Dissimilatory amino Acid metabolism in human colonic bacteria

The abilities of slurries of human faecal bacteria to ferment 20 different amino acids were investigated in batch culture incubations. Ammonia, short chain fatty acids, and in some cases, amines, were the principal products of dissimilatory metabolism. The types of SCFA produced were dependent on the chemical compositions of the test substrates. Thus, acetate and butyrate were formed from the acidic amino acid glutamate, while acetate and propionate predominated in aspartate fermentations. Breakdown of the basic amino acids lysine and arginine was rapid, and yielded butyrate and acetate, and ornithine and citrulline, respectively. The major products of histidine deamination were also acetate and butyrate. However, fermentation of sulphur-containing amino acids was slow and incomplete. Acetate, propionate and butyrate were formed from cysteine, whereas the main products of methionine metabolism were propionate and butyrate. The simple aliphatic amino acids alanine and glycine were fermented to acetate, propionate and butyrate, and acetate and methylamine, respectively. Branched-chain amino acids were slowly fermented by colonic bacteria, with the main acidic products being branched-chain fatty acids one carbon atom shorter than the parent amino acid. Low concentrations of amines were also detected in these fermentations. Aliphatic-hydroxy amino acids were rapidly deaminated by large intestinal microorganisms. Serine was primarily fermented to acetate and butyrate, while threonine was mainly metabolised to propionate. Proline was poorly utilized by intestinal bacteria, but hydroxyproline was efficiently fermented to acetate and propionate. The aromatic amino acids tyrosine, phenylalanine and tryptophan were broken down to a range of phenolic and indolic compounds.