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Inhibition of Bacteria by the Soybean Isoflavonoids Glyceollin and Coumestrol. W. F. Fett, Research plant pathologist, Eastern Regional Research Center, USDA, ARS, Philadelphia, PA 19118; S. F. Osman, research chemist, Eastern Regional Research Center, USDA, ARS, Philadelphia, PA 19118. Phytopathology 72:755-760. Accepted for publication 20 October 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1982.. DOI: 10.1094/Phyto-72-755.

The soybean isoflavonoids glyceollin and coumestrol were tested for inhibitory activity against various strains of bacteria including the soybean bacterial leaf pathogens Pseudomonas syringae pv. glycinea and Xanthomonas campestris pv. glycines. Six different bioassays were used to test for effects of glyceollin and two for coumestrol. Glyceollin inhibited growth of most bacteria, including P. syringae pv. glycinea and X. campestris pv. glycines, and was shown to be bactericidal towards strains of Bacillus subtilis, B. licheniformis, Staphylococcus aureus, and Corynebacterium flaccumfaciens pv. flaccumfaciens. Three bioassays with agar media were evaluated; direct spotting onto agar media seeded with bacteria was the most efficient method for demonstrating the inhibitory activity of glyceollin. Only bioassays in liquid media demonstrated inhibitory activity of glyceollin against P. syringae pv. glycinea. Two strains of P. syringae pv. glycinea were shown to differ markedly in sensitivity to glyceollin. Coumestrol was bactericidal towards strains of B. subtilis, B. licheniformis, S. aureus, Streptococcus thermophilus, and X. campestris pv. glycines but did not inhibit growth of four P. syringae pv. glycinea strains. The results indicate a possible role of glyceollin and coumestrol in inhibiting the bacterial pathogen X. campestris pv. glycines and secondary bacterial invaders.