Enumeration of amino acid fermenting bacteria in the human large intestine: effects of pH and starch on peptide metabolism and dissimilation of amino acids

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.

Shaping the Metabolism of Intestinal Bacteroides Population through Diet to Improve Human Health

“…The genus Bacteroides displays a high flexibility to adapt to the nutritional conditions of the intestinal environment (Comstock and Coyne, 2003), being able to use dietary or host-derived glycans according to the nutrient availability (Sonnenburg et al, 2005). Bacteroides can also incorporate amino acids from outside (Smith and MacFarlane, 1998) which could be used to maintain cell structures and as an energy source.…”

“…The pH in the caecum is about 5.7 but gradually increases to 6.7 in the rectum. Dietary fiber fermentation promotes a slight decrease of the luminal pH whereas high protein/amino acids fermentation, favored by low carbohydrate availability, causes slightly pH increases (Smith and MacFarlane, 1998). Interestingly, a recent study with mice demonstrated that diet-microbiome interactions are driven by the pattern of protein and carbohydrate intake (Holmes et al, 2017).…”

Ammonia production by human faecal bacteria, and the enumeration, isolation and characterization of bacteria capable of growth on peptides and amino acids

“…Protein is an abundant substrate for bacterial growth in the human intestine, possibly more so than carbohydrate in the distal colon [1]. Some of the protein may be of dietary origin, but large intestinal fermentation probably depends more on endogenous sources, including mucus and host proteins and bacterial protein resulting from bacterial cell turnover.…”

“…Proteolytic species from the human colon have been well characterized [1,8,9], and some aspects of the metabolism of peptides are known [1,10]. Bacterial species able to grow on individual amino acids as N and energy source are fairly well understood [11].…”

“…They include many of the ‘putrefactive’ Clostridium , Peptostreptococcus and Fusobacterium species [11,12]. Some evidence that gut bacteria can also use Stickland reactions, which involves the coupled oxidation and reduction of pairs of amino acids to organic acids [13], was obtained by Smith and Macfarlane [1]. However, bacteria able to grow on a mixture of protein breakdown products, although known to be numerous [11], have not been characterized.…”