Encapsulation of <i>Olea europaea</i> Leaf\nPolyphenols in Liposomes: A Study on Their Antimicrobial Activity\nto Turn a Byproduct into a Tool to Treat Bacterial Infection

According to the innovative and sustainable perspective\nof the\ncircular economy model, <i>Olea europaea</i> leaves, a solid\nbyproduct generated every year in large amounts by the olive oil production\nchain, are considered a valuable source of bioactive compounds, such\nas polyphenols, with many potential applications. In particular, the\nfollowing study aimed to valorize olive leaves in order to obtain\nproducts with potential antibacterial activity. In this study, olive\nleaf extracts, rich in polyphenols, were prepared by ultrasound-assisted\nextraction using green solvents, such as ethanol and water. The extracts\nwere found to be rich in polyphenols up to 26.7 mg<sub>GAE</sub>/g<sub>leaves</sub>; in particular, hydroxytyrosol-hexose isomers (up to\n6.6 mg/g<sub>dry extract</sub>) and oleuropein (up to 324.1 mg/g<sub>dry extract</sub>) turned out to be the most abundant polyphenolic\ncompounds in all of the extracts. The extracts were embedded in liposomes\nformulated with natural phosphocholine and cholesterol, in the presence\nor in the absence of a synthetic galactosylated amphiphile. All liposomes,\nprepared according to the thin-layer evaporation method coupled with\nan extrusion protocol, showed a narrow size distribution with a particle\ndiameter between 79 and 120 nm and a good polydispersity index (0.10–0.20).\nFurthermore, all developed liposomes exhibited a great storage stability\n up to 90 days at 4 °C and at different pH values, with no significant\nchanges in their size and polydispersity index. The effect of the\nencapsulation in liposomes of <i>O. europaea</i> leaf extracts\non their antimicrobial activity was examined <i>in vitro</i> against two strains of <i>Staphylococcus aureus</i>: ATCC\n25923 (wild-type strain) and ATCC 33591 (methicillin-resistant <i>S. aureus</i>, MRSA). The extracts demonstrated good antimicrobial\nactivity against both bacterial strains under investigation, with\nthe minimum inhibitory concentration ranging from 140 to 240 μg<sub>extract</sub>/mL and the minimum bactericidal concentration ranging\nfrom 180 to 310 μg<sub>extract</sub>/mL, depending on the specific\nextract and the bacterium tested. Moreover, a possible synergistic\neffect between the bioactive compounds inside the extracts tested\nwas highlighted. Notably, their inclusion in galactosylated liposomes\nhighlighted comparable or slightly increased antimicrobial activity\ncompared to the free extracts against both bacterial strains tested.