5,010 research outputs found
Automatic estimation of flux distributions of astrophysical source populations
In astrophysics a common goal is to infer the flux distribution of
populations of scientifically interesting objects such as pulsars or
supernovae. In practice, inference for the flux distribution is often conducted
using the cumulative distribution of the number of sources detected at a given
sensitivity. The resulting "-" relationship can be used to
compare and evaluate theoretical models for source populations and their
evolution. Under restrictive assumptions the relationship should be linear. In
practice, however, when simple theoretical models fail, it is common for
astrophysicists to use prespecified piecewise linear models. This paper
proposes a methodology for estimating both the number and locations of
"breakpoints" in astrophysical source populations that extends beyond existing
work in this field. An important component of the proposed methodology is a new
interwoven EM algorithm that computes parameter estimates. It is shown that in
simple settings such estimates are asymptotically consistent despite the
complex nature of the parameter space. Through simulation studies it is
demonstrated that the proposed methodology is capable of accurately detecting
structural breaks in a variety of parameter configurations. This paper
concludes with an application of our methodology to the Chandra Deep Field
North (CDFN) data set.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS750 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Muon-spin rotation measurements of the penetration depth of the Mo_3Sb_7 superconductor
Measurements of the magnetic field penetration depth \lambda in
superconductor Mo_3Sb_7 (T_c~2.1 K) were carried out by means of
muon-spin-rotation. The absolute values of \lambda, the Ginzburg-Landau
parameter \kappa, the first H_{c1} and the second H_{c2} critical fields at T=0
are \lambda(0)=720(100)nm, \kappa(0)=55(9), \mu_0H_{c1}(0)=1.8(3)mT, and
\mu_0H_{c2}(0)=1.9(2)T. The zero temperature value of the superconducting
energy gap \Delta(0) was found to be 0.35(1)meV corresponding to the ratio
2\Delta(0)/k_BT_c=3.83(10). At low temperatures \lambda^{-2}(T) saturates and
becomes constant below T~0.3T_c, in agreement with what is expected for s-wave
BCS superconductors. Our results suggest that Mo_3Sb_7 is a BCS superconductor
with the isotropic energy gapComment: 5 pages, 4 figure
In vitro assessment of Clostridium difficile PCR ribotype 002: the most prevalent C. difficile ribotype in the United Kingdom.
Simon Baines, Iye Ameh, Jane Freeman, W.N. Fawley, M.H. Wilcox, âIn vitro assessment of Clostridium difficile PCR ribotype 002: the most prevalent C. difficile ribotype in the United Kingdomâ, poster presented at the 25th European Congress on Clinical Microbiology and Infectious Diseases, Copenhagen, Denmark, 25-28 August, 2015.Background: Clostridium difficile infection (CDI) causes substantial morbidity and healthcare expenditure across Europe. UK prevalence of C. difficile PCR ribotype 027 (NAP1) has declined dramatically recently and other ribotypes have emerged, including ribotype 002 (CD002); now the most prevalent UK ribotype. CD002 is also responsible for CDI in many countries across Europe, including: France, Germany, Ireland, and The Netherlands. We assessed the in vitro phenotypic characteristics of CD002 from across Europe to determine traits that may contribute to its increasing clinical prevalence. Material/methods: Sixty CD002 were studied: UK isolates from 2007-2008 (geographically distinct, N=15), UK isolates from 2011-2013 (19 locations, N=22), and non-UK European isolates from 2012-2014 (N=23, 20 locations). Antimicrobial susceptibilities (13 antimicrobials) were evaluated using an agar incorporation method. Maximum specific growth rates (ÎŒmax) were calculated and cytotoxin titres (log10-relative units, RU) determined using Vero cell cytotoxicity assays. Biofilm formation was quantified using 96-well microtitre plate assays and sporulation capacities assessed in liquid culture by quantifying spore-formation over 120 h (CFU/mL). Results: All isolates were susceptible metronidazole, vancomycin, tetracycline and linezolid (MICs â€2 mg/L). Clindamycin resistance (MIC â„8 mg/L) was more common in non-UK CD002 (30%) than UK strains (5-13%). Resistance to erythromycin, clarithromycin, nitrofurantoin, chloramphenicol, and moxifloxacin was uncommon (5-7%). MICs for penicillinâs remained