Abstract:Shigella flexneri is the major pathogen causing bacillary dysentery in the developing countries. Besides phage-encoded glucosylation and/or O-acetylation, O-antigen of S. flexneri modification with phosphoethanolamine (PEtN) has been identified lately. The plasmid-carried opt gene encodes the phospholethanolamine transferase adding a PEtN on O-antigen rhammose II and/or rhammose III. O-antigen PEtN modification of serotype X, 4a and Y strains convert the hosts into MASF IV-1 positive “variant” Xv, 4av and Yv serotypes respectively. The plasmidcarried opt gene is able to transform all S. flexneri serotypes to make them express MASF IV-1 epitope. Among various serotypes, the PEtN modification of O-antigen may interact with glucosylation and O-acetylation process. Now S. flexneri - O antigen ethanolamine phosphate modification mechanism and its interaction with glycosylation, acetylated modification were reviewed.