 A common subdivision system of E. coli , but not based on evolutionary relatedness , is by serotype , which is based on major surface antigens ( O antigen : part of lipopolysaccharide layer ; H : <unk> ; K antigen : capsule ) , e.g. O157 : H7 ) . It is , however , common to cite only the <unk> , i.e. the O @-@ antigen . At present , about 190 <unk> are known . The common laboratory strain has a mutation that prevents the formation of an O @-@ antigen and is thus not <unk> . 
