ENUMERATION OF AMINO-ACID FERMENTING BACTERIA IN THE HUMAN LARGE-INTESTINE - EFFECTS OF PH AND STARCH ON PEPTIDE METABOLISM AND DISSIMILATION OF AMINO-ACIDS

Proteins and trichloroacetic acid-soluble peptides were present in hig h concentrations in human intestinal contents and faeces. Free amino a cids were also detected in millimolar amounts in proximal and distal c olon contents, with hydroxyproline, alanine, lysine and valine predomi nating, 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 ch ain fatty acids) demonstrated that these substances occurred in all re gions of the large bowel. Amino acid fermenting nn populations were en umerated in faeces obtained from five healthy donors by most probable number analysis. Counts ranged from 10(10) to 10(11) per gram dry weig ht faeces. Acetate, propionate and butyrate were the principal ferment ation acids in the most probable number tubes. Bacteria forming branch ed chain fatty acids as major end products of metabolism ranged from 0 .6% (isovalerate/2-methylbutyrate) to 40% (isobutyrate) of total pepti de and amino fermenting populations. Plate counts also gave high value s for peptide fermenting communities in the region of 10(11) per gram dry weight faeces, though considerably lower numbers of organisms grew on plates containing either single amino acids or Stickland pairs. Cl ostridia and anaerobic Gram-positive cocci were the predominant isolat es 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 o f 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 fermentatio n vessels. Starch reduced initial rates of production of branched chai n fatty acids by approximately 35% in peptide fermentations, however, culture pH was a more significant determinant affecting formation of t hese metabolites. Comparisons of branched chain fatty acid formation b y faecal bacteria at pH 6.8 and 5.5 showed that their production was r educed by over 60% in pH 5.5 cultures. These data demonstrate that by increasing bacterial requirements for organic N-containing compounds f or use in biosynthetic reactions. and through fermentation acid produc tion, carbohydrate availability plays a major role in regulating dissi milatory metabolism of peptides and amino acids in the human large int estine. (C) 1998 Federation of European Microbiological Societies. Pub lished by Elsevier Science B.V.