This is the peer reviewed version of the following article:Louis, P. and Flint, H. J. (2017), Formation of propionate and butyrate by the human colonic microbiota. Environ Microbiol, 19: 29–41., which has been published in final form at doi:10.1111/1462-2920.13589. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.