The extra genes in the pathogenic E. coli O157:H7 strain are organized into pathogenicity islands (O-islands) and many of them code for toxins and other proteins that are involved in the pathogenicity of O157:H7.The two E. coli genomes were later on compared with the genome of Shigella flexneri serotype 2a (another enteric pathogen that causes diarrhea) when it was sequenced [55].