Antimicrobial and antibiofilm activities of pomegranate peel phenolic compounds: Varietal screening through a multivariate approach

Pomegranates are rich in phenolic compounds and known for their antioxidant, anti-inflammatory, and anticancer properties. The highest concentration of these compounds is found in the peel (exocarp and mesocarp), which constitutes about 50% of the whole fresh fruit. These bioactive phytochemicals exhibit a broad spectrum of antimicrobial effects against both Gram-negative and Gram-positive bacteria, as well as fungi. In the present paper, the chemical composition and antimicrobial activity of the peel (exocarp and mesocarp) from seven Punica granatum varieties (Wonderful, Mollar de Elche, Primosole, Sassari 1, Sassari 2, Sassari 3, and Arbara Druci) grown in Sardinia (Italy) were evaluated. Polar phenols, flavonoids, condensed tannins, and anthocyanin contents were evaluated by extraction with water at 20 and 40 °C. Orthogonal projections to latent structures discriminant analysis (OPLS-DA) was used to characterize each variety according to the chemical composition of the pomegranate peel extracts (PPEs). The antimicrobial and antibiofilm activities of each PPE were further tested in vitro against Staphyloccocus aureus, Listeria monocytogenes, Salmonella bongori, Escherichia coli, Lacticaseibacillus casei and Limosilactobacillus reuteri. Gram-positive species were more sensitive than Gram-negative to the extracts tested. Antimicrobial activity was shown against S. aureus and L. monocytogenes strains, whereas less, even no activity was found against S. bongori and E. coli strains. The PPEs from Mollar de Elche, Primosole, and Sassari 3 showed the highest antimicrobial activities at concentrations that varied from 0.19 to 1.50 mg/mL, with biofilm activity being reduced by more than 70%. These activities were positively related to the punicalagin, flavonoid, and chlorogenic acid content of the extracts. Finally, regarding the pro-technological bacterial strains, La. casei and Li. reuteri showed very low, even no sensitivity to the used of the specific PPEs with high concentrations. This study proposes a formulation of pomegranate peel extract that valorizes agro-industrial waste in the context of sustainability and circular economy. Pomegranate extracts should be considered potential sources of natural, plant-derived antimicrobials, providing an alternative to artificial antimicrobial products.