Antimicrobial activity of flavan-3-ols and oligomeric proanthocyanidins from pea seed coats and grape seeds against food and gut bacteria

Abstract

Proanthocyanidins and their monomeric subunits flavan-3-ols belong to the polyphenol group of flavonoids. They have been proposed to exert health promoting effects against cancer, Type 2 diabetes and coronary disease among other conditions. These health promoting effects are believed to be related to the modulation of gastric transit, the inhibition of digestive enzymes, the modulation of the gut microbiota and the direct health promoting effects upon absorption, but the mechanisms remain unknown. This thesis focuses on the properties of modulation of gut microbiota as well as the inhibition of digestive enzymes. The proanthocyanidin extracts from pea seed coats were analyzed using UPCL-ESI-MS prior to their separation using High Speed Counter Current Chromatography (HSCCC), a technique with several advantages over other chromatographic methods. No flavan-3-ols or proanthocyanidins were identified in these extracts, hypothesizing that these molecules underwent oxidation and are now highly polymerized. Commercial grape seed extract was selected as the raw material for the separation of proanthocyanidins by their degree of polymerization. It was possible to isolate 3 fractions: Fraction I with trimeric proanthocyanidins, Fraction II with trimeric and dimeric proanthocyanidins and Fraction III with dimeric proanthocyanidins as well as a polymeric proanthocyanidin fraction; flavan-3-ols were commercially available. The antimicrobial effect of flavan-3-ols increased with the number of hydroxyl groups in the molecule, specifically by OH groups and gallate moieties located on the B ring of the monomer. The antimicrobial effect of oligomeric proanthocyanidins from grape seed extract does not increase with the length of the oligomer. The enzyme inhibition properties of flavan-3-ols is increased with the number of hydroxyl groups present in the monomer, but there was no evidence that this trait increased with the length of the oligomer in the case of grape seed proanthocyanidins. Gut microbiota modulation and enzyme inhibition properties of flavan-3-ols and proanthocyanidins cannot entirely explain the health promoting effects exerted by these compounds. Therefore, further research needs to be done in the modulation of the gastric transit by flavan-3-ols and oligomeric proanthocyanidins as well as their direct effects upon absorption; the determination of the antimicrobial effect of flavan-3-ol metabolites will be useful for a more thoroughly understanding of the modulation of gut microbiota by flavan-3-ols and proanthocyanidins.