The inhibitory effect of all fractions was bactericidal against all mouse gut bacteria along with C. jejuni FUA1211 (Table S2). The antibacterial effect was bacteriostatic for B. subtilis, E. coli, En. faecalis and S. pasteuri. Lm. reuteri FUA3400 was resistant to all compounds tested. The antibacterial effect of the grape seed extract procyanidin fractions was not affected by the degree of polymerization of the oligomer. However, the inhibitory effect of monomers seems to be higher than the oligomeric procyanidins, especially in the case of EGCG.