Table 1. Gut microbiome-produced metabolites, their biological functions, and their effects on pigs.

Metabolites	Producing bacteria	Biological functions and effects on pigs	References
Short-chain fatty acids (acetate, butyrate, and propionate)	RuminococcaceaeRuminococcusLachnospiraceaeBlautiaRoseburiaLactobacillaceaeClostridiumEubacteriumFaecalibacteriumBifidobacteriumBacteroides	•Energy source for colonocytes•Stimulate cell proliferation•Suppress pro-inflammatory cytokines•Reduce gut permeability through upregulation of tight junction proteins•Regulate lipid metabolism•Act as precursor for neurotransmitters•Improve feed efficiency and lower feed intake•Improve average daily gain•Improve meat quality	[6,29,33,35–55]
Branched-chain fatty acids (isobutyrate and isovalerate)	ClostridiumPropionibacteriumStreptococcusBacteroides	•Energy source for colonocytes (less preferred)•Suppress pro-inflammatory cytokines (dose-dependent)•Impair gut barrier function•Upregulate pro-inflammatory cytokines	[56–62]
Lactate	Lactic acid bacteriaBifidobacterium	•Important metabolite for cross-feeding mechanism•Reduce pH thus preventing the growth of potentially pathogenic bacteria	[6,63,64]
Bile acids (primary and secondary bile acids)	Clostridium speciesEubacteriumParabacteroidesLachnospiraceae	•Regulate glucose and energy homeostasis, and lipid metabolism•Suppress pro-inflammatory cytokines •Have antimicrobial and cytotoxic activities•Decrease cell proliferation and gene expression of tight junction proteins•Impair oxidative stress coping mechanism	[65–74]
Polyamines (putrescine, spermine,and spermidine)	ClostridiumRuminococcusRoseburiaEnterococcusStreptococcusLactococcusLactobacillusBifidobacterium BacteroidesFusobacterium	•Improve fetal development•Stimulate intestinal maturation•Suppress pro-inflammatory cytokines•Enhance oxidative stress resistance•Increase growth •Alleviate diarrhea•May cause damage to the intestinal morphology	[75–85]
Indolic and phenolic compounds	ClostridiumPeptostreptococcusLactobacillusBifidobacteriumEscherichiaBacteroides	•Indole and indole derivatives•Upregulate of aryl hydrocarbon receptors and increased tight junction proteins•Attenuate oxidative stress•Causes ‘boar-taint’ in barrows (skatole)•Phenols and phenol derivatives•Induce ‘leaky-gut’•Disrupt colonic cell respiration (p-cresol)•Reduce growth performance (p-cresol)	[60,86–99]
Ammonia	Amino acid fermenting commensalsHelicobacter	•By-product of amino acid fermentation•Inhibits mitochondrial respiration•Inhibits short-chain fatty acid oxidation	[56,87,100,101]
Hydrogen sulfide	ClostridiumFusobacteriumDesulfovibrioEnterobacterEscherichiaSalmonella	•Energy source for colonocytes•Acts as a signaling molecule•Inhibits cytochrome oxidase activity•Hinders cell proliferation and induces intestinal inflammation	[87,88,102,103]
Neurotransmitters (glutamate, dopamine, acetylcholine, γ-aminobutyric acid, norepinephrine, and serotonin)	BacillusBifidobacteriumLactobacillusEscherichiaKlebsiella	•Improve cognition and behavior•Ameliorate tail biting	[104–107]
Vitamins (B vitamins and vitamin K)	BacteroidesLactobacillus	•Serve as coenzymes in neurological processes (B vitamins)•Essential vitamin for proper blood clotting (vitamin K)	[108–110]