Fig. 5
Organic sulfur metabolism by gut bacteria may be a key mechanism linking a western diet and CRC risk. The degradation of sulfomucins by mucolytic bacteria is a key source of inorganic sulfate for sulfate-reducing bacteria. At micromolar concentrations, basal production of H2S through inorganic sulfate reduction exerts beneficial effects including gut barrier protection and fermentative hydrogen disposal. Intake of a western diet abundant in red and processed meat amplifies the production of taurine conjugated bile acids and increases colonic exposure to dietary sulfur amino acids (taurine, methionine, cysteine). In the context of a western diet, the metabolism of organic sulfur amino acids by gut microbes drives the production of H2S to genotoxic and pro-inflammatory levels (mM concentration). Simplified pathways demonstrate genes for sulfur metabolism that were significantly associated with CRC. The symbol '^' indicates inorganic sulfur sources primarily provided by sulfated bile acids and sulfamucins. The '^^' symbol indicates organic sulfur sources provided by dietary sulfur amino acids and conjugated bile acids