9,290 research outputs found
PBP4: A New Perspective on Staphylococcus aureus β-Lactam Resistance.
β-lactam antibiotics are excellent drugs for treatment of staphylococcal infections, due to their superior efficacy and safety compared to other drugs. Effectiveness of β-lactams is severely compromised due to resistance, which is widespread among clinical strains of Staphylococcus aureus. β-lactams inhibit bacterial cells by binding to penicillin binding proteins (PBPs), which perform the penultimate steps of bacterial cell wall synthesis. Among PBPs of S. aureus, PBP2a has received the most attention for the past several decades due to its preeminent role in conferring both high-level and broad-spectrum resistance to the entire class of β-lactam drugs. Studies on PBP2a have thus unraveled incredible details of its mechanism of action. We have recently identified that an uncanonical, low molecular weight PBP of S. aureus, PBP4, can also provide high-level and broad-spectrum resistance to the entire class of β-lactam drugs at a level similar to that of PBP2a. The role of PBP4 has typically been considered not so important for β-lactam resistance of S. aureus, and as a result its mode of action remains largely unknown. In this article, we review our current knowledge of PBP4 mediating β-lactam resistance in S. aureus
The Stochastic Reach-Avoid Problem and Set Characterization for Diffusions
In this article we approach a class of stochastic reachability problems with
state constraints from an optimal control perspective. Preceding approaches to
solving these reachability problems are either confined to the deterministic
setting or address almost-sure stochastic requirements. In contrast, we propose
a methodology to tackle problems with less stringent requirements than almost
sure. To this end, we first establish a connection between two distinct
stochastic reach-avoid problems and three classes of stochastic optimal control
problems involving discontinuous payoff functions. Subsequently, we focus on
solutions of one of the classes of stochastic optimal control problems---the
exit-time problem, which solves both the two reach-avoid problems mentioned
above. We then derive a weak version of a dynamic programming principle (DPP)
for the corresponding value function; in this direction our contribution
compared to the existing literature is to develop techniques that admit
discontinuous payoff functions. Moreover, based on our DPP, we provide an
alternative characterization of the value function as a solution of a partial
differential equation in the sense of discontinuous viscosity solutions, along
with boundary conditions both in Dirichlet and viscosity senses. Theoretical
justifications are also discussed to pave the way for deployment of
off-the-shelf PDE solvers for numerical computations. Finally, we validate the
performance of the proposed framework on the stochastic Zermelo navigation
problem
Computer aided synthesis: a game theoretic approach
In this invited contribution, we propose a comprehensive introduction to game
theory applied in computer aided synthesis. In this context, we give some
classical results on two-player zero-sum games and then on multi-player non
zero-sum games. The simple case of one-player games is strongly related to
automata theory on infinite words. All along the article, we focus on general
approaches to solve the studied problems, and we provide several illustrative
examples as well as intuitions on the proofs.Comment: Invitation contribution for conference "Developments in Language
Theory" (DLT 2017
The Einstein-Cartan-Elko system
The present paper analyses the Einstein-Cartan theory of gravitation with
Elko spinors as sources of curvature and torsion. After minimally coupling the
Elko spinors to torsion, the spin angular momentum tensor is derived and its
structure is discussed. It shows a much richer structure than the Dirac
analogue and hence it is demonstrated that spin one half particles do not
necessarily yield only an axial vector torsion component. Moreover, it is
argued that the presence of Elko spinors partially solves the problem of
minimally coupling Maxwell fields to Einstein-Cartan theory.Comment: 12 pages, no figure
Gravitational Slingshot of Young Massive Stars in Orion
The Orion Nebula Cluster (ONC) is the nearest region of massive star
formation and thus a crucial testing ground for theoretical models. Of
particular interest amongst the ONC's ~1000 members are: \theta^1 Ori C, the
most massive binary in the cluster with stars of masses 38 and 9 MSun (Kraus et
al. 2009); the Becklin-Neugebauer (BN) object, a 30 km/s runaway star of ~8
MSun (Tan 2004); and the Kleinmann-Low (KL) nebula protostar, a
highly-obscured, ~15 MSun object still accreting gas while also driving a
powerful, apparently "explosive" outflow (Allen & Burton 1993). The unusual
behavior of BN and KL is much debated: How did BN acquire its high velocity?
