Our study showed that strict anaerobic members of intestinal microbiota were very sensitive to flavan-3-ols and grape seed procyanidins. Flavan-3-ols and oligomeric procyanidins bound bacterial cell wall components, resulting in a variation of their effect on different bacterial species. In addition, the metabolism of intestinal strict anaerobes is reliant on iron-dependent anaerobic electron transfer chains , whereas anaerobic metabolism of lactic acid bacteria does not require iron or electron transfer chains. This highly selective antimicrobial activity of procyanidins potentially impacts the composition of intestinal microbiota by selective inhibition of specific species.