The invention relates to Salmonella typhi Ty21a comprising core-linked Shigella dysenteriae serotype 1 O-specific polysaccharide (O-Ps) and DNA encoding O antigen biosynthesis, said DNA selected from the group consisting of: a) the DNA sequence set out in any one of SEQ ID NOs: 1 and 2 and species homologs thereof； b) DNA encoding Shigella dysenteriae serotype 1 polypeptides encoded by any one of SEQ ID NOs: 1 and 2, and species homologs thereof； and c) DNA encoding a O antigen biosynthesis gene product that hybridizes under moderately stringent conditions to the DNA of (a) or (b)； and related sequences, compositions of matter, vaccines, methods of using, and methods of making.
