Escherichia coli is a Gram-negative straight rod, which either uses peritrichous flagella for mobility or is nonmotile. It is a facultatively anaerobic chemoorganotroph capable of both respiratory and fermentative metabolism. E.coli serves a useful function in the body by suppressing the growth of harmful bacterial species and by synthesising appreciable amounts of vitamins. It is an important component of the biosphere. It colonizes the lower gut of animals and survives when released to the natural environment, allowing widespread dissemination to new hosts. Pathogenic E.coli strains are responsible for infection of the enteric, urinary, pulmonary and nervous systems. Comparison of 20 E.coli/Shigella strains shows the core genome to be about 2000 genes while the pan-genome has over 18,000 genes. There are multiple, striking integration hotspots that are conserved across the genomes, corresponding to regions of abundant and parallel insertions and deletions of genetic material.

E. coli O55:H7 strain CB9615 was isolated from an infant with diarrhea in Germany in 2003. It belongs to the same sequence type (ST11) as O157:H7 clones by multilocus sequence typing. O55:H7 is considered to be an atypical EPEC that does not contain the EAF plasmid found in typical EPEC strains, although it does carry a plasmid, pO55, which has pO157 as its nearest relative. Data suggest that the O157:H7 strains rose from and O55:H7 ancestor in the recent past (adapted from PMID 20090843.