Table 1.

Microbial species and metabolites from the gut that have been associated with skin effects.

A. Microbial species
Organism		Effects on skin	Mechanism		References
Faecalibacterium prausnitzii, Akkermansia muciniphila and Ruminoccocus		Protection against psoriasis	Prevention of colonization of pathogenic flora on skin by competitive inhibition and the SCFAs production		158,159,169,213
Helicobacter pylori		Rosacea-related signs and symptoms	Production of cytotoxin and by proliferating the production of reactive oxygen species-nitric oxide [NO], which causes gut mucosal inflammation and changes physiological processes in the skin including vasodilation, inflammation and immunomodulation.		194
Faecalibacterium prausnitzii		Chronic atopic dermatitis progression resulting in gut epithelial barrier impairment	Dysregulation of gut epithelial inflammation		180
Lactobacillus casei		Decrease skin inflammation	Alteration of the number of cytotoxic CD8 + T cells		171
Lactobacillus paracasei		Reduce the size of acne lesions as well as inflammation	Inhibition of mast cell degranulation, TNF-α release, edema and vasodilation, and thereby speeding up the restoration of barrier function		145,171 136
Bifidobacterium animalis subsp. lactis [LKM512]		Reduce the scratching behavior in atopic dermatitis	Increase of levels of the kynurenic acid metabolite		223
Bacteroides thetaiotaomicron		Alleviate the allergic symptoms of atopic dermatitis as well as Crohn’s disease like other chronic inflammatory diseases	Anti-inflammatory action		228
Larger number of Clostridium difficile and Escherichia coli		Onset of atopic dermatitis symptoms in childhood	Immune dysregulation as a result of decreased Treg cell inducing beneficial bacteria		16,224
Decrease in Firmicutes and increase in Bacteroides		Development of acne vulgaris	Dysbiosis by altering the serological cytokine levels promoting inflammation		15,188
B. Metabolites
Metabolites	Effects on skin		Mechanism	References
SCFAs	Increase the epithelial barrier function and skin-inflammation		Development of Tregs within the colon, DCs precursors, and IL-10 production	120,229 13,106
GABA	Itch restriction		Inhibition of neurons which are responsible for itch-signaling in the spinal cord	230,223
Tryptophan	Regulate skin inflammation		Activation of AhR and inhibition of TSLP production in keratinocytes	231
Dopamine	Inhibition of hair growth		Through the stimulation of catagen induction	98
Serotonin	Involved in skin pigmentation		Modulation of melatonin	16,223
Acetylcholine	Barrier function		Not reported	16
Phenol & p-cresol	Impaired epidermal barrier function		Skin hydration reduction and disruption of keratinization	171,232
Propionic acid	Promote skin homeostasis by reducing inflammation		Antimicrobial effects	2
Sodium butyrate	Treat psoriasis and other hyperproliferative skin diseases		Modulation of several key cellular processes including differentiation, proliferation, and apoptosis.	137
Galactoligosaccharides and fructooligosaccharides	Reduction of infant eczema and allergy		Through the stimulation of Tregs	13,107
Polysaccharide A and retinoic acid	Suppress inflammation		Induction of accumulation of Tregs	153
Saturated fats and higher amount of glycemic load	Development of acne		Impairment in nutrient signaling. SREBP-1 overexpression and increased sebum synthesis of fatty acids (e.g., free oleic acid) and triglycerides which promotes flourishing. P. acnes growth	107
High-peptides and unsaturated omega-3 fatty acids	Act against hypersensitivity (allergies) and asthma		Through the development of Tregs	13
High-fat and alcohol	Promote skin inflammation and oxidative stress. Impairment of colonic epithelial integrity and barrier function.		Increase of pro-inflammatory cytokines secretion	122,123