Escherichia coli (E. coli) is a broad and diverse group of gram-negative bacterium, present in the environment and within the gut microflora. The bacterium grows up to a length of 2 micrometers and is protected by with a cell wall containing lipopolysaccharides (LPS). Its circular DNA genome is 4.6 Mbp, containing over 4200 protein-coding genes. However, only an estimated 20% of a typical E. coli genome is shared among other sequenced strains. While most strains of E. coli are harmless, commensal, or symbiotic there are a number of known pathogenic strains. Strains such as O157:H7 and O104:H4 produce shiga toxin, which can cause severe inflammations of the gastrointestinal tracts. Antigens in the E. coli lipopolysaccharide layer, flagellin, and capsule are used in serotype classification.