Table 1.

List of bacterial metabolites that may be found in the intestine

Type of metabolite	Metabolites
Metabolites derived from bacterial energy metabolism	‘Terminal’ metabolites from carbohydrate fermentationSCFA: formate, acetate, propionate, butyrate, longer-chain fatty acidsBranched-chain fatty acids
	‘Intermediate’ metabolites from carbohydrate fermentationPartially degraded oligomeric carbohydrates (disaccharides, oligosaccharides, complex proteoglycans from mucins, etc.)Alcohols: methanol, ethanol, etc.
	Gaseous metabolitesFermentation gases: hydrogen, methane, carbon dioxideHighly volatile compounds: hydrogen sulfide
	Metabolites of fatty acid and lipid bioconversionLong-chain aldehydesFatty acids
	Metabolites from protein fermentationBranched-chain fatty acidsAmmonia and aminesAromatic derivatives of amino acids: phenols, cresols, indoles, etc.
Metabolites derived from bioconversion of plant secondary compounds	Products of lignin/polyphenols bioconversion: equol, enterolactone, etc.
Metabolites from bacterial cytosolic compartment or secondary metabolism (spilled over by excess production, efflux or upon cell lysis)	Vitamins and cofactors (often in very small concentrations) Peptides (quorum-sensing signals of Gram-positive bacteria) Homoserine lactone (quorum-sensing signals of Gram-negative bacteria) Nucleic acids (free DNA, microRNA, etc.) Bacteriocins
Metabolites of the enterohepatic circulation	Bile acids Cholesterol, coprostanol Hormones and derivatives Glucuronide conjugates
Enzymes	Reductases Glucuronidases Glycohydrolases
Bacterial cell wall components |(of which several are immunoactive)	Lipopolysaccharide Polysaccharide A Peptidoglycan-derived structures Capsular polysaccharides (glycocalix)