Escherichia coli O121 is a serotype of Escherichia coli, a species of bacteria that lives in the lower intestines of mammals. The presence of many serotypes of E. coli in animals is beneficial or does not cause disease. However, E. coli O121 has been recognized as a potential disease causing serotype of bacteria. Unlike Escherichia coli O157:H7, another pathogenic serotype of E. coli, little is known about the public health significance of O121.
Escherichia coli O121 is also known as a type of "non-O157 Shiga toxin-producing E. coli" or "non-O157 STEC".