Evaluation of the Antimicrobial Effects of Olive Mill Wastewater Extract Against Food Spoiling/Poisoning, Fish-Pathogenic and Non-Pathogenic Microorganisms

Although antibiotics are the main therapy for bacterial infections, the reports showed that the overuse (or misuse) of antibiotics will results in several problems such as the development of antibiotic-resistant strains, persistence of drug residues, and numerous environmental concerns. Therefore, finding antibiotic alternatives is considered of vital importance. Investigation of the antimicrobial properties of several plant substances and extracts is of great value to replace antibiotics. With this objective, this study aimed to evaluate the antimicrobial activities of an ethanolic extract prepared from olive mill wastewater (OMWW), which is a by-product of olive oil production with considerable environmental burden, against 38 bacterial strains, including fish-associated pathogens, non-pathogenic isolates, collection strains, and one yeast strain, <i>Candida albicans</i>. Disk diffusion, minimum inhibitory concentration (MIC), and minimum bactericidal/fungicidal concentration (MBC/MFC) tests were used to determine the antimicrobial activity of the OMWWE. According to the results, OMWWE provoked strong inhibitory effects against <i>Shewanella baltica</i> strain SY-S145. It also showed a moderate inhibitory effect on <i>Plesiomonas shigelloides</i> strain SY-PS16 and <i>Vibrio anguillarum</i> strain SY-L24. The MIC and MBC of OMWWE on <i>Shewanella baltica</i> SY-S145, <i>Vibrio gigantis</i> strain C24, and <i>V. anguillarum</i> strain SY-L24 were 500 µg/mL. The MIC and MBC on <i>V. parahaemolyticus</i> ATCC 17802 were 1000 µg/mL, whereas the values for <i>Aeromonas salmonicida</i> ATCC 33658 were 500 µg/mL and 1000 µg/mL, respectively. To put it briefly, the OMWW extract showed high antimicrobial activity and can act as an environmentally friendly additive for the control and prevention of diseases caused by <i>A. veronii</i>, <i>A. hydrophila</i>, <i>P. shigelloides</i>, <i>S. baltica</i>, <i>V. anguillarum</i>, and <i>V. parahaemolyticus.</i> Its active agents also prevented infections of both fish-associated pathogens and food spoiling bacteria, which means it can not only help in the disease control mechanism but also in improving the safety of food by reduction of the microbial contamination.