Xanthan gum is a long chain polysaccharide - a polymer made up of sugar subunits, like starch or cellulose. Xanthan gum is composed of glucose, mannose, and glucuronic acid linked together to form a strand of cellulose with trisaccharide (three sugar) sidechains branching off every fifth cellulose sugar. This gives Xanthan gum the consistency of corn starch while making it undigestible (and thus more stable) like cellulose. Xanthan gum is produced by fermenting corn sugar with Xanthomonas campestris bacteria which produce Xanthan as part of their cell walls. This makes Xanthan gum a natural food just like wine, cheese, and yogurt which are also the products of bacterial fermentation.
source:
http://www.madsci.org/posts/archives/1999-02/918851605.Bc.r.html