below resistance breakpoints, regardless of origin, in all but one isolate (ampicillin MIC 2 mg/L). All CD002 were resistant to trimethoprim (MICs >128 mg/L) and ciprofloxacin (MICs â„8 mg/L). One MDR strain (UK, 2007) was observed that was macrolide, fluoroquinolone, ampicillin, and nitrofurantoin resistant. Significantly faster ÎŒmax was seen in non-UK CD002 (0.92 ±0.058 h-1) than recent/older UK strains (0.76 ±0.063/0.69 ±0.028 h-1 respectively) (P<0.001). Cytotoxin production did not differ significantly (median titres 2-3 RU) between CD002 groups. Recent UK/non-UK CD002 formed significantly greater biofilms by 3 days than asynchronous UK CD002 (P<0.001). Sporulation studies demonstrated that recent UK/non-UK CD002 sporulated more at 24 h than older UK CD002; 18.6-fold/31.2-fold respectively (P<0.05), but by 120 h sporulation did not differ. Conclusions: Recent CD002 from diverse European locations were assessed for traits that may help to explain emergence of CD002 in the UK and compared to asynchronous CD002. Previous studies demonstrated elevated CD002 ÎŒmax compared to hypervirulent ribotypes 027/078; and the present study demonstrated that recent non-UK CD002 ÎŒmax were significantly further elevated vs. UK isolates. Non-UK CD002 were more clindamycin resistant, but other antimicrobial susceptibilities were similar between CD002 groups. Recent CD002 demonstrated significantly increased sporulation capacities at 24 h and more extensive 3 day biofilm formation compared to asynchronous UK CD002, which could enhance their survival and transmission early in an episode CDI. Further phenotypic and genetic studies are required to evaluate further characteristics of CD002 that may be associated with its emergence in the UK.Peer reviewedFinal Published versio
Spatial and Temporal Dynamics of Pacific Oyster Hemolymph Microbiota across Multiple Scales
Unveiling the factors and processes that shape the dynamics of host associated microbial communities (microbiota) under natural conditions is an important part of understanding and predicting an organism's response to a changing environment. The microbiota is shaped by host (i.e., genetic) factors as well as by the biotic and abiotic environment. Studying natural variation of microbial community composition in multiple host genetic backgrounds across spatial as well as temporal scales represents a means to untangle this complex interplay. Here, we combined a spatially-stratified with a longitudinal sampling scheme within differentiated host genetic backgrounds by reciprocally transplanting Pacific oysters between two sites in the Wadden Sea (Sylt and Texel). To further differentiate contingent site from host genetic effects, we repeatedly sampled the same individuals over a summer season to examine structure, diversity and dynamics of individual hemolymph microbiota following experimental removal of resident microbiota by antibiotic treatment. While a large proportion of microbiome variation could be attributed to immediate environmental conditions, we observed persistent effects of antibiotic treatment and translocation suggesting that hemolymph microbial community dynamics is subject to within-microbiome interactions and host population specific factors. In addition, the analysis of spatial variation revealed that the within-site microenvironmental heterogeneity resulted in high small-scale variability, as opposed to large-scale (between-site) stability. Similarly, considerable within-individual temporal variability was in contrast with the overall temporal stability at the site level. Overall, our longitudinal, spatially-stratified sampling design revealed that variation in hemolymph microbiota is strongly influenced by site and immediate environmental conditions, whereas internal microbiome dynamics and oyster-related factors add to their long-term stability. The combination of small and large scale resolution of spatial and temporal observations therefore represents a crucial but underused tool to study host-associated microbiome dynamics
Evidence of Titan's Climate History from Evaporite Distribution
Water-ice-poor, 5-m-bright material on Saturn's moon Titan has
previously been geomorphologically identified as evaporitic. Here we present a
global distribution of the occurrences of the 5-m-bright spectral unit,
identified with Cassini's Visual Infrared Mapping Spectrometer (VIMS) and
examined with RADAR when possible. We explore the possibility that each of
