Antioxidant and Antimicrobial Activity of Hydroethanolic Leaf Extracts from Six Mediterranean Olive Cultivars

Abstract

Phenolic profiles, antioxidant, and antimicrobial activities of hydroethanolic olive leaf extracts from six Mediterranean olive cultivars (Croatian: <i>Lastovka</i>, <i>Levantinka</i>, <i>Oblica</i>; Italian: <i>Moraiolo</i>, <i>Frantoio</i>, <i>Nostrana di Brisighella</i>) were investigated. As expected, various distributions of phenolic levels were observed for each cultivar and the total phenolic content showed high variability (ranging from 4 to 22 mg GAE/g of dry extract), with the highest amount of phenolics found in the <i>Oblica</i> sample, which also provided the highest antiradical (ORAC) and reducing activity (FRAP). The screening of individual compounds was performed by HPLC-PDA-ESI-QTOF-MS and the main detected compounds were oleuropein, hydroxytyrosol, oleoside/secologanoside, verbascoside, rutin, luteolin glucoside, hydroxyoleuropein, and ligstroside. While the antioxidant activity of the samples was relatively high, they showed no bactericidal and bacteriostatic activity against <i>E. coli</i> and <i>S.</i> Typhimurium; weak activity against <i>Staphylococcus aureus</i>, <i>Bacillus cereus</i>, and <i>Listeria innocua</i>; and inhibitory effects against <i>Campylobacter jejuni</i> at 0.5 mg dry extract/mL. The obtained results support the fact that olive leaf extracts, and especially those from the <i>Oblica</i> cultivar, could potentially be applied in various industries as natural preservatives and effective and inexpensive sources of valuable antioxidants.