全文获取类型
收费全文 | 1548篇 |
免费 | 208篇 |
国内免费 | 27篇 |
出版年
2020年 | 12篇 |
2018年 | 16篇 |
2016年 | 21篇 |
2015年 | 44篇 |
2014年 | 41篇 |
2013年 | 73篇 |
2012年 | 72篇 |
2011年 | 79篇 |
2010年 | 52篇 |
2009年 | 52篇 |
2008年 | 52篇 |
2007年 | 58篇 |
2006年 | 81篇 |
2005年 | 75篇 |
2004年 | 60篇 |
2003年 | 66篇 |
2002年 | 57篇 |
2001年 | 38篇 |
2000年 | 58篇 |
1999年 | 48篇 |
1998年 | 27篇 |
1997年 | 25篇 |
1996年 | 21篇 |
1995年 | 14篇 |
1994年 | 21篇 |
1993年 | 14篇 |
1992年 | 37篇 |
1991年 | 33篇 |
1990年 | 32篇 |
1989年 | 37篇 |
1988年 | 23篇 |
1987年 | 37篇 |
1986年 | 28篇 |
1985年 | 24篇 |
1984年 | 30篇 |
1983年 | 17篇 |
1982年 | 13篇 |
1981年 | 19篇 |
1980年 | 27篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 23篇 |
1976年 | 19篇 |
1975年 | 12篇 |
1974年 | 19篇 |
1973年 | 12篇 |
1972年 | 17篇 |
1969年 | 10篇 |
1966年 | 10篇 |
1965年 | 10篇 |
排序方式: 共有1783条查询结果,搜索用时 31 毫秒
1.
OBJECTIVE: To assess the relation of smoking and alcohol and coffee consumption to active Helicobacter pylori infection. DESIGN: Cross sectional study of patients attending a general practitioner. Active H pylori infection was measured by the 15C-urea breath test and detailed quantitative information on smoking and on alcohol and coffee consumption was obtained by a standardised self administered questionnaire. SETTING: One general practice in Germany. SUBJECTS: 447 patients aged 15-79 who had not had peptic ulcer disease or treatment for H pylori infection. MAIN OUTCOME MEASURES: Prevalence of H pylori infection according to smoking and alcohol and coffee consumption. RESULTS: Overall prevalence of infection was 21% (94/447). There was no significant relation between smoking and active H pylori infection. Alcohol consumption showed a negative dose-response relation and coffee consumption a positive dose-response relation with active infection. After adjustment for potential confounders, the odds ratios for patients who drank < or = 75 g and > 75 g of ethanol a week compared with non-drinkers were 0.90 (95% confidence interval 0.55 to 1.59) and 0.33 (0.16 to 0.68), respectively (P value for trend 0.005, assuming that 1 litre of beer and 0.51 of wine contain on average 50 g of ethanol in south Germany). Adjusted odds ratios for patients who drank < 3 cups and > or = 3 cups of coffee per day compared with those who did not drink coffee were 1.49 (0.71 to 3.12) and 2.49 (1.23 to 5.03), respectively (P value for trend 0.007). CONCLUSION: These results suggest a protective effect of alcohol consumption against active infection with H pylori and an opposite effect of coffee consumption. 相似文献
2.
