TABLE 1.

Contribution of intestinal bacteria to metabolic pathways that influence health and disease

Source	Bacteria involved	Microbial metabolite(s)	Effects on host
			Beneficial when in normal concentrations	Potentially deleterious when in abnormal concentrations
Dietary carbohydrates	Various (eg, Faecalibacterium, Bifidobacterium)	Fermentation to short‐chain fatty acids	Anti‐inflammatory properties	Abnormal SCFA ratio can activate virulence factors of enteropathogens (eg, Salmonella invasion genes, Escherichia coli motility)
			Improve barrier function
			Regulate intestinal motility
			Provide systemic and local energy
Primary bile acids from liver	Mostly Clostridium hiranonis in dogs and cats	Transformation to secondary bile acids (BA)	Anti‐inflammatory	Increased primary BA can lead to secretory diarrhea
			Secondary BA are a major regulator of normal microbiome, also inhibit growth of C difficile, C perfringens, E coli
Tryptophan from diet	Various	Indole metabolites	Anti‐inflammatory, maintain intestinal barrier function	In increased concentrations cytotoxic, putrefactive indoxyl sulfate acts as uremic toxin
Dietary carnitine and choline	Various (eg, E coli)	Trimethylamine N‐oxide (TMAO)	n/a	Altered cholesterol metabolism associated with heart disease

Abbreviation: n/a, not applicable.

Source: Adapted from Ziese AL and Suchodolski JS, Vet Clin North Am Small Anim Pract. 2021