Concentrations of phenolic compounds in human gut contents were more than fourfold higher in the distal colon (6.2 mmol kg-1) compared to the proximal bowel (1.4 mmol kg-1). Tryptophan metabolites were never found in more than trace amounts in large intestinal contents and phenol substituted fatty acids were the major products of aromatic amino acid fermentation that accumulated in the proximal colon, whereas phenol and p-cresol were more important in the distal gut, accounting for 70% of all products of dissimilatory aromatic amino acid metabolism. In vitro incubations of colonic material showed that phenol was produced most rapidly (1.0 mumol g-1 h-1), whereas indole was formed comparatively slowly (0.06 mumol g-1 h-1). Most probable number (MPN) estimations demonstrated that large populations of phenol and indole producing bacteria occur in the large intestine (range log10 9.8-11.5 (g dry wt faeces)-1, mean 10.6, N = 7). With respect to phenolic compounds, phenylacetate and phenylpropionate producers predominated, while indoleacetate-forming bacteria were the major tryptophan-utilizing organisms. Quantitation of products of dissimilatory aromatic amino acid metabolism in MPN tubes showed that phenol and phenylpropionate mainly accumulated at low sample dilutions, whereas phenylacetate, p-cresol, indoleacetate and indolepropionate were formed in greatest amounts at high sample dilutions. The significance of pH and carbohydrate availability with respect to aromatic amino acid metabolism was shown in batch culture fermentation studies, where net production of phenolic compounds by mixed populations of intestinal bacteria was reduced by approximately 33% during growth at pH 5.5 compared to pH 6.8, and by 60% in the presence of a fermentable carbohydrate. Experiments with 16 species of intestinal bacteria belonging to six different genera showed that environmental factors such as low pH and high carbohydrate availability markedly reduced dissimilatory aromatic amino acid metabolism in some organisms, but stimulated this process in others. A three-stage continuous culture model of the colon was used to investigate the effect of system retention time (27.1 or 66.7 h) on aromatic amino acid fermentation. Qualitative and quantitative increases in phenol production occurred from vessel 1 to vessel 3 in this model. Concentrations of phenolic compounds in vessel 3 were three times greater at R = 66.7 h compared to R = 27.1 h. Phenol and p-cresol were not detected in vessel 1, though formation of these metabolites increased from vessel 2 to vessel 3, in a pattern similar to that observed in the distal colon.

No abstract

Proteins and trichloroacetic acid‐soluble peptides were present in high concentrations in human intestinal contents and faeces. Free amino acids were also detected in millimolar amounts in proximal and distal colon contents, with hydroxyproline, alanine, lysine and valine predominating, showing that a wide variety of organic N‐containing compounds was available for fermentation by intestinal bacteria. Measurements of products of dissimilatory amino acid metabolism (ammonia, branched chain fatty acids) demonstrated that these substances occurred in all regions of the large bowel. Amino acid fermenting populations were enumerated in faeces obtained from five healthy donors by most probable number analysis. Counts ranged from 1010 to 1011 per gram dry weight faeces. Acetate, propionate and butyrate were the principal fermentation acids in the most probable number tubes. Bacteria forming branched chain fatty acids as major end products of metabolism ranged from 0.6% (isovalerate/2‐methylbutyrate) to 40% (isobutyrate) of total peptide and amino fermenting populations. Plate counts also gave high values for peptide fermenting communities in the region of 1011 per gram dry weight faeces, though considerably lower numbers of organisms grew on plates containing either single amino acids or Stickland pairs. Clostridia and anaerobic Gram‐positive cocci were the predominant isolates in these studies. Physiological investigations on the effects of pH and carbohydrate availability on peptide and amino acid fermentation by intestinal bacteria showed that two environmental characteristics of the proximal colon (low pH, high carbohydrate availability), reduced the rate and net ammonia production from peptides, while carbohydrate (starch) was more important in this respect in amino acid fermentation vessels. Starch reduced initial rates of production of branched chain fatty acids by approximately 35% in peptide fermentations, however, culture pH was a more significant determinant affecting formation of these metabolites. Comparisons of branched chain fatty acid formation by faecal bacteria at pH 6.8 and 5.5 showed that their production was reduced by over 60% in pH 5.5 cultures. These data demonstrate that by increasing bacterial requirements for organic N‐containing compounds for use in biosynthetic reactions, and through fermentation acid production, carbohydrate availability plays a major role in regulating dissimilatory metabolism of peptides and amino acids in the human large intestine.

No abstract

No abstract