) asymptomatic for CDAD and negative for both C. difficile toxin and culture. The age and gender of each group were compared, and the colonizing strains were ribotyped and toxinotyped. Serum antibodies (IgG and IgM) were measured by ELISA using different antigen preparations: EDTA extract (containing cell-surface proteins and carbohydrates), guanidine hydrochloride extract (surface-layer proteins), aqueous phenol-extracted lipocarbohydrate (LC); crude toxin (dialysis culture supernatant) and purified toxin A. LPS from Escherichia coli was used as a control antigen. Antibodies were also tested for toxin neutralization on tissue monolayers and for binding to EDTA-extracted antigens by Western blotting. IgG antibody measurements to cytomegalovirus (CMV) were included as an indicator of potential immunosenescence. Results showed that the patient groups were well matched by age and gender, and the colonizing strains were similar in cases and carriers, being predominantly ribotype 001 and toxinotype 0. By ELISA, IgG levels to most of the antigens were highest in the cases and lowest in the controls, with the exception of antibodies to the LC, which were higher in the controls than the cases. Levels in the carriers tended to be of intermediate level or similar to the controls. For all antigens, the levels of IgM were not significantly different among cases, carriers and controls. Serum from all groups was able to neutralize the cytotoxic action of toxin on both Vero and Caco2 cells, and all to a similar extent. Western blots showed an overall higher level of IgG antibodies to the EDTA-extracted antigens in the cases. The results of the CMV ELISA showed that specific IgG was detected in more cases (78 %) than carriers and controls (both 65 %), but this difference in seropositivity was not significant. The conclusion is that, during symptomatic infection, patients respond to protein antigens of C. difficile in a manner typical of a secondary antibody response, with no evidence that an inability to respond predisposes to the appearance of symptoms.
Clostridium difficile TxAC314 and SLP-36kDa enhance the immune response toward a co-administered antigen
Authors: Brun Paola , Scarpa Melania , Grillo Alessia , Palù Giorgio , Mengoli Carlo , Zecconi Alfonso , Spigaglia Patrizia , Mastrantonio Paola , Castagliuolo Ignazio
Published: May 2008

This study evaluated the in vivo adjuvant activity of two peptides derived from Clostridium difficile: a fragment of the receptor-binding domain of toxin A (TxAC314) and a fragment of the 36 kDa surface-layer protein (SLP-36kDa) from strain C253. Their ability to affect the magnitude, distribution and polarization of the immune response against fibronectin-binding protein A (FnbpA), a protective vaccine antigen against Staphylococcus aureus, was evaluated using two different routes of immunization: intranasal and subcutaneous. It was shown that (i) the route of immunization affected the magnitude of the immune response; (ii) both peptides enhanced the production of circulating anti-FnbpA IgG and IgA; (iii) following mucosal immunization TxAC314 was more effective than SLP-36kDa at inducing antibody in the gastrointestinal tract; (iv) the adjuvant influenced the Th1/Th2 balance; and (v) TxAC314 was more effective than SLP-36kDa in inducing a cell-mediated response. These studies provide insight into the ability of different C. difficile-derived peptides to differentially affect and polarize the activity of the immune system and on their potential use as adjuvants in newly developed vaccines.
Fluoroquinolone resistance in Clostridium difficile isolates from a prospective study of C. difficile infections in Europe
Authors: Spigaglia Patrizia , Barbanti Fabrizio , Mastrantonio Paola , Brazier Jon S. , Barbut Frédéric , Delmée Michel , Kuijper Ed , R. Poxton Ian ,   on behalf of the European Study Group on  (ESGCD)
Published: May 2008

The European Study Group on Clostridium difficile (ESGCD) conducted a prospective study in 2005 to monitor and characterize C. difficile strains circulating in European hospitals, collecting 411 isolates. Eighty-three of these isolates, showing resistance or intermediate resistance to moxifloxacin (MX), were selected for this study to assess susceptibility to other fluoroquinolones (FQs) and to analyse the gyr genes, encoding the DNA gyrase subunits GyrA and GyrB. Twenty MX-susceptible isolates from the surveillance study were included for comparison. Overall, one amino acid substitution in GyrA (Thr82 to Ile) and four different substitutions in GyrB (Ser416 to Ala, Asp426 to Asn, Asp426 to Val and Arg447 to Lys) were identified. A high level of resistance (MIC ≥32 μg ml−1) to MX, ciprofloxacin (CI), gatifloxacin (GA) and levofloxacin (LE) was found in 68 isolates showing the amino acid substitution Thr82 to Ile in GyrA, in eight isolates with the substitutions Thr82 to Ile in GyrA and Ser416 to Ala in GyrB, in two isolates showing the substitution Asp426 to Asn in GyrB and in one isolate with Asp426 to Val in GyrB. The remaining four isolates showed high MICs for CI and LE, but different MIC levels for MX and GA. In particular, intermediate levels of resistance to MX were shown by two isolates, one with the substitution Thr82 to Ile in GyrA, and one showing Asp426 to Asn in GyrB. The substitution Arg447 to Lys in GyrB was found in two strains resistant to MX, CI and LE but susceptible to GA. No substitutions in GyrA were found in the FQ-susceptible strains, whereas two strains showed the amino acid change Ser416 to Ala in GyrB. Thr82 to Ile was the most frequent amino acid change identified in the C. difficile isolates examined. In contrast to previous observations, 10 % of the isolates showed this substitution in association with Ser416 to Ala in GyrB. The other amino acid changes found were characteristic of a few strains belonging to certain types and/or countries. Two new substitutions for C. difficile, Ser416 to Ala and Arg447 to Lys, were found in GyrB. Whereas the former does not seem to have a key role in resistance, since it was also detected in susceptible strains, the latter substitution occurred in the same position where other amino acid variations take place in resistant Escherichia coli and other C. difficile strains. A large number of C. difficile isolates now show an alarming pattern of resistance to the majority of FQs currently used in hospitals and outpatient settings, therefore judicious use of these antibiotics and continuous monitoring of in vitro resistance are necessary.
Comparative analysis of BI/NAP1/027 hypervirulent strains reveals novel toxin B-encoding gene (tcdB) sequences
Authors: Stabler Richard A. , Dawson Lisa F. , Phua Leslie T. H. , Wren Brendan W.
Published: May 2008

