406 research outputs found
Alanine 501 Mutations in Penicillin-Binding Protein 2 from Neisseria gonorrhoeae : Structure, Mechanism, and Effects on Cephalosporin Resistance and Biological Fitness
Resistance of Neisseria gonorrhoeae to expanded-spectrum cephalosporins such as ceftriaxone and cefixime has increased markedly in the past decade. The primary cephalosporin resistance determinant is a mutated penA gene, which encodes the essential peptidoglycan transpeptidase, penicillin-binding protein 2 (PBP2). Decreased susceptibility and resistance can be conferred by mosaic penA alleles containing upward of 60 amino acid changes relative to wild-type PBP2, or by nonmosaic alleles with relatively few mutations, the most important of which occurs at Ala501 located near the active site of PBP2. Recently, fully cefixime- and ceftriaxone-resistant clinical isolates that harbored a mosaic penA allele with an A501P mutation were identified. To examine the potential of mutations at Ala501 to increase resistance to expanded-spectrum cephalosporins, we randomized codon 501 in a mosaic penA allele and transformed N. gonorrhoeae to increased cefixime resistance. Interestingly, only five substitutions of Ala501 (A501V, A501T, A501P, A501R, and A501S) that increased resistance and preserved essential transpeptidase function were isolated. To understand their structural implications, these mutations were introduced into the nonmosaic PBP2-6140CT, which contains four C-terminal mutations present in PBP2 from the penicillin-resistant strain FA6140. The crystal structure of PBP2-6140CT-A501T was determined and revealed ordering of a loop near the active site and a new hydrogen bond involving Thr501 that connects the loop and the SxxK conserved active site motif. The structure suggests that increased rigidity in the active site region is a mechanism for cephalosporin resistance mediated by Ala501 mutations in PBP2
Control of gdhR Expression in Neisseria gonorrhoeae via Autoregulation and a Master Repressor (MtrR) of a Drug Efflux Pump Operon
ABSTRACT The MtrCDE efflux pump of Neisseria gonorrhoeae contributes to gonococcal resistance to a number of antibiotics used previously or currently in treatment of gonorrhea, as well as to host-derived antimicrobials that participate in innate defense. Overexpression of the MtrCDE efflux pump increases gonococcal survival and fitness during experimental lower genital tract infection of female mice. Transcription of mtrCDE can be repressed by the DNA-binding protein MtrR, which also acts as a global regulator of genes involved in important metabolic, physiologic, or regulatory processes. Here, we investigated whether a gene downstream of mtrCDE , previously annotated gdhR in Neisseria meningitidis , is a target for regulation by MtrR. In meningococci, GdhR serves as a regulator of genes involved in glucose catabolism, amino acid transport, and biosynthesis, including gdhA , which encodes an l -glutamate dehydrogenase and is located next to gdhR but is transcriptionally divergent. We report here that in N. gonorrhoeae , expression of gdhR is subject to autoregulation by GdhR and direct repression by MtrR. Importantly, loss of GdhR significantly increased gonococcal fitness compared to a complemented mutant strain during experimental murine infection. Interestingly, loss of GdhR did not influence expression of gdhA , as reported for meningococci. This variance is most likely due to differences in promoter localization and utilization between gonococci and meningococci. We propose that transcriptional control of gonococcal genes through the action of MtrR and GdhR contributes to fitness of N. gonorrhoeae during infection. IMPORTANCE The pathogenic Neisseria species are strict human pathogens that can cause a sexually transmitted infection ( N. gonorrhoeae ) or meningitis or fulminant septicemia ( N. meningitidis ). Although they share considerable genetic information, little attention has been directed to comparing transcriptional regulatory systems that modulate expression of their conserved genes. We hypothesized that transcriptional regulatory differences exist between these two pathogens, and we used the gdh locus as a model to test this idea. For this purpose, we studied two conserved genes ( gdhR and gdhA ) within the locus. Despite general conservation of the gdh locus in gonococci and meningococci, differences exist in noncoding sequences that correspond to promoter elements or potential sites for interacting with DNA-binding proteins, such as GdhR and MtrR. Our results indicate that implications drawn from studying regulation of conserved genes in one pathogen are not necessarily translatable to a genetically related pathogen
PAMELA Measurements of Cosmic-ray Proton and Helium Spectra
Protons and helium nuclei are the most abundant components of the cosmic
radiation. Precise measurements of their fluxes are needed to understand the
acceleration and subsequent propagation of cosmic rays in the Galaxy. We report
precision measurements of the proton and helium spectra in the rigidity range 1
GV-1.2 TV performed by the satellite-borne experiment PAMELA. We find that the
spectral shapes of these two species are different and cannot be well described
by a single power law. These data challenge the current paradigm of cosmic-ray
acceleration in supernova remnants followed by diffusive propagation in the
Galaxy. More complex processes of acceleration and propagation of cosmic rays
are required to explain the spectral structures observed in our data.Comment: 13 pages, 4 figures, link to SOM (with tables) in the references.
