Table 1.

Summary of gut microbiota–immune axis in various diseases.

Diseases	Reference	Findings
Gastrointestinal Infections	Singer et al., 2019 [222]	Provide resistance against colonization and invasion by pathobiont.
	Tovaglieri et al., 2019 [223]	Human gut microbiome metabolites induce expression of flagellin (a bacterial protein) increases EHEC motility and epithelial injury.
IBD	Lee and chang, 2021 [224]	Gut microbiota dysbiosis of IBD patients is consistently marked by an overgrowth in Proteobacteria.
	Furusawa et al., 2013 [110]	SCFA confers protection against IBD by maintaining gut barrier integrity, promoting Treg cell differentiation, and inhibiting histone deacetylases.
Colorectal carcinoma	Sepich-Poore et al., 2021 [225]	Generation of genotoxin such as Bacteroides fragilis toxin (Bft), cytolethal distending toxin (CDT), and colibactin.
	Hale et al., 2017 [226]	Bacterial-derived secondary bile acids and hydrogen sulfide promote proinflammatory milieu that increases CRC risk.
	Yeoh et al., 2020 [227]	Bacteria such as F. nucleatum can adhere to colon tumors and aggravate tumorigenesis.
Hepatocellular carcinoma	Lin et al., 1995 [228]	Systemic translocation of LPS promotes chronic liver injury and predisposes to HCC.
	Singh et al., 2018 [229]	Excess butyrate production promotes HCC progression.
	Yoshimoto et al., 2013 [230]	Secondary bile acids promote carcinogenesis and impede anti-tumor immunosurveillance in the liver.
Cardiometabolic disease	Cani et al., 2007 [231] Guasch-Ferré et al., 2017 [232] Millard et al., 2018 [233]	LPS and other microbial ligands drive low-grade chronic inflammation and predispose to CVD.
		Bacterial trimethylamine and its conversion to trimethylamine-N-oxide in the liver increases the risk of coronary artery disease, metabolic syndrome, stroke, and vascular inflammation.
Rheumatoid Arthritis	Scher et al., 2013 [234]	Prevotella spp. Abundance is positively associated with new-onset rheumatoid arthritis.
Allergic Diseases	Fazlollahi et al., 2018 [235] Bunyavanich et al., 2016 [236]	Gut microbiota dysbiosis increases risk for allergic disease, e.g., food allergy and asthma.