Gastro-enteric methane versus sulphate and volatile fatty acid production.

The breakdown of low digestible components present in food during passage through the human and animal gastro-intestinal (GI) tract is performed by the highly diverse microbial community present in this ecosystem. Fermentation of these substances yields, besides CO2 and volatile fatty acids, H2, which is used as a substrate by three different H2-consuming bacteria. Sulphate-reducing bacteria (SRB) use H2 to reduce SO inf4 (sup2-) to H2S, hydrogenotrophic methane-producing bacteria (MPB) use H2 to reduce CO2 to CH4 and reductive acetogens (RAC) use H2 to reduce CO2 to CH3COOH. A competition between these three bacterial groups exists for the common H2 substrate. This results generally in the dominance of one group above the other two.