Figure 2.

The integrity and selectivity of the blood–brain barrier in terms of microbial metabolites.

The blood–brain barrier restrains the passage of most microbially produced molecules from the circulation to the central nervous system. Only certain amino acids, carnitine, and phenolic derivatives together with secondary bile acids and products of carbohydrate metabolism may cross the blood–brain barrier. Carrier proteins such as monocarboxylate transporters, receptor-mediated or adsorptive transcytosis facilitate the metabolite’s translocation. Instead of crossing the blood–brain barrier, metabolites may alter the blood–brain barrier integrity resulting into increased permeability and translocation. Abbreviations: 5-AVAB, 5-aminovaleric acid betaine; GABA, ɣ-aminobutyric acid; SCFA, short-chain fatty acid; 3M-4-TMAB, 3-methyl-4-(trimethylammonio)butanoate; 4-TMAP, 4-(trimethylammonio)pentanoate; TMA(O), trimethylamine (-N-oxide). (Figure created with Biorender.com)