Escherichia coli O157:H7, or simply E. coli O157, is a pathogenic E. coli serotype known for causing food-bourne hemorrhagic diarrhea. Infections in the young and elderly may lead to a complication called hemolytic uremic syndrome. The virulence of this serotype stems mainly in part from acquisition of genes encoding for shiga toxins (stxs), as well as acquisition of the pO157 plasmid. In addition, O157 has evolved three systems of acid resistance, enabling it to effectively colonize the low pH environment of the stomach, even at low doses. The O157:H7 serotype is distinguished by its lipoplysaccharide O-antigen and flagellar H-antigen.