Exploring olive mill wastewater as a reservoir of antimicrobial and antioxidant compounds: insights from a cross-regional Moroccan study.

Olive oil producing countries, including Morocco, generate large volumes of olive mill wastewater (OMW), which is characterized by high organic and phenolic loads, leading to serious environmental concerns. The present study aimed to evaluate this by-product by investigating its antioxidant and antimicrobial potential. Twelve OMW samples collected from five Moroccan regions were subjected to this study. Polyphenol extraction yielded total phenolic contents ranging from 183.49 ± 2.18 to 736.56 ± 2.94 mg GAE/g extract and total flavonoid contents between 43.22 ± 0.42 and 341.92 ± 1.99 mg QE/g extract. HPLC-DAD analysis identified 17 phenolic compounds, with hydroxytyrosol being the predominant compound, reaching concentrations of up to 135.16 ± 0.01 mg/g in OMWExt 2. All extracts exhibited strong antioxidant activity, with total antioxidant capacities between 112.79 ± 0.37 and 515.00 ± 1.84 mg EAA/g extract. The FRAP values ranged from 69.78 ± 1.27 mg ET/g extract to 284.73 ± 0.85 mg ET/g, and the optimal IC₅₀ values were 11.99 ± 1.19 μg/mL (DPPH) and 11.91 ± 0.55 μg/mL (ABTS) for OMWExt 2. Antibacterial activity was observed against both Gram-negative (Escherichia coli K12, Pseudomonas aeruginosa CIP 82.114, and Klebsiella pneumoniae CIP A22) and Gram-positive bacteria (Bacillus subtilis ATCC 6633 and Staphylococcus aureus ATCC 29213), with MIC values ranging from 625 μg/mL to 2.5 mg/mL. Antifungal activity was detected particularly in extracts with high hydroxytyrosol contents, especially OMWExt 2, which presented MIC values between 1.15 and 25 mg/mL against Candida albicans ATCC 10231, Aspergillus flavus MTCC 9606, Aspergillus niger MTCC 282, and Fusarium oxysporum MTCC 9913.