If this hypothesis were correct, it follows that CH4 formation by CH4 producing faeces should be reduced appreciably by admixture with nonmethanogenic faeces, and this reduction should be reversed in the presence of a large excess of H2. This concept was tested in the present study by incubating CH4 producing and non-methanogenic faeces, individually or mixed together, with and without the addition of H2 and/or sulphate. Surprisingly, we found that methanogenesis actually was enhanced by the presence of non-methanogenic faeces, even when large quantities of sulphate were provided for SRB metabolism.Methane (CH4), a metabolic product of a group of anaerobic bacteria, is excreted consistently in appreciable quantity by some subjects but not others. In various population groups the prevalence of CH4 excretors has been found to range from 24%' to 95%.2 Since CH4 is not metabolised in man, the ability of subjects to excrete this gas reflects the number or activity, or both, of the methanogenic flora present in the colon.3 Why only select subjects harbour a CH4 producing flora has piqued the interest of numerous investigators.The sole source of energy of most species of methane producing bacteria is via the oxidation of H2 produced by other organisms and the activity of methanogens is limited by low H2 availability.' Methanogenesis consumes 4 moles of H2 to reduce 1 mole of CO2 to CH4, a process that greatly reduces the volume of gas that would otherwise be present in the colon. Thus, understanding the factors that regulate the activity of the CH4 producing flora could provide both clinically relevant information with regard to flatulence, as well as basic knowledge concerning the factors that regulate the proliferation and/or activity of colonic bacteria. Methods FAECAL HOMOGENATESWe studied faeces from eight healthy adult volunteers who were all on a conventional diet and who had not taken antibiotics during the two months before the study. On the basis of previous results, faeces of four of them were known to produce large quantities of CH4, while faeces of the other four produced little or no CH4.Faecal homogenates were prepared by homogenising faeces (1:5 w/v) in 0-1 M phosphate buffer (pH 7.0). Strict anaerobiosis was maintained during the procedure and all vessels, syringes, and solutions were exhaustively purged with argon before use. The four CH4 producing faecal samples were paired with the four nonproducing samples and a series of four experiments were then carried out in which faeces from the producer and the non-producer were incubated singly or mixed together. Incubations were carried out in 12 50 ml gas-tight syringes sealed with stopcocks. Four syringes contained 5 ml aliquots of the CH4 producing homogenate plus 5 ml of phosphate buffer, four
Increased H2S release is a relatively localized metabolic aberration of ulcerative colitis feces. This increased H2S may reflect abnormalities of the fecal bacteria and/or substrate availability.
Methanogens and sulphate reducing bacteria compete for H2 in the human colon, and, as a result, faeces usually contain high concentrations of just one of these two organisms. There is controversy over which of these organisms wins the competition for H2, although theoretical data suggest that sulphate reducing bacteria should predominate. To elucidate this question experiments were undertaken in which sulphate enriched homogenates of human sulphate reducing faeces and methane producing faeces were incubated separately or mixed together. Co-incubation of sulphate reducing faeces with methanogenic faeces resulted in a sixfold reduction in the activity of the sulphate reducing bacteria (measured as sulphide production), whereas methane production was not inhibited by co-incubation with sulphate reducing bacteria. Methanogenic faeces also consumed H2 more rapidly and reduced the H2 tension of the homogenate to a lower value than did sulphate reducing faecal samples. In these experiments, methanogens seem to outcompete sulphate reducing bacteria for H2. (Gut 1994; 35: 1098- If a sulphate reducing flora can utilise H2 more efficiently than a methanogenic flora, one would expect that, in the presence oflimited H2, co-incubation of faeces containing each type of flora would result in an inhibition of CH4 production. In a previous report, however,7 we found that CH4 liberation continued unabated when methanogenic and non-methanogenic faeces were co-incubated, and thus concluded that methanogens predominated. This study has been criticised because no evidence was provided that the non-methanogenic faeces contained sulphate reducing bacteria.5In response to this criticism, we now report H2 competition studies utilising faecal samples in which the presence of strong sulphate reducing activity was documented. These studies indicate that despite their apparent theoretical inferiority, methanogens experimentally outcompete sulphate reducing bacteria for H2. Methods EXPERIMENTAL DESIGNStudies were carried out using freshly passed faeces from six healthy adult volunteers, three of whom were known from previous studies to have faeces with methanogenic activity and three to have strong sulphate reducing activity. The ability of the latter samples to utilise H2 for this reduction reaction was studied by comparing the rates of H2 disappearance from a 10% head space in the presence or absence of 20 mM sodium molybdate, an inhibitor of sulphate reduction.8 Hydrogen consumption was expressed in units of ml consumed-24 hours-dlog mean [H2] -1 because of the linear relation between H2 consumption and H2 concentration in the range of H2 tensions observed in these experiments.9 In the absence of molybdate, the H2 consumption rate of the three samples was 3 0, 2-6, and 0-38 ml-24 h-1 rnM-1, falling to, respectively, 0-41, 0-42, and 0 07 ml-24 h-'mM-1 in the presence of molybdate.An individual experiment consisted of incubating nine syringes, three containing aliquots of a methanogenic homogenate, three containing a sulphate reducing...
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