C. PRADO E CASTRO A. SERRANO P. MARTINS DA SILVA M. D. GARCÍA 《Medical and veterinary entomology》2012,26(4):417-431
Information on Diptera community, seasonality and successional patterns in every geographical region is fundamental for the use of flies as forensic indicators of time of death. In order to obtain these data from the Lisbon area (Portugal), experiments were conducted during the four seasons of the year, using piglet carcasses as animal models. Five stages were recognized during the decomposition process. The stages, besides visually defined, could be separated taking into account the occurrence and abundance of the specific groups of Diptera collected. In general, the bloated stage recorded higher abundance and species‐richness values. Seventy‐one species were identified, belonging to 39 families, in a total of 20 144 adult Diptera collected. Autumn yielded the highest values of species richness, whereas winter had the lowest. In all seasons of the year, Calliphoridae was the dominant family; Muscidae and Fanniidae were very abundant as well. Calliphora vicina Robineau‐Desvoidy, Calliphora vomitoria (Linnaeus), Chrysomya albiceps (Wiedemann), Lucilia ampullacea Villeneuve, Lucilia caesar (Linnaeus), Lucilia sericata (Meigen) (Calliphoridae), Hydrotaea ignava (Harris), Muscina prolapsa (Harris), Synthesiomyia nudiseta (van der Wulp) (Muscidae), Piophila megastigmata McAlpine, Stearibia nigriceps (Meigen) (Piophilidae) and Nemopoda nitidula (Fallén) (Sepsidae) were revealed to be very important members of the Diptera community collected. The necrophagous behaviour, demonstrated by their immatures, using carrion as a food source makes them useful forensic indicator species. Also of relevance is the presence of Chrysomya megacephala (Fabricius), S. nudiseta and P. megastigmata, foreign species established in the local carrion communities. This study also marks the first record of S. nudiseta in Portugal. 相似文献
3.
Nucleotide sequence of the gene encoding the hydrogenase from Desulfovibrio vulgaris (Hildenborough) 总被引:22,自引:0,他引:22
The nucleotide sequence of the 4.7-kb SalI/EcoRI insert of plasmid pHV 15 containing the hydrogenase gene from Desulfovibrio vulgaris (Hildenborough) has been determined with the dideoxy chain-termination method. The structural gene for hydrogenase encodes a protein product of molecular mass 45820 Da. The NH2-terminal sequence of the enzyme deduced from the nucleic acid sequence corresponds exactly to the amino acid sequence determined by Edman degradation. The nucleic acid sequence indicates that a N-formylmethionine residue precedes the NH2-terminal amino acid Ser-1. There is no evidence for a leader sequence. The NH2-terminal part of the hydrogenase shows homology to the bacterial [8Fe-8S] ferredoxins. The sequence Cys-Ile-Xaa-Cys-Xaa-Xaa-Cys-Xaa-Xaa-Xaa-Cys-Pro-Xaa-Xaa-Ala-(Ile) occurs twice both in the hydrogenase and in [8Fe-8S] ferredoxins, where the Cys residues have been shown to coordinate two [4Fe-4S] clusters [Adman, E. T., Sieker, L. C. and Jensen, L. H. (1973) J. Biol. Chem. 248, 3987-3996]. These results, therefore, suggest that two electron-transferring ferredoxin-like [4Fe-4S] clusters are located in the NH2-terminal segment of the hydrogenase molecule. There are ten more Cys residues but it is not clear which four of these could participate in the formation of the third cluster, which is thought to be the hydrogen binding centre. Another gene, encoding a protein of molecular mass 13493 Da, was found immediately downstream from the gene for the 46-kDa hydrogenase. The nucleic acid sequence suggests that the hydrogenase and the 13.5-kDa protein belong to a single operon and are coordinately expressed. Since dodecylsulfate gel electrophoresis of purified hydrogenase indicates the presence of a 13.5-kDa polypeptide in addition to the 46-kDa component, it is proposed that the hydrogenase from D. vulgaris (Hildenborough) is a two-subunit enzyme. 相似文献
4.
