Glycans as legislators of host-microbial interactions: spanning the spectrum from symbiosis to pathogenicity

The number of microbes associated with our gut likely exceeds our total num ber of somatic and germ cells. Despite their numbers, almost nothing is kno wn about the molecular mechanisms that determine whether the interaction be tween a microbial species and its host will be beneficial. Recent results o btained from in vivo models have revealed critical roles for glycoconjugate s in helping define the outcome of two such host-microbial relationships. I n one case, attachment of Helicobacter pylori to fucosylated or sialylated glycans produced by various gastric epithelial lineages and their progenito rs skews the destiny of colonization toward pathogenicity. In the second ca se, a molecular dissection of how Bacteroides thetaiotaomicron, a normal in habitant of the distal small intestine, is able to communicate with intesti nal epithelial cells has revealed a novel role for host fucosylated glycans in forging a mutually beneficial relationship. These observations lend sup port to the hypothesis that the capacity to synthesize diverse carbohydrate structures may have arisen in part from our need to both evade pathogenic relationships and to coevolve symbiotic relationships with our nonpathogeni c resident microbes.