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排序方式: 共有239条查询结果,搜索用时 15 毫秒
71.
Croucher NJ Harris SR Fraser C Quail MA Burton J van der Linden M McGee L von Gottberg A Song JH Ko KS Pichon B Baker S Parry CM Lambertsen LM Shahinas D Pillai DR Mitchell TJ Dougan G Tomasz A Klugman KP Parkhill J Hanage WP Bentley SD 《Science (New York, N.Y.)》2011,331(6016):430-434
Epidemiological studies of the naturally transformable bacterial pathogen Streptococcus pneumoniae have previously been confounded by high rates of recombination. Sequencing 240 isolates of the PMEN1 (Spain(23F)-1) multidrug-resistant lineage enabled base substitutions to be distinguished from polymorphisms arising through horizontal sequence transfer. More than 700 recombinations were detected, with genes encoding major antigens frequently affected. Among these were 10 capsule-switching events, one of which accompanied a population shift as vaccine-escape serotype 19A isolates emerged in the USA after the introduction of the conjugate polysaccharide vaccine. The evolution of resistance to fluoroquinolones, rifampicin, and macrolides was observed to occur on multiple occasions. This study details how genomic plasticity within lineages of recombinogenic bacteria can permit adaptation to clinical interventions over remarkably short time scales. 相似文献
72.
Lee M Katz HE Erben C Gill DM Gopalan P Heber JD McGee DJ 《Science (New York, N.Y.)》2002,298(5597):1401-1403
A major challenge to increasing bandwidth in optical telecommunications is to encode electronic signals onto a lightwave carrier by modulating the light up to very fast rates. Polymer electro-optic materials have the necessary properties to function in photonic devices beyond the 40-GHz bandwidth currently available. An appropriate choice of polymers is shown to effectively eliminate the factors contributing to an optical modulator's decay in the high-frequency response. The resulting device modulates light with a bandwidth of 150 to 200 GHz and produces detectable modulation signal at 1.6 THz. These rates are faster than anticipated bandwidth requirements for the foreseeable future. 相似文献
73.
The Tennessee Valley Authority (TVA) and the Electric Power Research Institute (EPRI) have developed software to assist utilities in coordinating power and reservoir operations while balancing multiple objectives. The INTEGRAL project consists of a Decision Support System (DSS) and a Power and Reservoir System Scheduling Model (PRSYM). The DSS, as tailored to the TVA power and reservoir system, is the TVA-EPRI River Resource Aid (TERRA). TERRA is a decision support system that integrates tracking, display, and modeling tools. TERRA provides historical, current, and forecast data to users at several locations to help, resolve problems in operation, forecasting, and planning. Graphical displays are featured for individual power plants and reservoirs and on a system-wide basis. TERRA tracks constraints that affect operations and flags non-compliance. Constraints consist of flow, dissolved oxygen, and temperature limits for regulatory compliance, recreation, lake improvement, and special operations. Numerical models may be run from TERRA to predict water temperatures and flows at selected power plants to aid in reservoir operations to comply with thermal limits and minimum flow requirements. TERRA uses a geographic information system (GIs) background map of the Tennessee River Valley. The TERRA software is adaptable to other reservoir systems by changing the GIs background map and reconfiguring other system features. 相似文献
74.
Winter-hardy faba bean (Vicia faba L.) from northern Europe is represented by a rather narrow gene pool. Limited selection gains for overwintering below ?25 °C have restricted the adoption of this crop. Therefore, the faba bean collection maintained by the USDA-ARS National Plant Germplasm System (NPGS) was utilized to broaden this genetic base by identifying potentially new sources of winter-hardiness using a modified mass selection scheme. From an initial source population, bulk-harvested micro-plots of 466 NPGS accessions, four different bulks were formed by harvesting seeds from plants that survived at four locations representing a range of overwintering selection environments across southeastern Washington. These four bulk populations were then mass selected for three cycles based on winter survival along with a selected group of advanced northern European populations or breeding lines at two southeastern Washington locations with divergent climates. This procedure generated breeding materials with winter-hardiness comparable to the northern European populations. We observed that Northern European populations showed slower annual gains in percent survival (<5%) than NPGS bulks (>5%). Further, the frequency of large seeded populations (>80 g × 100 seed?1) was reduced over time, suggesting an association between seed size and overwintering. The breeding materials generated by mass selection are useful for the future improvement of faba bean as a fall-sown pulse or cover crop. 相似文献
75.
