Antibacterial Mechanism of (−)-Nortrachelogenin in Escherichia coli O157

(−)-Nortrachelogenin is a lignan belonging to group of polyphenolic compounds. Its biological properties in mammalian cells are well-studied; however, its biological effects in microorganisms remain poorly understood. Its efficacy against pathogenic bacteria, including antibiotic-resistant strains, was investigated and it was found that bacteria are highly susceptible to the antibacterial effects of this compound. To investigate the antibacterial mode of action(s) against Escherichia coli O157, its effect on the penetration of SYTOX green into bacterial cells was assayed. The penetration of SYTOX Green into a bacterial cell is a measure of permeability of the plasma membrane. An increase in fluorescence intensity using bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC₄(3)] and 3,3′-dipropylthiacarbocyanine iodide [DiSC₃(5)] was also observed, indicating membrane depolarization. Potassium ion efflux from the cytosol into the extracellular matrix showed that cellular damage due to (−)-nortrachelogenin treatment resulted in the loss of intracellular components. While cells were damaged by (−)-nortrachelogenin, large unilamellar vesicles containing fluorescein isothiocyanate-dextran were perturbed to migrate molecules between 3.3 and 4.8 nm. The release of calcein from giant unilamellar vesicles, occurring as a result of disruption in artificial membranes, was visualized. Taken together, our results indicate that (−)-nortrachelogenin exerts its antibacterial effect by disorganizing and perturbing the cytoplasmic membrane, demonstrating the potential of this compound as a candidate for antibiotic drug development.