Summary: Most probable number counts showed that denitrifying species were the numerically predominant NO reducing bacteria in the faeces of five methanogenic individuals [about 10 bacteria (g dry wt faeces)]. In faecal slurries, however, denitrification was a relatively minor route of NO dissimilation, since only about 3% of the NO was converted to gaseous products, with NO being mainly reduced to NO and NH . When KNO was added to the slurries, denitrification became quantitatively more significant with approximately 23% of the NO being lost as gaseous products. The addition of KNO (10 mM) to slurries containing either starch or casein significantly decreased H and CH production. The effect of NO on methanogenesis was twofold: firstly, H accumulation decreased due to diversion of electrons towards NO /NO reduction, and as a result of H being used as an electron donor for NO reduction, resulting in the removal of the methanogenic substrate; secondly, there was direct inhibition of methane-producing bacteria by NO and NO . In starch-containing slurries, acetate: butyrate molar ratios were increased when NO was added but this effect was not observed when casein replaced starch. These results show that the ability of NO /NO to act as an electron sink can significantly influence the major products of the human colonic fermentation.


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