10,671 research outputs found
Pre-determining the location of electromigrated gaps by nonlinear optical imaging
In this paper we describe a nonlinear imaging method employed to spatially
map the occurrence of constrictions occurring on an electrically-stressed gold
nanowire. The approach consists at measuring the influence of a tightly focused
ultrafast pulsed laser on the electronic transport in the nanowire. We found
that structural defects distributed along the nanowire are efficient nonlinear
optical sources of radiation and that the differential conductance is
significantly decreased when the laser is incident on such electrically-induced
morphological changes. This imaging technique is applied to pre-determined the
location of the electrical failure before it occurs.Comment: 3 figure
Saturation Mutagenesis of Lysine 12 Leads to the Identification of Derivatives of Nisin A with Enhanced Antimicrobial Activity
peer-reviewedIt is becoming increasingly apparent that innovations from the “golden age” of antibiotics are becoming ineffective, resulting in a pressing need for novel therapeutics. The bacteriocin family of antimicrobial peptides has attracted much attention in recent years as a source of potential alternatives. The most intensively studied bacteriocin is nisin, a broad spectrum lantibiotic that inhibits Gram-positive bacteria including important food pathogens and clinically relevant antibiotic resistant bacteria. Nisin is gene-encoded and, as such, is amenable to peptide bioengineering, facilitating the generation of novel derivatives that can be screened for desirable properties. It was to this end that we used a site-saturation mutagenesis approach to create a bank of producers of nisin A derivatives that differ with respect to the identity of residue 12 (normally lysine; K12). A number of these producers exhibited enhanced bioactivity and the nisin A K12A producer was deemed of greatest interest. Subsequent investigations with the purified antimicrobial highlighted the enhanced specific activity of this modified nisin against representative target strains from the genera Streptococcus, Bacillus, Lactococcus, Enterococcus and Staphylococcus.This work was supported by the Irish Government under the National Development Plan; by the Irish Research Council for Science Engineering and Technology (IRCSET); by Enterprise Ireland; and by Science Foundation Ireland (SFI), through the Alimentary Pharmabiotic Centre (APC) at University College Cork, Ireland, which is supported by the SFI-funded Centre for Science, Engineering and Technology (SFI-CSET) and provided P.D.C., C.H. and R.P.R. with SFI Principal Investigator funding
Efficacy of nisin A and nisin V semi-purified preparations alone and in combination with plant essential oils to control Listeria monocytogenes
peer-reviewedThe foodborne pathogenic bacterium Listeria is known for relatively low morbidity and high mortality rates reaching up to 25-30%. Listeria is a hardy organism and its control in foods represents a significant challenge. Many naturally occurring compounds, including the bacteriocin nisin and a number of plant essential oils, have been widely studied and are reported to be effective as antimicrobial agents against spoilage and pathogenic microorganisms. The aim of this study was to investigate the ability of semi-purified preparations (spp) containing either nisin A or an enhanced bioengineered derivative nisin V, alone and in combination with low concentrations of the essential oils thymol, carvacrol and trans-cinnamaldehyde, to control L. monocytogenes in both laboratory media and model food systems. Combinations of nisin V-containing spp (25 μg/ml) with thymol (0.02%), carvacrol (0.02%) or cinnamaldehyde (0.02%) produced a significantly longer lag phase than any of the essential oil/nisin A combinations. In addition, the log reduction in cell counts achieved by the nisin V + carvacrol or nisin V + cinnamaldehyde combinations was twice that of the equivalent nisin A + essential oil treatment. Significantly, this enhanced activity was validated in model food systems against L. monocytogenes strains of food origin. We conclude that the fermentate form of nisin V in combination with carvacrol and cinnamaldehyde offers significant advantages as a novel, natural and effective means to enhance food safety by inhibiting foodborne pathogens such as L. monocytogenes.This work was supported by the Irish Government under the National Development Plan, through Science Foundation Ireland Investigator awards to C.H. and R.P.R. (10/IN.1/B3027), and C.H., R.P.R. and P.D.C. (06/IN.1/B98)
Constraining the orbit of the possible companion to Beta Pictoris: New deep imaging observations
We recently reported on the detection of a possible planetary-mass companion
to Beta Pictoris at a projected separation of 8 AU from the star, using data
taken in November 2003 with NaCo, the adaptive-optics system installed on the
Very Large Telescope UT4. Eventhough no second epoch detection was available,
there are strong arguments to favor a gravitationally bound companion rather
than a background object. If confirmed and located at a physical separation of
8 AU, this young, hot (~1500 K), massive Jovian companion (~8 Mjup) would be
the closest planet to its star ever imaged, could be formed via core-accretion,
and could explain the main morphological and dynamical properties of the dust
disk. Our goal was to return to Beta Pic five years later to obtain a
second-epoch observation of the companion or, in case of a non-detection,
constrain its orbit. Deep adaptive-optics L'-band direct images of Beta Pic and
Ks-band Four-Quadrant-Phase-Mask (4QPM) coronagraphic images were recorded with
NaCo in January and February 2009. We also use 4QPM data taken in November
2004. No point-like signal with the brightness of the companion candidate
(apparent magnitudes L'=11.2 or Ks ~ 12.5) is detected at projected distances
down to 6.5 AU from the star in the 2009 data. As expected, the non-detection
does not allow to rule out a background object; however, we show that it is
consistent with the orbital motion of a bound companion that got closer to the
star since first observed in 2003 and that is just emerging from behind the
star at the present epoch. We place strong constraints on the possible orbits
of the companion and discuss future observing prospects.Comment: 8 pages, 8 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
Scattering functions of knotted ring polymers
We discuss the scattering function of a Gaussian random polygon with N nodes
under a given topological constraint through simulation. We obtain the Kratky
plot of a Gaussian polygon of N=200 having a fixed knot for some different
knots such as the trivial, trefoil and figure-eight knots. We find that some
characteristic properties of the different Kratky plots are consistent with the
distinct values of the mean square radius of gyration for Gaussian polygons
with the different knots.Comment: 4pages, 3figures, 3table
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