Antibacterial activity of tannins isolated from Sapium baccatum extract and use for control of tomato bacterial wilt

In the search for new antibacterial agents from natural sources, we revealed that a crude methanol extract of Sapium baccatum was highly active against Ralstonia solanacearum, a causal agent of a serious disease called bacterial wilt of tomato. The bioassay-guided fractionation of this extract resulted in the isolation of seven known active compounds, including gallic acid, methyl gallate, corilagin, tercatain, chebulagic acid, chebulinic acid, and quercetin 3-O-α-L-arabinopyranoside. Their chemical structures were determined by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy. An in vitro antibacterial bioassay using a broth microdilution method revealed that, except for quercetin 3-O-α-L-arabinopyranoside (MIC = 250 μg/mL), the isolated compounds exhibited strong antibacterial activity against R. solanacearum (MIC = 26–52 μg/mL). Among the seven compounds, methyl gallate exhibited the strongest broad-spectrum activity against most of the plant pathogenic bacteria tested (MIC = 26–250 μg/mL). In the in vivo experiments, the crude extract of S. baccatum at 2000 and 1000 μg/mL reduced the development of tomato bacterial wilt by 83 and 63%, respectively, under greenhouse conditions after 14 days of infection. The results suggested that the extracts of S. baccatum or isolated tannins could be used as natural bactericides for the control of bacterial wilt of tomato.