Escherichia coli (E. coli) is a rod shaped, gram negative bacterium most well known for its role as a laboratory model organism. They are extremely diverse, with only roughly 20% of its genes shared among other strains. Most strains of E. coli are harmless and can live commensally or symbiotically in the intestines along with other microbial flora. However, pathogenic strains can cause disease and infections in humans. Shiga toxin-producing strains, such as O157:H7 and O104:H4, can cause gastrointestinal conditions and can lead to severe complications. Classification of E. coli serotypes is based on antigens in its lipopolysaccharide layer (LPS), flagellin, and capsule.