Mode of antibacterial action of retrochalcones from Glycyrrhiza inflata.

Licochalcone A-D and echinatin, retrochalcones isolated from the roots of Glycyrrhiza inflata, showed antimicrobial activity. Among them, licochalcone A and C had potent activity against some Gram-positive bacteria. These retrochalcones inhibited oxygen consumption in susceptible bacterial cells. The oxidation of NADH in bacterial membrane preparations was also inhibited by them. NADH-cytochrome c reductase was inhibited by licochalcones, while cytochrome c oxidase was not. NADH-CoQ reductase and NADH-FMN oxidoreductase were not inhibited. The site of respiratory inhibition of licochalcones was thought to be between CoQ and cytochrome c in the bacterial respiratory electron transport chain.