Biol. Pharm. Bull. 29(11) 2226—2235 (2006)

have revealed that polyphenols of various foods and herbs have benefits to human health, and some extracts of polyphenol-rich plants, such as green tea and grape seed, have been added to foods or supplements. The antibacterial activity of various plant polyphenols and plant extracts have also been evaluated in several pharmaceutical studies. Although tea polyphenols and their extracts have been examined in detail, other extracts of polyphenol-rich plants have not been properly evaluated, because the activity of their chemical constituents has not been clearly demonstrated. Many reports on the antibacterial activity of pure polyphenols have been published, however, each result was not able to be compared directly because different methods of evaluation were applied and various bacterial species were used. In order to overcome this problem, we compared the activity of a wide variety of polyphenols against many bacterial species to clarify their antibacterial spectrum, using the same standard method of the minimum inhibitory concentration (MIC). As a result, we found a relatively simple structure–activity relationship, and it was applied to the plant extracts containing various types of polyphenols.