Table 1

Gut bacteria and the metabolites they contribute.

MetabolitesRelated bacteriaPotential biological functionsReferences
Short-chain fatty acids: acetate, propionate, butyrate, isobutyrate, 2-methylpropionate, valerate, isovalerate, hexanoateClostridial clusters IV and XIVa of Firmicutes, including species of Eubacterium, Roseburia, Faecalibacterium, and CoprococcusDecreased colonic pH, inhibit the growth of pathogens; stimulate water and sodium absorption; participate in cholesterol synthesis; provide energy to the colonic epithelial cells, implicated in human obesity, insulin resistance and type 2 diabetes, colorectal cancer.(13, 24, 57)
Bile acids: cholate, hyocholate, deoxycholate, chenodeoxycholate, α-muricholate, β-muricholate, ω-muricholate, taurocholate, glycocholate, taurochenoxycholate, glycochenodeoxycholate, taurocholate, tauro-α-muricholate, tauro-β-muricholate, lithocholate, ursodeoxycholate, hyodeoxycholate, glycodeoxylcholate, taurohyocholate, taurodeoxylcholateLactobacillus, Bifidobacteria, Enterobacter, Bacteroides, ClostridiumAbsorb dietary fats and lipid-soluble vitamins, facilitate lipid absorption, maintain intestinal barrier function, signal systemic endocrine functions to regulate triglycerides, cholesterol, glucose and energy homeostasis.(30, 31, 33)
Choline metabolites: methylamine, dimethylamine, trimethylamine, trimethylamine-N-oxide, dimethylglycine, betaineFaecalibacterium prausnitzii, BifidobacteriumModulate lipid metabolism and glucose homeostasis.
Involved in nonalcoholic fatty liver disease, dietary induced obesity, diabetes, and cardiovascular disease.
(29, 58)
Phenolic, benzoyl, and phenyl derivatives: benzoic acid, hippuric acid, 2-hydroxyhippuric acid, 2-hydroxybenzoic acid, 3-hydroxyhippuric acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, 3-hydroxyphenylpropionate, 4-hydroxyphenylpropionate, 3-hydroxycinnamate, 4-methylphenol, tyrosine, phenylalanine, 4-cresol, 4-cresyl sulfate, 4-cresyl glucuronide, 4-hydroxyphenylacetate, 3,4-dihydroxyphenylacetate, phenylacetylglycine, phenylacetylglutamine, phenylacetylglycine, phenylacetate, phenylpropionate, phenylpropionylglycine, cinnamoylglycineClostridium difficile, F. prausnitzii, Bifidobacterium, Subdoligranulum, LactobacillusDetoxification of xenobiotics; indicate gut microbial composition and activity; utilize polyphenols.
Urinary hippuric acid may be a biomarker of hypertension and obesity in humans. Urinary 4-hydroxyphenylacetate, 4-cresol, and phenylacetate are elevated in colorectal cancer. Urinary 4-cresyl sulfate is elevated in children with severe autism.
(59, 60)
Indole derivatives: N-acetyltryptophan, indoleacetate, indoleacetylglycine (IAG), indole, indoxyl sulfate, indole-3-propionate, melatonin, melatonin 6-sulfate, serotonin, 5-hydroxyindoleClostridium sporogenes, E. coliProtect against stress-induced lesions in the GI tract; modulate expression of proinflammatory genes, increase expression of anti-inflammatory genes, strengthen epithelial cell barrier properties. Implicated in GI pathologies, brain-gut axis, and a few neurological conditions.(6163)
Vitamins: vitamin K, vitamin B12, biotin, folate, thiamine, riboflavin, pyridoxineBifidobacteriumProvide complementary endogenous sources of vitamins, strengthen immune function, exert epigenetic effects to regulate cell proliferation.(64, 65)
Polyamines: putrescine, cadaverine, spermidine, spermineCampylobacter jejuni, Clostridium saccharolyticumExert genotoxic effects on the host, anti-inflammatory and antitumoral effects. Potential tumor markers.(66, 67)
Lipids: conjugated fatty acids, LPS, peptidoglycan, acylglycerols, sphingomyelin, cholesterol, phosphatidylcholines, phosphoethanolamines, triglyceridesBifidobacterium, Roseburia, Lactobacillus, Klebsiella, Enterobacter, Citrobacter, ClostridiumImpact intestinal permeability, activate intestine-brain-liver neural axis to regulate glucose homeostasis; LPS induces chronic systemic inflammation; conjugated fatty acids improve hyperinsulinemia, enhance the immune system and alter lipoprotein profiles. Cholesterol is the basis for sterol and bile acid production.(42, 68)
Others: D-lactate, formate, methanol, ethanol, succinate, lysine, glucose, urea, α-ketoisovalerate, creatine, creatinine, endocannabinoids, 2-arachidonoylglycerol (2-AG), N-arachidonoylethanolamide, LPS, etc.Bacteroides, Pseudobutyrivibrio, Ruminococcus, Faecalibacterium, Subdoligranulum, Bifidobacterium, Atopobium, Firmicutes, LactobacillusDirect or indirect synthesis or utilization of compounds or modulation of linked pathways including endocannabinoid system.(43, 60, 69)