**4. Antigenic Structure of** *E. coli*

*E. coli* is classified into 150–200 serotypes or serogroups based on 3 antigens, somatic (O) or cell wall antigen, capsular (K) antigen, and flagellar (H) antigen. Seventy five types of the H or flagellar antigen and 173 types of O or somatic antigens 103 types of the K or capsular antigens have been recognized [16] (**Figure 2**).
