Table 1.

Gut microbiota and its derived molecules in functional gastrointestinal disorders (FGIDs), currently known as disorders of gut–brain interaction FGIDs

Gut microbiota derived molecules	Gut microbes	FGIDs/animal model	Key findings	Pathophysiological mechanism	References
Propionate and butyrate	Bifidobacterium adolescentis, Roseburia‐E.rectale	Constipation‐predominant irritable bowel syndrome (IBS) (IBS‐C)	Bifidobacterium adolescentis increased and Roseburia‐E.rectale decreased in fecal samples of IBS‐C, possibly regulating the levels of butyrate and ropionate, resulting in slowed gut transit	Altered gut motility	Sun et al.32
Butyrate	Fecalibacterium prausnitzii	Diarrhea‐predominant IBS (IBS‐D)	Fecalibacterium prausnitzii increased in IBS‐D, which is a major producer of butyrate, resulting in accelerated gut transit. Butyrate might be considered a biomarker for IBS‐D	Altered gut motility	Sun et al.32
Butyrate	Blautia obeum ATCC 29174	IBS‐C	Blautia obeum ATCC 29174 were decreased in IBS‐C and correlated with butyrate levels	Altered gut motility	Bhattarai et al.17
Tryptamine	Ruminococcus gnavus, Clostridium sporogenes	IBS, slow transit constipation (STC)	Ruminococcus gnavus and Clostridium sporogenes identified as Tryptophan decarboxylase Tryptamine accelerates gut transit and increases colonic secretion by activating epithelial serotonin type 4 (5‐HT4) receptor Genetically engineered bacteria Bacteroides thetaiotaomicron produce tryptamine	Altered gut motility Abnormal secretion	Bhattarai et al.17 Williams et al.33
BAs (CDCA, CA)	Blautia wexlerae DSM19850	IBS‐C	Reduced bile acid (BA) levels correlated with delayed gut transit Chenodeoxycholic acid (CDCA) was the most frequent associated metabolite of Blautia wexlerae DSM19850	Altered gut motility Abnormal secretion	Mars et al.34 Camilleri et al.35
BAs (CDCA, CA)	Blautia wexlerae DSM19850	IBS‐D	Increased BA levels correlated with bowel movements, colonic transit and visceral pain Elevated CDCA levels increased water content in stools from IBS‐D	Altered gut motility	Mars et al.34
Methane	Methanobravibacter smithii	IBS‐C, STC	Methane (CH4) in the depletes gut serotonin resulting in slowed gut transit and constipation	Altered gut motility Visceral hypersensitivity	Triantafyllou et al.36
Hydrogen	Hydrogen‐producing bacteria	Small intestinal bacterial overgrowth (SIBO), IBS‐D	Hydrogen gas correlated with SIBO condition in IBS‐D patients	Altered gut motility Gut immune dysfunction Visceral hypersensitivity Abdominal bloating and distension	Ghoshal et al.37 Pimentel et al.38
Hydrogen sulfide	Escherichia coli, Salmonella enterica, Clostridia, and Enterobacter aerogenes	Diarrhea, SIBO, IBS‐D?	Probably due to formation of sulfuric acid in the cells? Hydrogen sulfide (H2S) might be considered a biomarker for SIBO	Altered gut motility Abnormal secretion Visceral hypersensitivity Gut immune dysfunction	Kalantar‐Zadeh et al.39 Pimentel et al.38 Banik et al.40
Lactic acid	Lactobacillus, Bifidobacterium	SIBO, IBS‐D	Possible link between SIBO, brain fogginess, and D‐lactic acidosis. Patients with brain fogginess presented a higher SIBO prevalence with bloating, pain, and distension	Dysregulated microbiota‐gut‐brain axis	Rao et al.39
Alcohol (Ethanol)	Klebsiella pneumonia	Need to explore in FGIDs and SIBO?	Alcohol abuse triggers gut microbiota dysbiosis, mucosal inflammation, and intestinal barrier derangement which corroborates liver damage	Impaired barrier dysfunction, gut immune dysfunction, dysregulated microbiota‐gut–brain axis	Meroni et al.41