Genomic View on the Evolution of Enterohemorrhagic Escherichia coli

Abstract

In this chapter, among the various Escherichia coli strains, the authors focus on strains belonging to Enterohemorrhagic E. coli (EHEC), particularly the O157:H7 serotype, and review the recent advancement in understanding of their evolution and diversification. Although the importance of horizontal gene transfer in the evolution of pathogenic E. coli strains was noticed before, its genome-wide view was first obtained by the genome sequence determination of two O157:H7 strains and following genomic comparison with K-12. To fully understand the genomic differences between O157 and non-O157 EHEC and the evolution pathways of each non-O157 EHEC, the genome sequencing of non-O157 EHEC strains is also required. The genome sequencing of O157 EHEC revealed that the O157 genome is a huge genetic mosaic generated by insertion of an extremely large amount of foreign DNA into the chromosome backbone shared by a benign strain K-12. A large number of virulence-related genes are encoded on this strain-specific DNA. These findings highlighted a surprisingly high level of genome plasticity in bacteria and reinforced the importance of horizontal gene transfer, especially that by bacteriophages, in the evolution of pathogenic bacteria. It was also revealed that gain and loss of foreign genes mediated by bacteriophages are still actively ongoing and continuously producing a wide variety of O157 strains and recombinant phages.