8,569 research outputs found
Efflux-mediated multiresistance in Gram-negative bacteria
ABSTRACTMultiresistance in Gram-negative pathogens, particularly Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Acinetobacter spp. and the Enterobacteriaceae, is a significant problem in medicine today. While multiple mechanisms often contribute to multiresistance, a broadly distributed family of three-component multidrug efflux systems is an increasingly recognised determinant of both intrinsic and acquired multiresistance in these organisms. Homologues of these efflux systems are also readily identifiable in the genome sequences of a wide range of Gram-negative organisms, pathogens and non-pathogens alike, where they probably promote efflux-mediated resistance to multiple antimicrobials. Significantly, these systems often accommodate biocides, raising the spectre of biocide-mediated selection of multiresistance in Gram-negative pathogens. While there is some debate as to the natural function of these efflux systems, only some of which are inducible by their antimicrobial substrates, their contribution to resistance in a variety of pathogens nonetheless makes them reasonable targets for therapeutic intervention. Indeed, given the incredible chemical diversity of substrates accommodated by these efflux systems, it is likely that many novel or yet to be discovered antimicrobials will themselves be efflux substrates and, as such, efflux inhibitors may become an important component of Gram-negative antimicrobial therapy
Reckoning Roanoke: A Historiographical Examination of the Lost Colony
The disappearance of the Lost Colony of Roanoke is an American mystery which has baffled historians for centuries. This paper takes a historiographical view of the works of academics Lee Miller and James Horn, comparing their research and conclusions on the topic. Miller’s belief that the colony was sabotaged by English secretary of state Sir. Francis Walsingham and Horn’s theory that an English desire for mineral wealth and poor preparation for survival in the New World brought about their demise are each analyzed for their legitimacy, research gaps, and possible biases. Through this analysis, it is concluded that the field of study on the attempted colonization of Roanoke and the settlers who disappeared there could benefit from a broader consideration for factors, outside of English politics and Native American relations, which may have contributed to the Lost Colonists\u27 disappearance
Nonlinear c-axis transport in Bi_2Sr_2CaCu_2O_(8+d) from two-barrier tunneling
Motivated by the peculiar features observed through intrinsic tunneling
spectroscopy of BiSrCaCuO mesas in the normal state,
we have extended the normal state two-barrier model for the c-axis transport
[M. Giura et al., Phys. Rev. B {\bf 68}, 134505 (2003)] to the analysis of
curves. We have found that the purely normal-state model reproduces all
the following experimental features: (a) the parabolic -dependence of
in the high- region (above the conventional pseudogap temperature),
(b) the emergence and the nearly voltage-independent position of the "humps"
from this parabolic behavior lowering the temperature, and (c) the crossing of
the absolute curves at a characteristic voltage . Our
findings indicate that conventional tunneling can be at the origin of most of
the uncommon features of the c axis transport in
BiSrCaCuO. We have compared our calculations to
experimental data taken in severely underdoped and slightly underdoped
BiSrCaCuO small mesas. We have found good agreement
between the data and the calculations, without any shift of the calculated
dI/dV on the vertical scale. In particular, in the normal state (above
) simple tunneling reproduces the experimental dI/dV quantitatively.
Below quantitative discrepancies are limited to a simple rescaling of
the voltage in the theoretical curves by a factor 2. The need for such
modifications remains an open question, that might be connected to a change of
the charge of a fraction of the carriers across the pseudogap opening.Comment: 7 pages, 5 figure
"Swarm relaxation": Equilibrating a large ensemble of computer simulations
It is common practice in molecular dynamics and Monte Carlo computer
simulations to run multiple, separately-initialized simulations in order to
improve the sampling of independent microstates. Here we examine the utility of
an extreme case of this strategy, in which we run a large ensemble of
independent simulations (a "swarm"), each of which is relaxed to equilibrium.
We show that if is of order , we can monitor the swarm's relaxation
to equilibrium, and confirm its attainment, within , where
is the equilibrium relaxation time. As soon as a swarm of this size
attains equilibrium, the ensemble of final microstates from each run is
sufficient for the evaluation of most equilibrium properties without further
sampling. This approach dramatically reduces the wall-clock time required,
compared to a single long simulation, by a factor of several hundred, at the
cost of an increase in the total computational effort by a small factor. It is
also well-suited to modern computing systems having thousands of processors,
and is a viable strategy for simulation studies that need to produce
high-precision results in a minimum of wall-clock time. We present results
obtained by applying this approach to several test cases.Comment: 12 pages. To appear in Eur. Phy. J. E, 201
Crystal Nucleation in a Supercooled Liquid with Glassy Dynamics
In simulations of supercooled, high-density liquid silica we study a range of
temperature T in which we find both crystal nucleation, as well as the
characteristic dynamics of a glass forming liquid, including a breakdown of the
Stokes-Einstein relation. We find that the liquid cannot be observed below a
homogeneous nucleation limit (HNL) at which the liquid crystallizes faster than
it can equilibrate. We show that the HNL would occur at lower T, and perhaps
not at all, if the Stokes-Einstein relation were obeyed, and hence that glassy
dynamics plays a central role in setting a crystallization limit on the liquid
state in this case. We also explore the relation of the HNL to the Kauzmann
temperature, and test for spinodal-like effects near the HNL.Comment: 4 pages, 4 figure
Simulator test to study hot-flow problems related to a gas cooled reactor
An advance study of materials, fuel injection, and hot flow problems related to the gas core nuclear rocket is reported. The first task was to test a previously constructed induction heated plasma GCNR simulator above 300 kW. A number of tests are reported operating in the range of 300 kW at 10,000 cps. A second simulator was designed but not constructed for cold-hot visualization studies using louvered walls. A third task was a paper investigation of practical uranium feed systems, including a detailed discussion of related problems. The last assignment resulted in two designs for plasma nozzle test devices that could be operated at 200 atm on hydrogen
Reducing Polarization Mode Dispersion With Controlled Polarization Rotations
One of the fundamental limitations to high bit rate, long distance,
telecommunication in optical fibers is Polarization Mode Dispersion (PMD). Here
we introduce a conceptually new method to reduce PMD in optical fibers by
carrying out controlled rotations of polarization at predetermined locations
along the fiber. The distance between these controlled polarization rotations
must be less than both the beat length and the mode coupling length of the
fiber. This method can also be combined with the method in which the fiber is
spun while it drawn. The incidence of imperfections on the efficiency of the
method is analysed.Comment: 4 page
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