Evaluation of in vitro antimicrobial and antioxidant properties of Ziziphus robertsoniana Beentje aqueous and methanol extracts

Plants are used in the manufacture of pharmaceuticals, cosmetics, and food supplements. In this study, we extracted secondary metabolites from the leaves of Ziziphus robertsoniana Beentje using methanol and water. The extracts were evaluated for their antimicrobial properties by disk diffusion assay, and antioxidant activities using 2, 2-diphenyl-1-picryl-hydrazyl-hydrate scavenging activity and ferricyanide reduction assay. The functional groups and compounds were determined using Fourier transform infrared spectroscopy and gas chromatography-mass spectroscopy. The screening of the extracts revealed the presence of saponins, tannins, flavonoids and alkaloids compounds, that are linked to various biological activities. The crude extracts showed antioxidant inhibition concentration (IC50) values of 47.88 and 33.59 µg/ml for the aqueous and methanol extracts respectively. The extracts exhibited concentration dependent ferric reducing power. Methanolic extract had inhibitory activity against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aures and Bacillus subtillis with inhibition zones of between 6 and 11 mm as compared to the aqueous extract which showed activity against P. aeruginosa at 50 and 100 mg/ml with zones of inhibition of 5.7 and 8.7 mm respectively. The aqueous extract showed no activity against any of the other three bacterial strains. Both aqueous and methanol extracts had no effect on Candida albicans at 25, 50, and 100 mg/ml. The minimum inhibition concentration values ranged from 25,000 to 3125 µg/ml for the extracts and from 0.5 to 0.03125 µg/ml for ciprofloxacin. These findings demonstrate the presence of beneficial compounds in Z. robertsoniana that can be used in the formulation of new products with antimicrobial and antioxidant activities.