Abstract: Provided herein is a novel E. coli O polysaccharide, O25B. Also provided herein are prokaryotic host cells containing enzymes (e.g., glycosyltransferases) used in O25B production. The host cells provided herein produce O25B bioconjugates, wherein said bioconjugates contain O25B linked to a carrier protein. Further provided herein are compositions, e.g., pharmaceutical compositions, including O25B and/or bioconjugates containing O25B. Such compositions can be used as vaccines against infection with ExPEC, and may further include one or more additional bioconjugates.