This manuscript has been accepted for publication in Science. This version
has not undergone final editing. Please refer to the complete version of
record at http://www.sciencemag.org/ [www.sciencemag.org
The cosmic-ray electron flux measured by the PAMELA experiment between 1 and 625 GeV
Precision measurements of the electron component in the cosmic radiation
provide important information about the origin and propagation of cosmic rays
in the Galaxy. Here we present new results regarding negatively charged
electrons between 1 and 625 GeV performed by the satellite-borne experiment
PAMELA. This is the first time that cosmic-ray electrons have been identified
above 50 GeV. The electron spectrum can be described with a single power law
energy dependence with spectral index -3.18 +- 0.05 above the energy region
influenced by the solar wind (> 30 GeV). No significant spectral features are
observed and the data can be interpreted in terms of conventional diffusive
propagation models. However, the data are also consistent with models including
new cosmic-ray sources that could explain the rise in the positron fraction.Comment: 11 pages, 3 figures, accepted for publication in PR
Observations of the December 13 and 14, 2006, Solar Particle Events in the 80 MeV/n - 3 GeV/n range from space with PAMELA detector
We present the space spectrometer PAMELA observations of proton and helium
fluxes during the December 13 and 14, 2006 solar particle events. This is the
first direct measurement of the solar energetic particles in space with a
single instrument in the energy range from 80 MeV/n up to 3
GeV/n. In the event of December 13 measured energy spectra of solar protons and
helium were compared with results obtained by neutron monitors and other
detectors. Our measurements show a spectral behaviour different from those
derived from the neutron monitor network. No satisfactory analytical fitting
was found for the energy spectra. During the first hours of the December 13
event solar energetic particles spectra were close to the exponential form
demonstrating rather significant temporal evolution. Solar He with energy up to
~1 GeV/n was recorded on December 13. In the event of December 14 energy of
solar protons reached ~600 MeV whereas maximum energy of He was below 100
MeV/n. The spectra were slightly bended in the lower energy range and preserved
their form during the second event. Difference in the particle flux appearance
and temporal evolution in these two events may argue for a special conditions
leading to acceleration of solar particles up to relativistic energies.Comment: Accepted for publication on Astrophysical journa
Hep-2–Adherent Escherichia coli Strains Associated with Acute Infantile Diarrhea, São Paulo, Brazil
In this paired case-control study of infants with diarrhea in São Paulo, we examined the association between HEp-2–adherent Escherichia coli strains and diarrhea. We tested isolates from stool specimens of infants with diarrhea and matched controls in an HEp-2 cell adherence assay; we then hybridized isolates with DNA probes and identified enteropathogenic E.
coli (EPEC), enteroaggregative E. coli (EAEC), and diffusely adherent E. coli (DAEC). From 100 patient-control pairs, we isolated 78 HEp-2–adherent strains; of these, 61 strains were single pathogens identified in stools of infants with diarrhea. While typical EPEC was significantly associated with diarrhea (p<0.001), EAEC was more frequently associated with diarrhea in clinical cases (20%) compared with healthy controls (3%) (p<0.001). Atypical EPEC, showing a localized adherence-like pattern, was also more common in patients than controls (p>0.1). DAEC was isolated with equal frequency from patients and controls (p>0.1)
PAMELA results on the cosmic-ray antiproton flux from 60 MeV to 180 GeV in kinetic energy
The satellite-borne experiment PAMELA has been used to make a new measurement
of the cosmic-ray antiproton flux and the antiproton-to-proton flux ratio which
extends previously published measurements down to 60 MeV and up to 180 GeV in
kinetic energy. During 850 days of data acquisition approximately 1500
antiprotons were observed. The measurements are consistent with purely
secondary production of antiprotons in the galaxy. More precise secondary
production models are required for a complete interpretation of the results.Comment: 11 pages, 3 figures, 1 table. Accepted for publication in Physical
Review Letter
In-flight validation of Metis Visible-light Polarimeter Coronagraph on board Solar Orbiter
Context. The Metis coronagraph is one of the remote-sensing instruments of
the ESA/NASA Solar Orbiter mission. Metis is aimed at the study of the solar
atmosphere and solar wind by simultaneously acquiring images of the solar
corona at two different wavelengths; visible-light (VL) within a band ranging
from 580 nm to 640 nm, and in the HI Ly-alpha 121.6 +/- 10 nm ultraviolet (UV)
light. The visible-light channel includes a polarimeter with electro-optically
modulating Liquid Crystal Variable Retarders (LCVRs) to measure the linearly
polarized brightness of the K-corona to derive the electron density.
Aims. In this paper, we present the first in-flight validation results of the
Metis polarimetric channel together with a comparison to the on-ground
calibrations. It is the validation of the first use in deep space (with hard
radiation environment) of an electro-optical device: a liquid crystal-based
polarimeter.
Methods. We used the orientation of the K-corona's linear polarization vector
during the spacecraft roll maneuvers for the in-flight calibration.
Results. The first in-flight validation of the Metis coronagraph on-board
Solar Orbiter shows a good agreement with the on-ground measurements. It
confirms the expected visible-light channel polarimetric performance. A final
comparison between the first pB obtained by Metis with the polarized brightness
(pB) obtained by the space-based coronagraph LASCO and the ground-based
coronagraph KCor shows the consistency of the Metis calibrated results.Comment: 8 pages, 13 figures, 3 tables, pape
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