The repeat units of heteropolymeric O antigen are synthesized at the cytosolic side of the inner bacterial membrane via the Wzx/Wzy-dependent assembly pathway. After being translocated across the membrane by Wzx, each repeat unit is polymerized by Wzy to form a glycan chain. In this study, we demonstrate the need of the corresponding enzyme trans- ferring the initial HexNAc to undecaprenol phosphate (lipid carrier), together with the corre- sponding O-antigen polymerase (Wzy), to produce the Aeromonas hydrophila O:34- antigen. We suggest, the concerted action of WecA or P enzyme (UDP-HexNAc: polypre- nol-P HexNAc-1-P transferase) and Wzy is involved in the mechanism responsible for the A . hydrophila O-antigen polymerization.