Chapter 19, Problem 6CHI

In 2007 a severe food-borne outbreak of enterohemorrhagic E. coli (EHEC) O157:H7 occurred in the United States. The strain that caused this outbreak seemed to be more virulent, causing more severe illness, than previous outbreaks. To test

if new, more virulent strains of EHEC might be evolving, SNP analysis was performed on 500 clinical isolates of the bacterium that were collected before, during, and after the 2007 outbreak. SNPs were detected in 96 loci, and 39 discrete SNP genotypes were identified. These could be separated into nine different clades, as shown in this phylogenetic tree. The amount of toxin produced and the severity of disease differed among clades. It was discovered that members of clade 8 caused the 2007 outbreak and this clade was associated with high levels of toxin production. Discuss the evolution of this pathogen. Specifically consider each of these mechanisms of genetic variation: horizontal gene transfer, mutation, gene amplification, gene deletion, and intragenic recombination. Which do you think is (or are) the most important? Explain your answer.

Read the original paper: Manning, S. D., et al. 2008. Variation in virulence among clades of Escherichia coli O157:H7 associated with disease outbreaks. Proc. Natl. Acad. Sci. USA. 105:4868.