epsilon-Poly-L-lysine (epsilon-PL)c(2) is widely used as an antibacterial agent because of its broad antimicrobial spectrum. However, the mechanism of epsilon-PL against pathogens at the molecular level has not been elucidated. This study investigated the antibacterial activity and mechanism of epsilon-PL against Escherichia coli O157:H7 CMCC44828.-Propidium monoazide-PCR test results indicated that the threshold condition of epsilon-PL for complete membrane lysis of E. coil O157:H7 was 10 mu g/mL (90% mortality for 5 mu g/mL). Further verification of the destructive effect of epsilon-PL on cell structure was performed by atomic force microscopy and transmission electron microscopy. Results showed a positive correlation between reactive oxygen species (ROS)(3) levels and epsilon-PL concentration in E. coli O157:H7 cells. Moreover, the mortality of E. coil O157:H7 was reduced when antioxidant N-acetylcysteine was added. Results from real-time quantitative PCR (RT-qPCR)(4) indicated that the expression levels of oxidative stress genes sodA and oxyR were up-regulated 4- and 16-fold, respectively, whereas virulence genes eaeA and espA were down-regulated after E-PL treatment. Expression of DNA damage response (SOS response)(5) regulon genes recA and lexA were also affected by E-PL. In conclusion, the antibacterial mechanism of E-PL against E. coli O157:H7 may be attributed to disturbance on membrane integrity, oxidative stress by ROS, and effects on various gene expressions, such as regulation of oxidative stress, SOS response, and changes in virulence. (C) 2013 Published by Elsevier Inc.