In one aspect, the invention relates to an immunogenic composition comprising modified O-polysaccharide molecules derived from E. coli lipopolysaccharides and conjugates thereof. Multivalent vaccines may be prepared by combining two or more monovalent immunogenic compositions for different E. coli serotypes. In one embodiment, the modified O-polysaccharide molecules are produced by a recombinant bacterium that includes a wzz gene.