Fig. 1

Strategies of pyruvate catabolism by the human gut microbiome. Carbohydrates are first degraded to pyruvate. Pyruvate may then be converted to succinate, lactate, acetyl CoA + formate/carbon dioxide + hydrogen, ethanol, or 2,3-butanediol. Succinate may, however, also be a direct product of carbohydrate fermentation. Succinate and lactate do not typically reach high concentrations in fecal samples, as they can be further catabolized to produce energy, but certain species do secrete them as their final fermentation end-product, which enables cross-feeding. Acetate is produced by two pathways; (1) through direct conversion of acetyl CoA for the generation of energy (brown) or (2) acetogenesis (red). Formate/carbon dioxide + hydrogen can also be substrates for methanogenesis. Propionate is produced by three pathways; (1) the succinate pathway (orange), (2) the acrylate pathway (green), or (3) the 1,2-propanediol pathway (blue). 1,2-Propanediol is synthesized from lactaldehyde or dihydroxyacetone phosphate, which both are products of deoxy sugar fermentation (e.g., fucose, rhamnose). Alternatively, lactaldehyde can be produced from lactate, or 1,2-propanediol can be fermented to propanol. Propionate can be coupled with ethanol for fermentation to valerate (gray). The precursor for butyrate, butyryl CoA, is generated from either acetyl CoA or succinate. Butyrate is then produced by two pathways; (1) the butyrate kinase pathway (pink) or (2) the butyryl CoA:acetyl CoA transferase pathway (purple). Butyrate-producing bacteria may also cross-feed on lactate, converting it back to pyruvate. Lactate may also be catabolized as part of sulfate reduction