Chance and necessity in the evolution of a bacterial pathogen

The combination of genomic, epidemiological and evolutionary analyses provides a powerful toolbox for understanding how pathogens adapt to their human hosts. By sequencing 112 Burkholderia dolosa genomes from an outbreak among patients with cystic fibrosis, a new study documents evolution in action and identifies a set of genes that contributed to the pathogen's adaptation.     

Developmental cheating in the social bacterium Myxococcus xanthus

Cheating is a potential problem in any social system that depends on cooperation and in which actions that benefit a group are costly to individuals that perform them. Genetic mutants that fail to perform a group-beneficial function but that reap the benefits of belonging to the group should have a within-group selective advantage, provided that the mutants are not too common. Here we show that social cheating exists even among prokaryotes. The bacterium Myxococcus xanthus exhibits several social behaviours, including aggregation of cells into spore-producing fruiting bodies during starvation. We examined a number of M. xanthus genotypes that were defective for fruiting-body development, including several lines that evolved for 1,000 generations under asocial conditions and others carrying defined mutations in developmental pathways, to determine whether they behaved as cheaters when mixed with their developmentally proficient progenitor. Clones from several evolved lines and two defined mutants exhibited cheating during development, being overrepresented among resulting spores relative to their initial frequency in the mixture. The ease of finding anti-social behaviours suggests that cheaters may be common in natural populations of M. xanthus.     

New data on excisions of Mu from E. coli MCS2 cast doubt on directed mutation hypothesis

According to the directed mutation hypothesis, certain mutations in bacteria occur more frequently in environments in which the resulting phenotype is selectively favoured than in non-selective environments. This hypothesis therefore challenges the fundamental tenet that mutations occur spontaneously, irrespective of effects on the organism's fitness. One purported case of directed mutation is the excision of a Mu sequence from Escherichia coli strain MCS2 in minimal lactose-arabinose medium. Here, we show that this case can be more simply explained by an accelerated rate of excision mutation in response to non-specific physiological stresses of starvation and by slight growth of MCS2 on minimal lactose-arabinose medium.     

Experimental evidence for an alternative to directed mutation in the bgl operon.

The directed mutation hypothesis suggests that some mutations occur more often when selectively advantageous than when neutral or disadvantageous, challenging the principle that the selective value of a mutation does not affect the rate of its occurrence. Mutations in the bgl operon of Escherichia coli have been reported to be a case of directed mutation. E. coli K12 strains chi342LD cannot grow on salicin but derivatives with two mutations in the bgl operon, an excision of IS150 (formally called IS103) from bglF and a point mutation or insertion in bflR, grow rapidly on this sugar. When chi342LD is grown on a medium containing salicin, bglF excision mutants accumulate to a frequency of greater than 1%, even though these mutants are reportedly unable to grown on salicin, and Sal+ double mutants subsequently attain a high frequency. Comparable accumulations of excision mutants and Sal+ double mutants are not observed in the absence of salicin. As salicin is not mutagenic, it has been suggested that excision mutations in bglF might serve only to create the potential for a secondary selectively advantageous mutation. We show here, however, that these double mutants can be accounted for by spontaneous mutation to intermediate genotypes in non-growing populations, coupled with slow growth of some of these intermediates on salicin, which enables their populations to reach a size where secondary mutations allowing rapid growth on salicin become common.     

Genome complexity, robustness and genetic interactions in digital organisms.

Digital organisms are computer programs that self-replicate, mutate and adapt by natural selection. They offer an opportunity to test generalizations about living systems that may extend beyond the organic life that biologists usually study. Here we have generated two classes of digital organism: simple programs selected solely for rapid replication, and complex programs selected to perform mathematical operations that accelerate replication through a set of defined 'metabolic' rewards. To examine the differences in their genetic architecture, we introduced millions of single and multiple mutations into each organism and measured the effects on the organism's fitness. The complex organisms are more robust than the simple ones with respect to the average effects of single mutations. Interactions among mutations are common and usually yield higher fitness than predicted from the component mutations assuming multiplicative effects; such interactions are especially important in the complex organisms. Frequent interactions among mutations have also been seen in bacteria, fungi and fruitflies. Our findings support the view that interactions are a general feature of genetic systems.     

苯酚-硫酸法测定藏木香中多糖含量

采用传统的水煮醇沉法从藏木香中提取可溶性多糖,通过Sevag法脱蛋白后,以葡萄糖为对照品,使用苯酚-硫酸法测定多糖含量.结果表明,在波长486nm处测定吸光度,10-100μg/mL范围内吸光度与被测含量之间具有良好的线性关系,藏木香中多糖的含量为64.37%.     

甘青青兰挥发性成分GC/MS分析

目的：分离鉴定出甘青青兰（Dracocephalum tanguticum Maxim.）挥发油的化学成分.方法：用气相色谱-质谱（GC/MS）联用技术及峰面积归一化法测定各组分的相对含量.结果：共鉴定出23种化合物,占总色谱峰总面积的87.46%.结论：甘青青兰挥发油中的化学成分主要为[-]-反-松香芹乙酯和桉油精,两者分别占总挥发油中化学成分的60.30%和9.31%.     

钢管相贯口变参数变结构建库与数据提取方法研究

零件的批处理和零件库的建立是计算机辅助设计的重要内容之一，以钢管相贯口为例应用UG的几种建库方法，通过分析电子表格、零件族、GRIP编程在变参变结构的复杂零件建库时存在的问题，提出了一种将零件族和GRIP编程相结合的建库方法来建立变参变结构的复杂零件的零件库。     

Evolution of a bacteria/plasmid association

Associations between bacteria and their accessory elements (viruses, plasmids and transposons) range from antagonistic to mutualistic. A number of previous studies have demonstrated that plasmid carriage reduces bacterial fitness in the absence of selection for specific functions such as antibiotic resistance. Many studies have demonstrated increased fitness of evolving microbial populations in laboratory environments, but we are aware of only one study in which fitness gains were partitioned between a plasmid and its host. Here, we examine the evolution of an association between a plasmid and its bacterial host. Carriage of the non-conjugative plasmid pACYC184 initially reduced the fitness of Escherichia coli B in the absence of antibiotic. We then cultured plasmid-bearing bacteria for 500 generations in the presence of antibiotic. The fitness of each combination of host and plasmid, with and without the culture history, was determined by competing it against a baseline strain. The results indicate adaptation by the host genome, but no plasmid adaptation. We also competed the evolved host, transformed with the baseline plasmid, against its isogenic plasmid-free counterpart. The plasmid now increased the fitness of its host.