Evolution of Shigella and Enteroinvasive Escherichia coli

Abstract

Shigellosis in humans is characterized by the destruction of the colonic epithelium provoked by the inflammatory response that is induced upon invasion of the mucosa by bacteria of Shigella spp. and enteroinvasive Escherichia coli (EIEC). Numerous phylogenetic analyses based on multilocus enzyme electrophoresis, ribotyping, and sequence comparison established that all members of the genus Shigella and EIEC strains belong to the species E. coli. Sequence analysis of chromosomal genes indicates that Shigella and EIEC strains belong to at least six phylogenetic groups, designated S1, S2, S3, SD1, SS, and EIEC. Informative sites used for the phylogenetic analysis of the virulence plasmid were clustered mostly in two genes (ipaD and ipgD), and a more complete view might come from the analysis of sequences of whole virulence plasmids. For the time being, there are two possible scenarios for the origin of Shigella and EIEC groups: (i) the arrival (or construction) of the virulence plasmid in an ancestral E. coli strain from which all Shigella and EIEC groups descend or (ii) multiple arrivals of the virulence plasmid(s) in different E. coli strains. The genomic sequence of five Shigella strains, including strains of S. flexneri 2a (Sf301 and 2457T), S. dysenteriae 1 (Sd197), S. boydii 4 (Sb227), and S. sonnei (Ss046), has been determined. The large number of genes deleted in Shigella and EIEC genomes, compared to the E. coli K-12 genome, is confirmed by comparative genomic hybridization analyses.