Metabolic interactions involving sulphate-reducing and methanogenic bacteria in the human large intestine

A comprehensive study involving 87 healthy human volunteers showed that dissimilatory sulphate-reducing bacteria (SRB) occurred in high numbers in faeces of non-methanogenic persons. Three broad population groupings were identified on the basis of SRB carriage rates. Individuals in Group A (n = 21) were strongly methanogenic and SRB were never detected in significant numbers in their faeces. In Group B (n = 9), methanogenesis occurred, how numbers of faecal SRB (ca. 105 g (wet wt. faeces)−1) were found. Volunteers in Group C (n = 57) had high counts of faecal SRB (ca. 109 (g wet wt. faeces)−1) and no methanogenesis. These data were confirmed by CH4 production and SO42− reduction rates in faecal slurries. The principal SRB were identified as lactate and H2 utilizing Desulfovibrio spp. (64–81%), acetate utilizing Desulfobacter spp. (9–16%), propionate and H2 utilizing Desulfobulbus spp. (5–8%), lactate utilizing Desulfomonas spp. (3–10%) and acetate and butyrate utilizing Desulfotomaculum spp. (2%). Measurements of SRB numbers and activities in digesta taken from the proximal and distal colons from 2 individuals who had died suddenly, showed no significant difference in SRB counts or the relative distribution of genera in these regions. However, in the methanogenic person, MB were higher by approximately 3 orders of magnitude in the distal gut. Inhibition experiments with 2-bromoethanesulphonic acid (BES) and sodium molybdate (Mo) showed that H2 production was higher in the distal colon compared to the proximal gut (33% and 263% respectively, in CH4 producing and non-CH4 producing digesta). Inhibition of methanogenesis had little qualitative or quantitative effect on short chain fatty acid (SCFA) formation, but inhibition of SRB resulted in an increase in accumulation of SCFA, together with succinate and lactate. These data demonstrate the greater metabolic diversity and capacity of human colonic SRB compared to MB, and suggest that SRB may play a significant role in the turnover of fatty acids in the colons of some individuals.