Ultrasound-Assisted Extraction of Syringa vulgarisMill., Citrus sinensisL. and Hypericum perforatumL.: 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 sinensisand Hypericum perforatumby 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. sinensisand 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. perforatumwhich 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. sinensisshowed 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. perforatumwhich had MIC of 0.625 mg/mL on Chromobacterium violaceumCV026, all extracts had MIC values of 1.25 mg/mL Chromobacterium violaceumCV026 and Chromobacterium violaceumCV12472. Only S. vulgarisextract exhibited anti-quorum sensing activity on C. violaceumCV026 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. violaceumCV12472 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 aeruginosaPA01. 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.