The article discusses the amelioration of bacterial genomes, focusing on how horizontally transferred genes change over time to match the base composition of the recipient genome. Bacterial genomes show variation in GC content, but genes within a species are relatively similar. Genes introduced through horizontal transfer often have unusual sequence characteristics and can be distinguished from ancestral DNA. Over time, these genes undergo amelioration, reflecting the DNA composition of the new genome due to mutational processes. This process is evident in genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the time required for foreign DNA to resemble native DNA. Models of amelioration can estimate the time of introgression of foreign genes. Analysis of a 1.43-megabase sequence in E. coli suggests that more than 600 kb of its chromosome contains horizontally transferred, protein-coding DNA. The rate of accumulation of this DNA is about 31 kb per million years, comparable to the amount of variant DNA introduced by point mutations. This rate predicts that E. coli and Salmonella enterica lineages have each gained and lost more than 3 megabases of novel DNA since their divergence. The study highlights four key features of bacterial base composition: variation among species, relationship to phylogeny, homogeneity across the chromosome, and characteristic base compositions in codon positions. Regions with atypical base compositions or codon usage patterns are considered evidence of horizontal transfer. The article also discusses the assumption that atypical features are not due to stochastic variation or selective factors, and that amelioration is influenced by mutational biases. The study examined the rates and patterns of evolution of genes in E. coli and Salmonella to monitor base composition changes and estimate the amount of acquired DNA. The results provide insights into the long-term evolution of enteric bacteria.The article discusses the amelioration of bacterial genomes, focusing on how horizontally transferred genes change over time to match the base composition of the recipient genome. Bacterial genomes show variation in GC content, but genes within a species are relatively similar. Genes introduced through horizontal transfer often have unusual sequence characteristics and can be distinguished from ancestral DNA. Over time, these genes undergo amelioration, reflecting the DNA composition of the new genome due to mutational processes. This process is evident in genes involved in host-cell invasion by enteric bacteria and can be modeled to predict the time required for foreign DNA to resemble native DNA. Models of amelioration can estimate the time of introgression of foreign genes. Analysis of a 1.43-megabase sequence in E. coli suggests that more than 600 kb of its chromosome contains horizontally transferred, protein-coding DNA. The rate of accumulation of this DNA is about 31 kb per million years, comparable to the amount of variant DNA introduced by point mutations. This rate predicts that E. coli and Salmonella enterica lineages have each gained and lost more than 3 megabases of novel DNA since their divergence. The study highlights four key features of bacterial base composition: variation among species, relationship to phylogeny, homogeneity across the chromosome, and characteristic base compositions in codon positions. Regions with atypical base compositions or codon usage patterns are considered evidence of horizontal transfer. The article also discusses the assumption that atypical features are not due to stochastic variation or selective factors, and that amelioration is influenced by mutational biases. The study examined the rates and patterns of evolution of genes in E. coli and Salmonella to monitor base composition changes and estimate the amount of acquired DNA. The results provide insights into the long-term evolution of enteric bacteria.