Production of short-chain fatty acids

A simplified diagram (Figure 1) outlines polysaccharide breakdown and the main routes of carbohydrate fermentation in the human large intestine. Bacteria and, to a lesser extent, archaea contribute to processes. Two distinct cross-feeding mechanisms operate in the gastrointestinal tract: one due to the consumption of fermentation end-products (lactate, acetate, succinate) and the other due to cross-feeding of partial breakdown products from complex substrates (Falony et al., 2006; Belenguer et al., 2007; Reichardt et al., 2014). Both mechanisms contribute to the production of butyrate and propionate.

Figure 1. Summary of short-chain fatty acid production by the human gut microbiota. Updated from Hoyles & Wallace (2010) to include propionate formation (Reichardt et al., 2014) and the bifid shunt, which is restricted to Bifidobacterium spp. in the human gut via the action of fructose 6-phosphate phosphoketolase (Pokusaeva et al., 2011). *Akkermansia muciniphila is thought to be produce propionate via the succinate pathway. †Species predicted from sequence analyses to be capable of reductive acetogenesis (Ohashi et al., 2007; Hylemon et al., 2018).

References
Belenguer, A., Duncan, S. H., Holtrop, G., Anderson, S. E., Lobley, G. E. & Flint, H. J. (2007). Impact of pH on lactate formation and utilization by human fecal microbial communities. Appl Environ Microbiol 73, 6526–6533.
​Falony, G., Vlachou, A., Verbrugghe, K. & De, Vuyst, L. (2006). Cross-feeding between Bifidobacterium longum BB536 and acetate-converting, butyrate-producing colon bacteria during growth on oligofructose. Appl Environ Microbiol 72, 7835–7841.
Hoyles, L. & Wallace, R. J. (2010). Gastrointestinal tract: intestinal fatty acid metabolism and implications for health. In Handbook of Hydrocarbon and Lipid Microbiology, pp. 3119–3132. Springer, Berlin.
Hylemon, P. B., Harris, S. C. & Ridlon, J. M. (2018). Metabolism of hydrogen gases and bile acids in the gut microbiome. FEBS Lett doi:10.1002/1873-3468.13064.
Ohashi, Y., Igarashi, T., Kumazawa, F. & Fujisawa, T. (2007). Analysis of acetogenic bacteria in human feces with formyltetrahydrofolate synthetase sequences. Biosci Microflora 26, 37–40.
Pokusaeva, K., Fitzgerald, G. F. & van Sinderen, D. (2011). Carbohydrate metabolism in bifidobacteria. Genes Nutr 6, 285–306.
Reichardt, N., Duncan, S. H., Young, P., Belenguer, A., McWilliam Leitch, C., Scott, K. P., Flint, H. J. & Louis, P. (2014). Phylogenetic distribution of three pathways for propionate production within the human gut microbiota. ISME J 8, 1323–1335.