Table 1.

Examples of postbiotics.

Microbial-metabolite pathways	Overview of bacterial transformation	Examples of clinical and translational investigations in liver disease
Bile acids	• Deconjugation: bile salt hydrolases  • Secondary bile acid formation: 7α dehydroxylation  • Epimerisation  • Desulphation	• Significant changes during alcohol-associated liver disease progression and alcohol cessation  • Changes with use of FXR modulators, FMT or rifaximin
Choline compounds (TMAO)	• Degraded by several taxa to TMA then oxidised in the liver to TMAO	• Associated with metabolic syndrome  • Linked with minimal hepatic encephalopathy  • Reduced pre-transplant but rebounds post-liver transplant
Indole derivatives	• Balance between human and microbial tryptophan degradation  • Specific taxa are unclear  • Metabolites: tryptamine, indole-3-pyruvate, indole, indole-3-acetamide, indole-3-acetaldehyde, indole-3-aldehyde, indole-3-acetic acid, indole-3-lactic acid, indole acrylic acid and indole-3-propionic acid  • Indole sulfation in the liver	• Preclinical evidence of strengthening gut barrier with selected metabolites  • High production of oxindole in patients with hepatic encephalopathy  • Associated with mood changes that co-exist with liver disease  • Higher in those who developed negative outcomes in cirrhosis in outpatient and inpatient setting
Short-chain fatty acids	• Fermentation of dietary fibre and colonic mucus	• Varying impact on obesity and high-fat diet response  • Butyrate levels reduce with advancing liver disease and alcohol-associated liver disease  • Varies with lactulose administration  • Restored/enhanced after FMT

FMT, faecal microbiota transplant; FXR, farnesoid X receptor; TMAO, tri-methylamine oxide.