Ultrasound-Assisted Extraction of Syringa vulgaris Mill., Citrus sinensis L. and Hypericum perforatum L.: Phenolic Composition, Enzyme Inhibition and Anti-quorum Sensing Activities

Phenolic compounds constitute a class of bioactive metabolites whose extraction from plants, characterization and medicinal applications are of significant importance. Phenolic extracts were prepared from Syringa vulgaris , Citrus sinensis and Hypericum perforatum by ultra-sound assisted extraction with ethanol:water (70%) as solvent. Using HPLC–DAD with twenty-six standards, the major constituents detected were p -coumaric acid (48.23 ± 0.33 µg/g), rutin (30.85 ± 0.88 µg/g), ferulic acid (27.72 ± 0.37 µg/g), and p -hydroxy benzoic acid (25.68 ± 0.31 µg/g) in S. vulgaris , hesperidin (242.3 ± 1.54 µg/g), rutin (62.51 ± 0.86 µg/g), and quercetin (20.85 ± 0.48 µg/g) in C. sinensis and quercetin (43.71 ± 0.68 µg/g), rutin (29.23 ± 0.43 µg/g), and coumarin (17.40 ± 0.28 µg/g) in H. perforatum . At 200 µg/mL, the most active extract was that of H. perforatum which inhibited AChE (45.84 ± 1.18%) and BChE (67.40 ± 0.98%) appreciably compared to the standard galantamine with inhibitions on AChE and BChE of 83.43 ± 0.67% and 76.51 ± 0.31%, respectively. At 200 µg/mL, C. sinensis showed good urease inhibition of 60.20 ± 0.84% compared to thiourea (79.45 ± 0.25%) and inhibition of tyrosinase of 41.59 ± 0.95% as against 75.27 ± 0.56% for kojic acid. Apart from H. perforatum which had MIC of 0.625 mg/mL on Chromobacterium violaceum CV026, all extracts had MIC values of 1.25 mg/mL Chromobacterium violaceum CV026 and Chromobacterium violaceum CV12472. Only S. vulgaris extract exhibited anti-quorum sensing activity on C. violaceum CV026 with inhibition zones of 10.5 ± 1.0 mm at MIC and 8.0 ± 0.5 mm at MIC/2. All extracts inhibited violacein production in C. violaceum CV12472 at MIC and MIC/2 and the highest activities were observed at MIC concentrations for S. vulgaris (100.0 ± 0.0%), C. sinensis (100.0 ± 0.0%) and H. perforatum (29.1 ± 0.8%). At the highest test concentration of 100 µg/mL, S. vulgaris (24.33 ± 1.05%), C. sinensis (8.90 ± 0.18%) and H. perforatum (43.58 ± 1.00%) inhibited swarming motilities on flagellated Pseudomonas aeruginosa PA01. These results indicate that phenolic extracts of the studied plants can be used to manage Alzheimer’s disease and reduce emergence of microbial resistance and severity during infections.