449 research outputs found
Antimicrobial resistance of Neisseria gonorrhoeae isolates in south-west Germany, 2004 to 2015: increasing minimal inhibitory concentrations of tetracycline but no resistance to third-generation cephalosporins
Introduction Numbers of gonorrhoea cases have increased, and the World Health
Organization has estimated 106 million new cases in adults worldwide for 2008,
which was 21% higher than numbers for 2005 [1]. At the same time, rising rates
of antimicrobial resistance of its causative agent, Neisseria gonorrhoeae,
have been reported in many parts of the world including Europe, even against
the third-generation cephalosporins, cefixime and ceftriaxone [2,3]. For this
reason, cefixime alone is no longer recommended as single-drug treatment for
gonorrhoea in Europe or the United States [4,5]. The European Gonococcal
Antimicrobial Surveillance Programme (EURO-GASP) was established by 12
European countries in 2004 in response to the emerging antimicrobial
resistance of N. gonorrhoeae, as part of the European Surveillance of Sexually
Transmitted Infections Project [6]. In 2016, EURO-GASP has participation from
laboratories from 21 European Union/European Economic Area countries, which
regularly report gonorrhoea susceptibility testing results and epidemiological
surveillance data, and submit gonococcal isolates for centralised testing or
participate in decentralised testing. Since 2009, EURO-GASP has been
coordinated by the European Centre for Disease Prevention and Control (ECDC).
Resistance data have been published regularly and in a timely manner [7,8],
but the numbers of isolates tested per country are relatively low (between 10
and 251 in 2011 [8]) and therefore most likely not representative of the
epidemiological situation of the individual countries. As cases of gonorrhoea
or antimicrobial resistance patterns of N. gonorrhoeae isolates are not
subject to reporting in Germany, data regarding current antimicrobial
susceptibility and its development over time are scarce. Only three individual
studies have addressed this issue in the past 10 years. Abraham et al.
analysed 50 isolates collected between 2001 and 2010 in Dresden, Saxony [9],
while Horn and colleagues have reported the results of a nationwide
surveillance study conducted by the Paul-Ehrlich-Society of Chemotherapy in
2010/2011 in which 213 isolates submitted by 23 laboratories were analysed
[10]. Additionally, minimum inhibitory concentrations (MICs) of selected
antibiotics for 65 N. gonorrhoeae isolates collected in 2004/2005 in southern
Germany have been reported [11]. None of these studies has reported
cephalosporin-resistant N. gonorrhoeae isolates. Data from EURO-GASP, however,
have provided evidence for the presence of cephalosporin-resistant N.
gonorrhoeae isolates in Germany, too [7,8]; in fact, an Austrian patient with
a cefixime-resistant N. gonorrhoeae isolate acquired his infection in Munich,
south Germany [12]. To gain more information on the antimicrobial
susceptibility of N. gonorrhoeae in Germany and elucidate possible changes in
antimicrobial resistance occurring over time, we analysed the susceptibility
patterns of all N. gonorrhoeae isolates identified and tested in our
laboratory between 2004 and 2015 (n = 434). Since age and sex have been
identified as risk factors for harbouring antimicrobial-resistant N.
gonorrhoeae isolates [13,14], we additionally analysed our data regarding
these parameters. Unfortunately, the study design chosen did not provide
information regarding other possible risk factors, e.g. working as
professional sex worker or being a man who has sex with men (MSM)
Small Angle Shubnikov-de Haas Measurements in Silicon MOSFET's: the Effect of Strong In-Plane Magnetic Field
Measurements in magnetic fields applied at small angles relative to the
electron plane in silicon MOSFETs indicate a factor of two increase of the
frequency of Shubnikov-de Haas oscillations at H>H_{sat}. This signals the
onset of full spin polarization above H_{sat}, the parallel field above which
the resistivity saturates to a constant value. For H<H_{sat}, the phase of the
second harmonic of the oscillations relative to the first is consistent with
scattering events that depend on the overlap instead of the sum of the spin-up
and spin-down densities of states.Comment: 4 pages; figures now inserted in text; additional referenc
Hall Coefficient of a Dilute 2D Electron System in Parallel Magnetic Field
Measurements in magnetic fields applied at a small angle with respect to the
2D plane of the electrons of a low-density silicon MOSFET indicate that the
Hall coefficient is independent of parallel field from H=0 to , the
field above which the longitudinal resistance saturates and the electrons have
reached full spin-polarization. This implies that the mobilities of the spin-up
and spin-down electrons remain comparable at all magnetic fields, and suggests
there is strong mixing of spin-up and spin-down electron states.Comment: 4 pages, 2 figure
Projected Multi-Agent Consensus Equilibrium (PMACE) for Distributed Reconstruction with Application to Ptychography
Multi-Agent Consensus Equilibrium (MACE) formulates an inverse imaging
problem as a balance among multiple update agents such as data-fitting terms
and denoisers. However, each such agent operates on a separate copy of the full
