Shigella flexneri RfbF protein is a putative dTDP-rhamnosyl transferase. dTDP rhamnosyl transferases of Shigella flexneri add rhamnose sugars to N-acetyl-glucosamine in the O-antigen tetrasaccharide repeat. Lipopolysaccharide O antigens are important virulence determinants for many bacteria. The variations of sugar composition, the sequence of the sugars and the linkages in the O antigen provide structural diversity of the O antigen.