Human symbiont Bacteroides thetaiotaomicron synthesizes 2-keto-3-deoxy-D-glycero-D- galacto-nononic acid (KDN).

The proper functioning of the human intestine is dependent on its bacterial symbionts, the most predominant of which belong to the Phylum Bacteroidetes. These bacteria are known to use variable displays of multiple capsular polysaccharides (CPs) to aid in their survival and foraging within the intestine. Bacteroides thetaiotaomicron is a prominent human gut symbiont and a remarkably versatile glycophile. The structure determination of the CPs, encoded by the eight CP loci, is the key to understanding the mechanism of this organism's adaptation on a molecular level. Herein, we report the bioinformatics-based discovery and chemical demonstration of a biosynthetic pathway that forms and cytidylates 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN), most likely for inclusion in the CP encoded by B. thetaiotaomicron CP locus 7.