J Cernicharo B Lefloch P Cox D Cesarsky C Esteban F Yusef-Zadeh DI Mendez J Acosta-Pulido Garcia Lopez RJ A Heras 《Science (New York, N.Y.)》1998,282(5388):462-465
The Trifid nebula is a young (10(5) years) galactic HII region where several protostellar sources have been detected with the infrared space observatory. The sources are massive (17 to 60 solar masses) and are associated with molecular gas condensations at the edges or inside the nebula. They appear to be in an early evolutionary stage and may represent the most recent generation of stars in the Trifid. These sources range from dense, apparently still inactive cores to more evolved sources, undergoing violent mass ejection episodes, including a source that powers an optical jet. These observations suggest that the protostellar sources may have evolved by induced star formation in the Trifid nebula. 相似文献
76.
PO Wennberg TF Hanisco L Jaegle DJ Jacob EJ Hintsa EJ Lanzendorf JG Anderson R Gao ER Keim SG Donnelly LAD Negro DW Fahey SA McKeen RJ Salawitch CR Webster RD May RL Herman MH Proffitt JJ Margitan EL Atlas SM Schauffler F Flocke CT McElroy TP Bui 《Science (New York, N.Y.)》1998,279(5347):49-53
The concentrations of the hydrogen radicals OH and HO2 in the middle and upper troposphere were measured simultaneously with those of NO, O3, CO, H2O, CH4, non-methane hydrocarbons, and with the ultraviolet and visible radiation field. The data allow a direct examination of the processes that produce O3 in this region of the atmosphere. Comparison of the measured concentrations of OH and HO2 with calculations based on their production from water vapor, ozone, and methane demonstrate that these sources are insufficient to explain the observed radical concentrations in the upper troposphere. The photolysis of carbonyl and peroxide compounds transported to this region from the lower troposphere may provide the source of HOx required to sustain the measured abundances of these radical species. The mechanism by which NO affects the production of O3 is also illustrated by the measurements. In the upper tropospheric air masses sampled, the production rate for ozone (determined from the measured concentrations of HO2 and NO) is calculated to be about 1 part per billion by volume each day. This production rate is faster than previously thought and implies that anthropogenic activities that add NO to the upper troposphere, such as biomass burning and aviation, will lead to production of more O3 than expected. 相似文献
77.
A previously unknown solid phase of H2O has been identified by its peculiar growth patterns, distinct pressure-temperature melting relations, and vibrational Raman spectra. Morphologies of ice crystals and their pressure-temperature melting relations were directly observed in a hydrothermal diamond-anvil cell for H2O bulk densities between 1203 and 1257 kilograms per cubic meter at temperatures between -10 degrees and 50 degreesC. Under these conditions, four different ice forms were observed to melt: two stable phases, ice V and ice VI, and two metastable phases, ice IV and the new ice phase. The Raman spectra and crystal morphology are consistent with a disordered anisotropic structure with some similarities to ice VI. 相似文献
78.
79.
T Grandin K McGee J L Lanier 《Journal of the American Veterinary Medical Association》1999,214(10):1531-1533
OBJECTIVE: To assess the prevalence of severe welfare problems in horses that arrive at slaughter plants and to identify horses that were unfit for travel. DESIGN: Prevalence survey. ANIMALS: 1,008 horses. PROCEDURE: Horses arriving at 2 slaughter plants were observed. The following were rated severe welfare problems in horses: body condition scores of 1 or 2 (emaciated) of 9; recumbency (down) or the inability to walk; fractured limbs or other foot or limb problems that extremely impaired mobility; severe wounds, such as deep cuts, extensive lacerations, abrasions on the head or back, eye injuries, neglected purulent lesions, and numerous bite and kick marks over extensive areas of the body; and dead on arrival. Bruises on carcasses were tabulated to further assess injuries. Horses that had been loaded with a fractured limb, arrived nonambulatory, had severe lameness that interfered with mobility, were weak and emaciated, or were dead on arrival or died shortly after arrival were considered unfit for travel. RESULTS: Ninety-two percent (930/1,008) of the horses arrived in good condition, and 7.7% (78) had a condition that was rated a serious welfare problem. Thirty horses (3%) had a body condition score of 1 or 2, 12 (1.0%) had foot and limb problems (other than fractures), 4 (0.4%) had fractured limbs, 18 (2.0%) had deep cuts, lacerations, or injuries from bites, 8 (0.8%) were nonambulatory or dead on arrival, 2 (0.2%) had deformities, 3 (0.3%) had extensive purulent lesions, and 1 (0.1%) had a behavior problem. Characteristic patterns of 51% of carcass bruises indicated that they were caused by bites or kicks. Fighting was the major cause of injuries that occurred during transport and marketing. Fifteen (1.5%) horses were unfit for travel. Abuse or neglect by owners was the cause of 77% of the severe welfare problems observed. CLINICAL IMPLICATIONS: To decrease the number of injuries that result from fighting when transporting horses to slaughter plants, aggressive mares and geldings that continually attack other horses must be segregated. 相似文献
80.