Reactive oxygen species depletion by silibinin stimulates apoptosis-like death in Escherichia coli

Silibinin is the major active component of silymarin, extracted from the medicinal plant Silybum marianum. Silibinin has potent antibacterial activity; however, the exact mechanism underlying its activity has not been elucidated. Here, we investigated the novel mechanism of silibinin against Escherichia coli. Time-kill kinetic assay showed that silibinin possess a bactericidal effect at minimal inhibitory concentration (MIC) and higher concentrations (2- and 4-fold MIC). At the membrane, depolarization and increased intracellular Ca2+ levels were observed, considered as characteristics of bacterial apoptosis. Additionally, cells treated with MIC and higher concentrations showed apoptotic features like DNA fragmentation, phosphatidylserine exposure, and caspase-like protein expression. Generally, apoptotic death is closely related with ROS generation; however, silibinin did not induce ROS generation but acted as a scavenger of intracellular ROS. These results indicate that silibinin dosedependently induces bacterial apoptosis-like death, which was affected by ROS depletion, suggesting that silibinin is a potential candidate for controlling bacteria.