How is this related to massive star formation in the KL nebula? Here we report
the results of a systematic survey using ~ 10^7 numerical experiments of
gravitational interactions of the \theta^1C and BN stars. We show that
dynamical ejection of BN from this triple system at its observed velocity
leaves behind a binary with total energy and eccentricity matching those
observed for \theta^1C. Five other observed properties of \theta^C are also
consistent with it having ejected BN and altogether we estimate there is only a
<~ 10^{-5} probability that \theta^1C has these properties by chance. We
conclude that BN was dynamically ejected from the \theta^1C system about 4,500
years ago. BN has then plowed through the KL massive-star-forming core within
the last 1,000 years causing its recently-enhanced accretion and outflow
activity.Comment: 16 pages, 9 figures, 1 table, accepted to Ap
Collapsing Perfect Fluid in Higher Dimensional Spherical Spacetimes
The general metric for N-dimensional spherically symmetric and conformally
flat spacetimes is given, and all the homogeneous and isotropic solutions for a
perfect fluid with the equation of state are found. These
solutions are then used to model the gravitational collapse of a compact ball.
It is found that when the collapse has continuous self-similarity, the
formation of black holes always starts with zero mass, and when the collapse
has no such a symmetry, the formation of black holes always starts with a mass
gap.Comment: Class. Quantum Grav. 17 (2000) 2589-259
Sex differences in DNA methylation and expression in zebrafish brain: a test of an extended ‘male sex drive’ hypothesis
The sex drive hypothesis predicts that stronger selection on male traits has resulted in masculinization of the genome. Here we test whether such masculinizing effects can be detected at the level of the transcriptome and methylome in the adult zebrafish brain. Although methylation is globally similar, we identified 914 specific differentially methylated CpGs (DMCs) between males and females (435 were hypermethylated and 479 were hypomethylated in males compared to females). These DMCs were prevalent in gene body, intergenic regions and CpG island shores. We also discovered 15 distinct CpG clusters with striking sex-specific DNA methylation differences. In contrast, at transcriptome level, more female-biased genes than male-biased genes were expressed, giving little support for the male sex drive hypothesis. Our study provides genome-wide methylome and transcriptome assessment and sheds light on sex-specific epigenetic patterns and in zebrafish for the first time
Chemotaxis When Bacteria Remember: Drift versus Diffusion
{\sl Escherichia coli} ({\sl E. coli}) bacteria govern their trajectories by
switching between running and tumbling modes as a function of the nutrient
concentration they experienced in the past. At short time one observes a drift
of the bacterial population, while at long time one observes accumulation in
high-nutrient regions. Recent work has viewed chemotaxis as a compromise
between drift toward favorable regions and accumulation in favorable regions. A
number of earlier studies assume that a bacterium resets its memory at tumbles
-- a fact not borne out by experiment -- and make use of approximate
coarse-grained descriptions. Here, we revisit the problem of chemotaxis without
resorting to any memory resets. We find that when bacteria respond to the
environment in a non-adaptive manner, chemotaxis is generally dominated by
diffusion, whereas when bacteria respond in an adaptive manner, chemotaxis is
dominated by a bias in the motion. In the adaptive case, favorable drift occurs
together with favorable accumulation. We derive our results from detailed
simulations and a variety of analytical arguments. In particular, we introduce
a new coarse-grained description of chemotaxis as biased diffusion, and we
discuss the way it departs from older coarse-grained descriptions.Comment: Revised version, journal reference adde
Functional limit theorems for random regular graphs
Consider d uniformly random permutation matrices on n labels. Consider the
sum of these matrices along with their transposes. The total can be interpreted
as the adjacency matrix of a random regular graph of degree 2d on n vertices.
We consider limit theorems for various combinatorial and analytical properties
of this graph (or the matrix) as n grows to infinity, either when d is kept
fixed or grows slowly with n. In a suitable weak convergence framework, we
prove that the (finite but growing in length) sequences of the number of short
cycles and of cyclically non-backtracking walks converge to distributional
limits. We estimate the total variation distance from the limit using Stein's
method. As an application of these results we derive limits of linear
functionals of the eigenvalues of the adjacency matrix. A key step in this
latter derivation is an extension of the Kahn-Szemer\'edi argument for
estimating the second largest eigenvalue for all values of d and n.Comment: Added Remark 27. 39 pages. To appear in Probability Theory and
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