Synergistic antimicrobial activities of aqueous extract derived from olive byproduct and their modes of action

Background

Plant-derived antimicrobials (PDAs) are considered a viable alternative to synthetic antimicrobial agents. Diverse antimicrobial mechanisms of PDAs significantly reduce the risk of developing antimicrobial resistance. Utilization of PDAs also offers economic and environmental advantages, as they can be derived from agricultural byproducts, such as olive pomace.

Results

In this study, a green, water-based, ultrasound-assisted extraction (UAE) was deployed to obtain aqueous olive pomace extract (OPE) from dry olive pomace. Total phenolic content, extraction yield, chemical compositions, and antimicrobial activities of OPE were evaluated. In addition, the potential synergistic interaction between the phenolic components in OPE and the antimicrobial mechanisms underlying the synergistic interaction were characterized. The results show that ca. 25 mg GAE/g of extraction yields were achieved by the UAE of dry olive pomace. Based on the high-performance liquid chromatography (HPLC) analysis, diverse phenolic compounds such as gallic acid (GA), hydroxytyrosol (HT), and 4-hydroxyphenylacetic acid (4-HPA) were identified in OPE. OPE exhibited strong antimicrobial activities, and 0.2 mg GAE/mL of OPE achieved > 5 log reductions of Escherichia coli O157:H7 and Listeria innocua cells within 30 min of treatment. A 3D isobologram analysis demonstrated that OPE exhibited strong synergistic antimicrobial activities, compared to those of individual phenolic components (GA, HT, or 4-HPA), showing interaction index (γ) of 0.092 and 0.014 against E. coli O157:H7 and L. innocua, respectively (γ < 1: synergistic activity). Antimicrobial mechanism analyses revealed that phenolic components in OPE exerted strong synergistic activities through diverse modes of action, and increased levels of oxidative stress, membrane damage, and decreased levels of metabolic activities were observed in the OPE-treated bacterial cells.

Conclusions

These findings demonstrate an approach for valorizing agricultural byproducts to develop plant byproduct-based antimicrobials with strong synergistic activities. Multiple modes of action of this byproduct extract may enable the control of diverse microbes in food and agriculture systems.

Graphical abstract