Competition for hydrogen between sulphate‐reducing bacteria and methanogenic bacteria from the human large intestine

Sulphate‐reducing activity in human faecal slurries was followed by measuring sulphide production. Sulphate‐reducing bacteria (SRB) were found to outcompete methanogenic bacteria (MB) for the mutual substrate hydrogen in faecal slurries from methane‐ and non‐methane‐producing individuals mixed together. When molybdate (20mmol/l) was added to these slurries, sulphate reduction was inhibited and methanogenesis became the major route of electron disposal. Sulphide production was stimulated by the addition of 20 mmol/1 sulphate in non‐methanogenic but not in methanogenic slurries. In methanogenic slurries that contained the methanogen inhibitor 2‐bromoethanesulphonic acid (BES), hydrogen accumulated whilst sulphide levels were unaffected, confirming the absence of SRB in methanogenic faeces. The addition of nitrate (10 mmol/l) to faecal slurries completely inhibited methanogenesis but only slightly reduced sulphate reduction. The sulphated mucopolysaccharides, chondroitin sulphate and mucin, strongly stimulated sulphide production in non‐methanogenic faecal slurries only, suggesting that these substances may be a potential source of sulphate in the large gut.