Major phenylpropanoid-derived metabolites in the human gut can arise from microbial fermentation of protein.
METHODS
Plant secondary metabolites, such as phenolic acids are commonly associated with benefits for human health. Two of the most abundant phenylpropanoid-derived compounds detected in human faecal samples are phenylacetic acid (PAA) and 4-hydroxylphenylacetic acid (4-hydroxyPAA). Although they have the potential to be derived from diets rich in plant-based foods, evidence suggests that these compounds can be derived from the microbial fermentation of aromatic amino acids (AAAs) in the colon.


RESULTS
To identify the bacteria responsible, 26 strains representing 25 of the dominant human colonic species were screened for phenyl metabolite formation. Seven strains produced significant amounts of both PAA and 4-hydroxyPAA. These included five out of seven Bacteroidetes (Bacteroides thetaiotaomicron, Bacteroides eggerthii, Bacteroides ovatus, Bacteroides fragilis, Parabacteroides distasonis), and two out of 17 Firmicutes (Eubacterium hallii and Clostridium bartlettii). These species also produced indole-3-acetic acid (IAA), the corresponding tryptophan metabolite, but C. bartlettii showed 100 times higher IAA production than the other six strains. Four strains were further tested and PAA formation was substantially increased by phenylalanine, 4-hydroxyPAA by tyrosine and IAA by tryptophan.


CONCLUSIONS
This study demonstrates that certain microbial species have the ability to ferment all three AAAs and that protein fermentation is the likely source of major phenylpropanoid-derived metabolites in the colon.
Molecular Nutrition Group, Rowett Institute of Nutrition and Health, University of Aberdeen, Aberdeen, UK. W.Russell@abdn.ac.uk
Wendy R
WR
Russell
Sylvia H
SH
Duncan
Lorraine
L
Scobbie
Gary
G
Duncan
Louise
L
Cantlay
A Graham
AG
Calder
Susan E
SE
Anderson
Harry J
HJ
Flint
D000596:Amino Acids / Q000378:metabolism / Q000494:pharmacology; D024322:Amino Acids, Aromatic / Q000378:metabolism; D001439:Bacteroides / Q000187:drug effects / Q000378:metabolism; D003106:Colon / Q000382:microbiology*; D005051:Eubacterium / Q000378:metabolism; D005285:Fermentation; D006801:Humans; D007210:Indoleacetic Acids / Q000378:metabolism; D064307:Microbiota; D010648:Phenylacetates / Q000378:metabolism*; D014364:Tryptophan / Q000378:metabolism / Q000494:pharmacology
D000596:Amino Acids; D024322:Amino Acids, Aromatic; D007210:Indoleacetic Acids; D010648:Phenylacetates; C008070:4-hydroxyphenylacetic acid; C030737:indoleacetic acid; D014364:Tryptophan; C025136:phenylacetic acid