Chapter 34 : Genomic View on the Evolution of Enterohemorrhagic Escherichia coli

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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.

Citation: Hayashi T, Ooka T, Ogura Y, Asadulghani. 2008. Genomic View on the Evolution of Enterohemorrhagic Escherichia coli, p 407-419. In Baquero F, Nombela C, Cassell G, Gutiérrez-Fuentes J (ed), Evolutionary Biology of Bacterial and Fungal Pathogens. ASM Press, Washington, DC. doi: 10.1128/9781555815639.ch34
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Image of Figure 1.
Figure 1.

Schematic representation of the evolutionary mechanism and intralineage diversification of O157 EHEC. Most of the virulence determinants of O157 were introduced by horizontal gene transfer. Bacteriophages played particularly important roles in this process. They, together with IS elements, are also major contributors in generating intralineage diversification of O157. The order of acquisition of phages and other mobile elements is not exactly understood. Only representative virulence determinants are shown here. For example, O157 contains as many as 15 sets of pilus biosynthesis genes.

Citation: Hayashi T, Ooka T, Ogura Y, Asadulghani. 2008. Genomic View on the Evolution of Enterohemorrhagic Escherichia coli, p 407-419. In Baquero F, Nombela C, Cassell G, Gutiérrez-Fuentes J (ed), Evolutionary Biology of Bacterial and Fungal Pathogens. ASM Press, Washington, DC. doi: 10.1128/9781555815639.ch34
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Image of Figure 2.
Figure 2.

Map positions of prophages (Sp1-18) and prophage-like elements (SpLE1-6) on the O157:H7 Sakai chromosome and alignment of lambdoid prophage regions containing tRNA and virulence-related genes with the corresponding regions of phage lambda. Tf and TfC denote tail fiber protein and C-terminal part of the tail fiber protein, respectively. In other cases, standard gene names of phage lambda and virulence-related genes are used.

Citation: Hayashi T, Ooka T, Ogura Y, Asadulghani. 2008. Genomic View on the Evolution of Enterohemorrhagic Escherichia coli, p 407-419. In Baquero F, Nombela C, Cassell G, Gutiérrez-Fuentes J (ed), Evolutionary Biology of Bacterial and Fungal Pathogens. ASM Press, Washington, DC. doi: 10.1128/9781555815639.ch34
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