The reported incidence and mortality of Clostridium difficile-associated disease has increased significantly, which in part is likely to be due to the emergence of a new, highly virulent strain in North America and Europe. This epidemic strain, referred to as BI/NAP1/027, has increased virulence, attributed to overexpression of the two toxin-encoding genes, tcdA and tcdB, which may be due to truncation of the negative regulator (tcdC) by a 1 bp deletion. In a previous study of whole-genome comparisons using microarray analysis of 75 C. difficile isolates, it was noted that the 20 027 strains, which formed a hypervirulent clade, possessed a unique hybridization pattern for the 7 toxin B microarray reporters. This unique pattern was conserved in all of these 027 strains. The pattern was different for the 55 non-027 strains tested. These data, along with the knowledge that 027 strains are toxinotype III (i.e. possess a complete tcdB gene of comparable size to toxin reference strain VPI 10463), suggest that the sequence of the N-terminal binding domain of toxin B must be divergent from C. difficile strain 630 (and the other 55 strains tested). Additionally, these 027 strains had comparable hybridization patterns across the whole microarray, as well as for tcdB. Therefore, it was suggested that they share a similar, novel N-terminal binding domain. The aim of this study was to ascertain the sequence variation in tcdB from eight characterized BI/NAP1/027 strains. The study confirmed significant sequence variation of tcdB from the sequenced strain 630 and slight variation in tcdB among the eight 027 strains. These results suggest that toxin B from 027 strains may have a different binding capacity compared with its less-virulent counterparts and may, in addition to the mutated tcdC regulator, be responsible for the increased virulence of 027 strains.
Assessing the role of p-cresol tolerance in Clostridium difficile
Authors: Dawson Lisa F. , Stabler Richard A. , Wren Brendan W.
Published: May 2008

Clostridium difficile is an important nosocomial pathogen, resulting in antibiotic-associated disease ranging from mild diarrhoea to the life-threatening pseudomembranous colitis. Upon antibiotic exposure, it is believed that the normal bowel microflora of patients is disrupted, allowing C. difficile to proliferate. Significantly, C. difficile is among only a few bacteria able to ferment tyrosine to p-cresol, a phenolic compound that is toxic to other microbes via its ability to interfere with metabolism. Therefore, the ability of different C. difficile strains to produce and tolerate p-cresol may play an important role in the development and severity of C. difficile-associated disease. In this study, it was demonstrated that two C. difficile hypervirulent 027 strains (Stoke Mandeville and BI-16) are more tolerant to p-cresol than other C. difficile strains including 630, CF4 and CD196. Surprising, it was shown that Clostridium sordellii also has a high tolerance to p-cresol, suggesting an overlap in the tolerance pathways in these clostridial species.
Dual KaiC-based oscillations constitute the circadian system of cyanobacteria
Authors: Kitayama Yohko , Nishiwaki Taeko , Terauchi Kazuki , Kondo Takao
Published: May 2008

In the cyanobacterium Synechococcus elongatus PCC 7942, the KaiA, KaiB, and KaiC proteins are essential for the generation of circadian rhythms. Both in vivo and in vitro, phosphorylation of KaiC is regulated positively by KaiA and negatively by KaiB and shows circadian rhythmicity. The autonomous circadian cycling of KaiC phosphorylation is thought to be the basic pacemaker of the circadian clock and to control genome-wide gene expression in cyanobacteria. In this study, we found that temperature-compensated circadian oscillations of gene expression persisted even when KaiC was arrested in the phosphorylated state due to kaiA overexpression. Moreover, two phosphorylation mutants showed transcriptional oscillation with a long period. In kaiA-overexpressing and phosphorylation-deficient strains, KaiC oscillated and transient overexpression of phosphorylation-deficient kaiC reset the phase of the rhythm. These results suggest that transcription- and translation-based oscillations in KaiC abundance are also important for circadian rhythm generation in cyanobacteria. Furthermore, at low temperature, cyanobacteria can show circadian rhythms only when both the KaiC phosphorylation cycle and the transcription and translation cycle are intact. Our findings indicate that multiple coupled oscillatory systems based on the biochemical properties of KaiC are important to maintain robust and precise circadian rhythms in cyanobacteria.