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

Authors: KIM, YOONBIN - University Of California, Davis; ZHAO, HEFEI - University Of California, Davis; AVENA-BUSTILLOS, ROBERTO - US Department Of Agriculture (USDA); WANG, SELINA - University Of California, Davis; NITIN, NITIN - University Of California, Davis

Submitted to: Chemical and Biological Technologies in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/24/2024
Publication Date: 8/28/2024
Citation: Kim, Y., Zhao, H., Avena-Bustillos, R., Wang, S.C., Nitin, N. 2024. Synergistic antimicrobial activities of aqueous extract derived from olive byproduct and their modes of action. Chemical and Biological Technologies in Agriculture. 11. Article 122. https://doi.org/10.1186/s40538-024-00634-5.
DOI: https://doi.org/10.1186/s40538-024-00634-5

Interpretive Summary: A plant byproduct-based antimicrobial was developed using olive pomace, and its synergistic antimicrobial activities were evaluated. Aqueous olive pomace extract was obtained using water-based, ultrasound-assisted extraction. The extract showed strong antimicrobial activities both against Gram-negative and -positive bacteria and achieved more than 5 log reductions of E. coli O157:H7 and L. innocua cells within 30 min. Extract components showed strong synergistic activities through diverse modes of action. Bacterial cells treated with olive pomace extracts exhibited a higher degree of oxidative stress, membrane damage, and inhibition of metabolic activities compared to those treated with a single phenolic component. The results of this study suggest a strong potential of utilizing plant byproduct-derived antimicrobials as a promising substitute for conventional antimicrobials. This could lead to the application of olive pomace extracts in the food industry as a natural preservative or in the agriculture sector for developing new biopesticides, thereby enhancing sustainability in the food and agriculture system.

Technical Abstract: 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. 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 (') of 0.092 and 0.014 against E. coli O157:H7 and L. innocua, respectively (' < 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. 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.