Starch Digestion by Gut Bacteria: Crowdsourcing for Carbs

Starch is a polymer of glucose and is one of the most abundant carbohydrates in a Western diet. Resistant starch escapes digestion by host small intestinal glucoamylases and transits the colon where it is degraded by the combined efforts of many gut bacteria. Bacterial metabolism and fermentation of resistant starch leads to increases in short-chain fatty acids, including the clinically beneficial butyrate. Here, we review the molecular machinery that gut bacteria use to degrade starch and how these functions may intersect to facilitate complete starch digestion. While the protein complexes that gut bacteria use to degrade starch differ across phyla, some molecular details converge to promote the optimal positioning of enzymes and substrate for starch degradation. Different types of starch influence the gut bacterial community which impacts health outcomes.While soluble starch is vulnerable to enzymatic degradation by human and most bacterial enzymes, resistant starch is minimally processed by host enzymes, so it transits to the colon where it is digested by few bacterial species.Starch prebiotics and amylase inhibitors can be utilized to modulate the gut microbiota to harness the increase in the clinically beneficial molecule butyrate upon shunting starch to the colon.Biochemical studies on the mechanics of soluble starch breakdown and utilization in gut bacteria revealed that starch is degraded in multiple steps by different bacteria via unique molecular strategies.The enzymatic machinery that may confer resistant starch utilization involves catalytic and carbohydrate-binding modules that synergize for efficient degradation.