Abstract: Four extracts including water extract, 95% ethanol extract, acetone extract and ethyl acetate extract from Dimocarpus longan seeds were determined for their total flavonoid contents by polyamide adsorption-aluminum nitrate color reaction method. Meanwhile, their antibacterial activities were evaluated by cylinder-plate method and minimal inhibitory concentration (MIC) method. In addition, the abilities of these extracts were tested to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals, hydroxyl radicals and superoxide anion radicals and inhibit lipid peroxidation. The results showed that the 95% ethanol extract had the highest content of total flavonoids, which was (3.90 ± 0.12)% among the four extracts. Its antibacterial activities against Escherichia coli, Proteus vulgaris, Bacillus subtilis, Sarcina lutea, Staphylococcus aureus, Erwinia carotorora and Xanthomonas campestris were stronger than the other extracts. Its inhibition zones were more than 15 mm at the concentration of 100 mg/mL. The MICs of the four extracts against all tested bacteria were not higher than 100 mg/mL. The four extracts superoxide anion radicals had a relatively weak scavenging effect on superoxide anion radicals but could strongly scavenge DPPH and hydroxyl free radicals and resist lipid peroxidation. In terms of IC50 (the concentration required for 50% inhibition), the scavenging effects of the four extracts against DPPH, hydroxyl and superoxide anion radicals ranked in the following sequence: the water extract, the 95% ethanol extract, the acetone extract and the ethyl acetate extract. The IC50 values of the water extract were 0.20, 0.15 mg/mL and 2.69 mg/mL, respectively. Therefore, it can be clearly seen that the 95% ethanol extract is a suitable source of botanical antibacterial agent, and the water extract is more suitable to be deeply developed and utilized as a source of botanical natural antioxidant.

Key words: Dimocarpus longan seeds, polar extract, total flavonoid, antibacterial activity, antioxidant activity