3.3. Inhibitory and bactericidal concentrations of grape seed procyanidin fractions MIC and MBC assays were performed with the fractions obtained from the commercial grape seed extract to assess their antimicrobial activity against the same microorganisms tested for flavan-3-ols (Table 4). All grape seed extract fractions inhibited the strict anaerobes. Among the facultative anaerobic organisms, S. pasteuri and C. jejuni strains were more sensitive to the procyanidin fractions than B. subtilis and E. coli AW1.7. Differential activity of the fractions was observed only for C. jejuni FUA1215, which was more resistant to Fraction III than to other fractions. Propionibacterium spp. showed a higher inhibition for polymeric fraction compared to the other three fractions tested. Polymeric fraction and fraction I showed a higher inhibitory effect than fraction II and III for both Enterococcus strains.