A E McDaniels R H Bordner P S Gartside J R Haines K P Brenner C C Rankin 《Applied and environmental microbiology》1985,50(4):755-762
Standard procedures for analyzing drinking water stress the need to adhere to the time and temperature conditions recommended for holding samples collected for microbiological testing. The National Drinking Water Laboratory Certification Program requires compliance with these holding limits, but some investigators have reported difficulties in meeting them. Research was conducted by standard analytical methods to determine if changes occurred when samples were held at 5 and 22 degrees C and analyzed at 0, 24, 30, and 48 h. Samples were analyzed for coliforms by the membrane filter and fermentation-tube procedures and for heterotrophs by the pour plate method. A total of 17 treated water samples were collected from a large municipal distribution system from August to December 1981, and 12 samples were collected from January to May 1983. The samples were dosed with coliforms previously isolated from the water system, Enterobacter cloacae in 1981 and Citrobacter freundii in 1983. The coliform counts declined linearly over time, and the rates of decline were significant at both 5 and 22 degrees C. Within 24 h at 22 degrees C, levels of E. cloacae and C. freundii decreased by 47 and 26%, respectively, and at 5 degrees C, E. cloacae numbers declined by 23%. Results from these representative laboratory-grown coliforms reinforced those previously obtained with naturally occurring coliforms under the same experimental conditions. Significantly, some samples with initially unacceptable counts (greater than 4/100 ml) met the safe drinking water limits after storage at 24 h at 5 and 22 degrees C and would have been classified as satisfactory.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
5.
6.
L. Pavl J. Vosáhlová H. Klierová M. Prouza K. Demnerová V. Brenner 《Journal of applied microbiology》1999,87(3):381-386
Two polychlorinated biphenyl-contaminated sites in the Czech Republic, a soil at Zamberk and a sediment sludge at Milevsko, were screened for the presence of chlorobenzoate degraders. Sixteen different chlorobenzoate degraders were isolated from the soil compared with only three strains isolated from the sediment. From these strains, only four soil degraders and one strain isolated from the sediment, respectively, were shown to possess a complete chlorobenzoate (CB) pathway. Bacteria isolated from the soil have expressed more flexibility for CB degradation, namely in the case of ortho-chlorinated benzoates. They all possessed large plasmids, the restriction patterns of which were compared. Plasmids in Pseudomonas sp. A7, A8, A18 and A19, respectively, were cured and found to encode at least part of the metabolic pathway involved in the growth on ortho-chlorinated benzoates. 相似文献
7.
8.
9.
10.
Maria Azrad Kui Zhang Robin T. Vollmer John Madden Thomas J. Polascik Denise C. Snyder Mack T. Ruffin Judd W. Moul Dean Brenner Robert W. Hardy Wendy Demark-Wahnefried 《PloS one》2012,7(12)
Previous observational studies have reported associations between prostate cancer and alpha-linolenic acid (ALA). However, few investigations have been able to study this relationship prospectively and in well-controlled settings. Moreover, no studies have determined whether single nucleotide polymorphisms (SNPs) that influence ALA metabolism are associated with this common cancer. The purpose of this study was to explore associations between prostatic levels of ALA, SNPs and prostate cancer-specific biomarkers in samples collected from a previous randomized clinical trial conducted using a presurgical model and which tested the effects of flaxseed supplementation, a rich source of ALA, prior to prostatectomy (n = 134). Serum prostate-specific antigen (PSA) was determined and immunohistochemistry was used to assess tumor proliferation rate (Ki67). Prostatic ALA was determined with gas chromatography. Seven previously identified SNPs associated with delta-6 desaturase activity (rs99780, rs174537, rs174545, rs174572, rs498793, rs3834458 and rs968567) were tested for associations with prostatic ALA, PSA and Ki67. Despite consuming seven times more ALA per day, men in the flaxseed arm had similar amounts of prostatic ALA relative to men not consuming flaxseed. In unadjusted analysis, there were significant positive associations between prostatic ALA and PSA (ρ = 0.191, p = 0.028) and Ki67 (ρ = 0.186, p = 0.037). After adjusting for covariates (flaxseed, age, race, BMI and statin-use) the association between ALA and PSA remained (p = 0.004) but was slightly attenuated for Ki67 (p = 0.051). We did not observe associations between any of the SNPs studied and prostatic ALA; however, in models for PSA there was a significant interaction between rs498793 and ALA and for Ki67 there were significant interactions with ALA and rs99780 and rs174545. Independent and inverse associations were observed between rs174572 and Ki67. This study provides evidence that prostatic ALA, independent of the amount of ALA consumed, is positively associated with biomarkers of aggressive prostate cancer and that genetic variation may modify this relationship. 相似文献