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Horizontal gene transfer plays a major role in microbial evolution. However, newly acquired sequences can decrease fitness unless integrated into preexisting regulatory networks. We found that the histone-like nucleoid structuring protein (H-NS) selectively silences horizontally acquired genes by targeting sequences with GC content lower than the resident genome. Mutations in hns are lethal in Salmonella unless accompanied by compensatory mutations in other regulatory loci. Thus, H-NS provides a previously unrecognized mechanism of bacterial defense against foreign DNA, enabling the acquisition of DNA from exogenous sources while avoiding detrimental consequences from unregulated expression of newly acquired genes. Characteristic GC/AT ratios of bacterial genomes may facilitate discrimination between a cell's own DNA and foreign DNA.  相似文献   
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The comparative effects of iron-supplemented levels on growth, tissue distribution, haematology and immunology of gilthead sea bream, Sparus aurata (2 g) were investigated, using four organic (50, 100, 200, 300 mg ORG/kg diet) and one inorganic iron source (200 INOR mg/kg diet). Fish were treated for 12 weeks with the experimental diets and maintained at a water temperature of 19–22°C. Growth (final weight and specific growth rate), tissue distribution (spleen, liver and muscle), haematological parameters (red blood cells, haematocrit, haemoglobin and mean corpuscular haemoglobin concentration) and non-specific immune indexes (respiratory burst activity and antibacterial activity of serum) were analysed. No significant differences were found in growth and iron tissue distribution among the tested groups. Red blood cell counting was statistically higher in fish given 50 ORG, 100 ORG, 200 ORG and 200 INOR feeds. However, haematocrit, haemoglobin and mean corpuscular haemoglobin concentration were not significantly affected by increasing dietary iron. Fish receiving the 100 ORG diet had the best performance with respect to the respiratory burst activity and significantly higher values for antibacterial activity of serum were obtained in fish fed with the 300 ORG diet. The present findings provided no clear evidence of the optimum iron concentration. However, there was adequate indication that iron supplementation enhanced the performance of gilthead sea bream, mainly from a haematological and immunological point of view.  相似文献   
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A method for the separation of the oil, emulsifier and polymer or salt fractions of adjuvants is described. The adjuvant is dissolved in 90% aqueous ethanol and filtered to remove polymers or salt. Light petroleum (distillation range 30–40°C) is added to the solution in a separating funnel. The light petroleum fraction is subsequently extracted with successive aliquots of 80, 70, 60 and 50% aqueous ethanol. The whole of the ethanol and light petroleum fractions are evaporated to dryness to yield the emulsifier and oil fractions respectively. Emulsifier, oil and polymer or salt are determined gravimetrically. The method successfully detected the presence of polymer or salt in adjuvants, but could not distinguish between them when both were present in the same product. The adjuvants could be classified into four broad classes; those containing > 70% emulsifier (wetter-spreaders), those containing > 75% oil (anti-evaporants), those containing a high proportion of polymer (film-forming substances), and those containing comparable amounts of all three components (multi-purpose adjuvants).  相似文献   
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Five commercially available organic Cu products and reagent-grade CuSO4 x 5H2O (Cu Sulf) were evaluated by polarographic analysis and solubility in 0.1 M K2HPO4-KH2PO4 buffer (pH 5), 0.2 M HCl-KCl buffer (pH 2), or deionized water. Fractions from these solubility tests were evaluated by gel filtration chromatography for structural integrity. The organic sources were Cu lysine complex (Cu Lys), Cu amino acid chelate (Cu AA), Cu proteinate A (Cu ProA), Cu proteinate B (Cu ProB), and Cu proteinate C (Cu ProC). Separation of peaks in the chromatograms for the soluble Cu fraction from deionized water indicated that 77, 31, 69, 94, and 16% of the Cu remained chelated for the above sources, respectively. Two experiments were conducted to estimate the relative bioavailability of Cu from the organic Cu supplements for chicks when added at high dietary concentrations to practical corn-soybean meal diets. Liver Cu concentration increased (P < 0.0001) as dietary Cu increased in both experiments. When Cu Sulf was assigned a value of 100% as the standard, linear regression slope ratios of log10 liver Cu concentration regressed on added dietary Cu concentration gave estimated relative bioavailability values of 124 +/- 5.1, 122 +/- 5.3, and 111 +/- 6.0 for Cu Lys, Cu AA, and Cu ProC, respectively, in Exp. 1. The bioavailability estimates for Cu Lys and Cu AA were greater (P < 0.05) than that for Cu Sulf. Values in Exp. 2 were 111 +/- 7.6, 109 +/- 8.4, and 105 +/- 7.5 for Cu Lys, Cu ProA, and Cu ProB, respectively, and all sources were similar in value for chicks. Solubility of Cu in pH 2 buffer provided the best prediction of bioavailability (r2 = 0.924). Other indicators of chelation integrity and solubility had little value as predictors of bioavailability (r2 < or = 0.445).  相似文献   
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Streptococcus uberis is an important cause of mastitis in dairy cows throughout the world, particularly during the dry period, around the time of calving, and during early lactation. Strategies for controlling S. uberis mastitis have not received adequate research attention and are therefore poorly defined and inadequate. Objectives of the present study were to evaluate the efficacy of extended therapy regimens with pirlimycin for treatment of experimentally induced S. uberis intramammary infections in lactating dairy cows during early lactation and to evaluate the usefulness of the S. uberis experimental infection model for evaluating antimicrobial efficacy in dairy cows. The efficacy of extended pirlimycin intramammary therapy regimens was investigated in 103 mammary glands of 68 dairy cows that became infected following experimental challenge with S. uberis during early lactation. Cows infected with S. uberis in one or both experimentally challenged mammary glands were randomly allocated to three groups, representing three different treatment regimens with pirlimycin, including 2-day (n = 21 cows, 31 mammary quarters), 5-day (n = 21 cows, 32 quarters), and 8-day (n = 26 cows, 40 quarters). For all groups, pirlimycin was administered at a rate of 50 mg of pirlimycin hydrochloride via intramammary infusion. A cure was defined as an experimentally infected mammary gland that was treated with pirlimycin and was bacteriologically negative for the presence of S. uberis at 7, 14, 21, and 28 days after treatment. Experimental S. uberis intramammary infections were eliminated in 58.1% of the infected quarters treated with the pirlimycin 2-day regimen, 68.8% for the 5-day regimen, and 80.0% for the 8-day regimen. Significant differences (P <.05) in efficacy were observed between the 2-day and 8-day treatment regimens. The number of somatic cells in milk decreased significantly following therapy in quarters for which treatment was successful in eliminating S. uberis. However, there was no evidence to suggest that extended therapy with pirlimycin resulted in a greater reduction in somatic cell counts in milk than the 2-day treatment. The S. uberis experimental infection model was a rapid and effective means of evaluating antimicrobial efficacy during early lactation at a time when mammary glands are highly susceptible to S. uberis intramammary infection.  相似文献   
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