The complete genome sequence of Escherichia coli strain MG1655 allows for the first comprehensive assessment of the impact of horizontal gene transfer on bacterial evolution. The study found that 755 of 4,288 ORFs (547.8 kb) have been introduced into the E. coli genome through at least 234 lateral transfer events since the species diverged from the Salmonella lineage 100 million years ago. The average age of these genes is 14.4 million years, indicating a transfer rate of 16 kb/Myr/lineage. Although many of these genes have been deleted, the remaining 18% of the chromosome have contributed to E. coli's ability to occupy new ecological niches. Horizontal gene transfer is a major factor in bacterial evolution, as it allows for the rapid adaptation to new environments. Transferred sequences can significantly influence bacterial evolution, such as the transformation of a benign E. coli strain into a pathogen or the acquisition of antibiotic resistance. The study also shows that horizontally transferred genes often have distinct base compositions and codon usage patterns compared to native genes, which can be used to identify them. The analysis of the E. coli genome revealed that horizontally transferred genes are often located near tRNA loci, suggesting that bacteriophages may be involved in their transfer. The study also identified that horizontally transferred genes can be ameliorated over time, adjusting to the base composition and codon usage of the host genome. This process allows for the estimation of the age of horizontally transferred genes. The study estimated that horizontally transferred genes have been accumulating in the E. coli genome at a rate of 16 kb/Myr, resulting in the acquisition of approximately 1,600 kb of novel genes since divergence from Salmonella 100 million years ago. This suggests that horizontal transfer has played a significant role in the diversification of enteric bacteria, with the potential for rapid adaptation and speciation. The findings support the hypothesis that bacterial speciation is driven by a high rate of horizontal transfer, which introduces novel genes and enables the rapid exploitation of competitive environments.The complete genome sequence of Escherichia coli strain MG1655 allows for the first comprehensive assessment of the impact of horizontal gene transfer on bacterial evolution. The study found that 755 of 4,288 ORFs (547.8 kb) have been introduced into the E. coli genome through at least 234 lateral transfer events since the species diverged from the Salmonella lineage 100 million years ago. The average age of these genes is 14.4 million years, indicating a transfer rate of 16 kb/Myr/lineage. Although many of these genes have been deleted, the remaining 18% of the chromosome have contributed to E. coli's ability to occupy new ecological niches. Horizontal gene transfer is a major factor in bacterial evolution, as it allows for the rapid adaptation to new environments. Transferred sequences can significantly influence bacterial evolution, such as the transformation of a benign E. coli strain into a pathogen or the acquisition of antibiotic resistance. The study also shows that horizontally transferred genes often have distinct base compositions and codon usage patterns compared to native genes, which can be used to identify them. The analysis of the E. coli genome revealed that horizontally transferred genes are often located near tRNA loci, suggesting that bacteriophages may be involved in their transfer. The study also identified that horizontally transferred genes can be ameliorated over time, adjusting to the base composition and codon usage of the host genome. This process allows for the estimation of the age of horizontally transferred genes. The study estimated that horizontally transferred genes have been accumulating in the E. coli genome at a rate of 16 kb/Myr, resulting in the acquisition of approximately 1,600 kb of novel genes since divergence from Salmonella 100 million years ago. This suggests that horizontal transfer has played a significant role in the diversification of enteric bacteria, with the potential for rapid adaptation and speciation. The findings support the hypothesis that bacterial speciation is driven by a high rate of horizontal transfer, which introduces novel genes and enables the rapid exploitation of competitive environments.