Isolation, Identification, and Antibacterial Activities of Flavonoids from Jujube ( Zizipus Jujuba Mill .) Fruit

ABSTRACTThe aim of this study was to isolate and purify flavonoids from Jujube (Zizipus jujuba Mill.) fruit and further evaluate antibacterial activities. Semi-preparative high-performance liquid chromatography (HPLC) was used to isolate and purify the flavonoids, while nuclear magnetic resonance spectroscopy (NMR) was employed for structural identification. Five flavonoids, including epicatechin, quercetin, rutin, isoquercitrin, and hyperin were identified. Antibacterial activity assays showed quercetin had the greatest inhibitory effect on Escherichia coli, Shigella, and Pseudomonas aeruginosa, and the minimum inhibitory concentration (MIC) against Escherichia coli or Pseudomonas aeruginosa was 250 μg/mL and against Shigella was 125 μg/mL. Rutin had the greatest inhibitory effect on Staphylococcus aureus, the MIC of which was 62.5 μg/mL. Hyperin had the greatest inhibitory effect on Bacillus subtilis, the MIC of which against Bacillus subtilis was 250 μg/mL, whereas that against Staphylococcus aureus was 62.5 μg/mL. We found that the antibacterial effects of the flavonoids were strong at a pH less than 6 and that they dropped sharply with increasing pH. Furthermore, the five metal ions Na+, Ca2+, K+, Fe2+, and Mg2+ generally increased the antibacterial activity of quercetin against Escherichia coli, Shigella, and Pseudomonas aeruginosa, and that of rutin against Staphylococcus aureus. For hyperin, Ca2+ and K+ had the strongest antibacterial effects. The present study suggests that the flavonoids extracted from Jujube (Zizipus jujuba Mill.) fruit were considered a promising candidate for natural and healthy antibacterial agents for the pharmaceutical and food industries.