Reid S D, Herbelin C J, Bumbaugh A C, Selander R K, Whittam T S
Institute of Molecular Evolutionary Genetics, Pennsylvania State University, University Park 16802, USA.
Nature. 2000 Jul 6;406(6791):64-7. doi: 10.1038/35017546.
The mechanisms underlying the evolution and emergence of new bacterial pathogens are not well understood. To elucidate the evolution of pathogenic Escherichia coli strains, here we sequenced seven housekeeping genes to build a phylogenetic tree and trace the history of the acquisition of virulence genes. Compatibility analysis indicates that more than 70% of the informative sites agree with a single phylogeny, suggesting that recombination has not completely obscured the remnants of ancestral chromosomes. On the basis of the rate of synonymous substitution for E. coli and Salmonella enterica (4.7 x 10(-9) per site per year), the radiation of clones began about 9 million years ago and the highly virulent pathogen responsible for epidemics of food poisoning, E. coli O157:H7, separated from a common ancestor of E. coli K-12 as long as 4.5 million years ago. Phylogenetic analysis reveals that old lineages of E. coli have acquired the same virulence factors in parallel, including a pathogenicity island involved in intestinal adhesion, a plasmid-borne haemolysin, and phage-encoded Shiga toxins. Such parallel evolution indicates that natural selection has favoured an ordered acquisition of genes and the progressive build-up of molecular mechanisms that increase virulence.
新细菌病原体的进化和出现背后的机制尚未得到充分理解。为了阐明致病性大肠杆菌菌株的进化，我们对七个管家基因进行了测序，构建了系统发育树，并追溯了毒力基因的获得历史。兼容性分析表明，超过70%的信息位点与单一系统发育一致，这表明重组并未完全掩盖祖先染色体的遗迹。根据大肠杆菌和肠炎沙门氏菌的同义替换率（每年每个位点4.7×10⁻⁹），克隆的辐射大约始于900万年前，而导致食物中毒流行的高毒力病原体大肠杆菌O157:H7，早在450万年前就从大肠杆菌K-12的一个共同祖先中分离出来。系统发育分析表明，大肠杆菌的古老谱系平行获得了相同的毒力因子，包括一个参与肠道黏附的致病岛、一个质粒携带的溶血素和噬菌体编码的志贺毒素。这种平行进化表明，自然选择有利于基因的有序获得以及增加毒力的分子机制的逐步积累。