image, leading to redundant memory use and slow convergence when each agent
affects only a small subset of the full image. In this paper, we extend MACE to
Projected Multi-Agent Consensus Equilibrium (PMACE), in which each agent
updates only a projected component of the full image, thus greatly reducing
memory use for some applications.We describe PMACE in terms of an equilibrium
problem and an equivalent fixed point problem and show that in most cases the
PMACE equilibrium is not the solution of an optimization problem. To
demonstrate the value of PMACE, we apply it to the problem of ptychography, in
which a sample is reconstructed from the diffraction patterns resulting from
coherent X-ray illumination at multiple overlapping spots. In our PMACE
formulation, each spot corresponds to a separate data-fitting agent, with the
final solution found as an equilibrium among all the agents. Our results
demonstrate that the PMACE reconstruction algorithm generates more accurate
reconstructions at a lower computational cost than existing ptychography
algorithms when the spots are sparsely sampled
Parallel magnetic field induced giant magnetoresistance in low density {\it quasi}-two dimensional layers
We provide a possible theoretical explanation for the recently observed giant
positive magnetoresistance in high mobility low density {\it quasi}-two
dimensional electron and hole systems. Our explanation is based on the strong
coupling of the parallel field to the {\it orbital} motion arising from the
{\it finite} layer thickness and the large Fermi wavelength of the {\it
quasi}-two dimensional system at low carrier densities.Comment: 4 pages with 4 figures. Accepted for Publication in Physical Review
Letter
Temperature-Dependence of the Resistivity of a Dilute 2D Electron System in High Parallel Magnetic Field
We report measurements of the resistance of silicon MOSFETs as a function of
temperature in high parallel magnetic fields where the 2D system of electrons
has been shown to be fully spin-polarized. A magnetic field suppresses the
metallic behavior observed in the absence of a magnetic field. In a field of
10.8 T, insulating behavior is found for densities up to n_s approximately 1.35
x 10^{11} cm^{-2} or 1.5 n_c; above this density the resistance is a very weak
function of temperature, varying less than 10% between 0.25 K and 1.90 K. At
low densities the resistance goes to infinity more rapidly as the temperature
is reduced than in zero field and the magnetoresistance diverges as T goes to
0.Comment: 4 pages, including 4 figures. References adde
Metal-insulator transition in a 2D electron gas: Equivalence of two approaches for determining the critical point
The critical electron density for the metal-insulator transition in a
two-dimensional electron gas can be determined by two distinct methods: (i) a
sign change of the temperature derivative of the resistance, and (ii) vanishing
activation energy and vanishing nonlinearity of current-voltage characteristics
as extrapolated from the insulating side. We find that in zero magnetic field
(but not in the presence of a parallel magnetic field), both methods give
equivalent results, adding support to the existence of a true zero-field
metal-insulator transition.Comment: As publishe
In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems
Using a novel technique, we make quantitative measurements of the spin
polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional
holes as a function of an in-plane magnetic field. As the field is increased
the system gradually becomes spin polarized, with the degree of spin
polarization depending on the orientation of the field relative to the crystal
axes. Moreover, the behavior of the system turns from metallic to insulating
\textit{before} it is fully spin polarized. The minority-spin population at the
transition is ~8*10^{9} cm^{-2}, close to the density below which the system
makes a transition to an insulating state in the absence of a magnetic field.Comment: 4 pages with figure
Magnetic Field Induced Spin Polarization of AlAs Two-dimensional Electrons
Two-dimensional (2D) electrons in an in-plane magnetic field become fully
spin polarized above a field B_P, which we can determine from the in-plane
magnetoresistance. We perform such measurements in modulation-doped AlAs
electron systems, and find that the field B_P increases approximately linearly
with 2D electron density. These results imply that the product |g*|m*, where g*
is the effective g-factor and m* the effective mass, is a constant essentially
independent of density. While the deduced |g*|m* is enhanced relative to its
band value by a factor of ~ 4, we see no indication of its divergence as 2D
density approaches zero. These observations are at odds with results obtained
in Si-MOSFETs, but qualitatively confirm spin polarization studies of 2D GaAs
carriers.Comment: 4 pages, 5 figure
Parallel Magnetic Field Induced Transition in Transport in the Dilute Two-Dimensional Hole System in GaAs
A magnetic field applied parallel to the two-dimensional hole system in the
GaAs/AlGaAs heterostructure, which is metallic in the absence of an external
magnetic field, can drive the system into insulating at a finite field through
a well defined transition. The value of resistivity at the transition is found
to depend strongly on density
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