these occurrences are evaporite deposits. The 5-m-bright material covers
1\% of Titan's surface and is not limited to the poles (the only regions with
extensive, long-lived surface liquid). We find the greatest areal concentration
to be in the equatorial basins Tui Regio and Hotei Regio. Our interpretations,
based on the correlation between 5-m-bright material and lakebeds, imply
that there was enough liquid present at some time to create the observed
5-m-bright material. We address the climate implications surrounding a
lack of evaporitic material at the south polar basins: if the south pole basins
were filled at some point in the past, then where is the evaporite
Comment on ``Muon-spin rotation studies of the superconducting properties of Mo_3Sb_7'', Phys.Rev.B 78, 172505 (2008)
In a recent article Tran et al. [Phys. Rev.B 78, 172505 (2008)] report on the
result of the muon-spin rotation (\muSR) measurements of Mo_3Sb_7
superconductor. Based on the analysis of the temperature and the magnetic field
dependence of the Gaussian relaxation rate \sigma_{sc} they suggest that
Mo_3Sb_7 is the superconductor with two isotropic s-wave like gaps. An
additional confirmation was obtained from the specific heat data published
earlier by partly the same group of authors in [Acta Mater. 56, 5694 (2008)].
The purpose of this Comment is to point out that from the analysis made by Tran
et al. the presence of two superconducting energy gaps in Mo_3Sb_7 can not be
justified. The analysis of \muSR data does not account for the reduction of
\sigma_{sc} with increasing temperature, and, hence, yields inaccurate
information on the magnetic penetration depth. The specific heat data can be
satisfactory described within the framework of the one-gap model with the small
residual specific heat component. The experimental data of Tran et al., as well
as our earlier published \muSR data [Phys. Rev. B 78, 014502 (2008)] all seem
to be consistent with is the presence of single isotropic superconducting
energy gap in Mo_3Sb_7.Comment: Comment on Phys.Rev.B 78, 172505 (2008
Para-cresol production by Clostridium difficile affects microbial diversity and membrane integrity of Gram-negative bacteria
Clostridium difficile is a Gram-positive spore-forming anaerobe and a major cause of antibiotic-associated diarrhoea. Disruption of the commensal microbiota, such as through treatment with broad-spectrum antibiotics, is a critical precursor for colonisation by C. difficile and subsequent disease. Furthermore, failure of the gut microbiota to recover colonisation resistance can result in recurrence of infection. An unusual characteristic of C. difficile among gut bacteria is its ability to produce the bacteriostatic compound para-cresol (p-cresol) through fermentation of tyrosine. Here, we demonstrate that the ability of C. difficile to produce p-cresol in vitro provides a competitive advantage over gut bacteria including Escherichia coli, Klebsiella oxytoca and Bacteroides thetaiotaomicron. Metabolic profiling of competitive co-cultures revealed that acetate, alanine, butyrate, isobutyrate, p-cresol and p-hydroxyphenylacetate were the main metabolites responsible for differentiating the parent strain C. difficile (630Îerm) from a defined mutant deficient in p-cresol production. Moreover, we show that the p-cresol mutant displays a fitness defect in a mouse relapse model of C. difficile infection (CDI). Analysis of the microbiome from this mouse model of CDI demonstrates that colonisation by the p-cresol mutant results in a distinctly altered intestinal microbiota, and metabolic profile, with a greater representation of Gammaproteobacteria, including the Pseudomonales and Enterobacteriales. We demonstrate that Gammaproteobacteria are susceptible to exogenous p-cresol in vitro and that there is a clear divide between bacterial Phyla and their susceptibility to p-cresol. In general, Gram-negative species were relatively sensitive to p-cresol, whereas Gram-positive species were more tolerant. This study demonstrates that production of p-cresol by C. difficile has an effect on the viability of intestinal bacteria as well as the major metabolites produced in vitro. These observations are upheld in a mouse model of CDI, in which p-cresol production affects the biodiversity of gut microbiota and faecal metabolite profiles, suggesting that p-cresol production contributes to C. difficile survival and pathogenesis.Peer reviewedFinal Published versio
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