Antibacterial activity and mode of action of ε-polylysine against Escherichia coli O157:H7.

Purpose. Gram-negative Escherichia coli O157: H7 were chosen as model bacteria to evaluate the antimicrobial mechanism of epsilon-polylysine (epsilon-PL). Methodology. The antibacterial activity of epsilon -PL was detected by measuring the minimum inhibitory concentration values as well as the time-kill curve. The membrane integrity was determined by ultraviolet (UV) absorption, membrane potential (MP) assay and flow cytometry (FCM) experiments. The permeability of the inner membrane was detected by beta-galactosidase activity assay. Furthermore, electron microscopy [scanning electron microscopy (SEM) and transmission electron microscopy (TEM)] was utilized to observe bacterial morphology. Key findings. These results demonstrated that epsilon-PL showed its antibacterial activity by changing the integrity and permeability of cell membranes, leading to rapid cell death. The electron microscopy analysis (SEM and TEM) results indicated that the bacterial cell morphology, membrane integrity and permeability were spoiled when the E. coli O157: H7 cells were exposed to minimum inhibitory concentrations of epsilon-PL (16 mu g ml(-1)). In addition, the bacterial membrane was damaged more severely when the concentration of epsilon-PL was increased. Conclusion. The present study investigated the antimicrobial mechanism of epsilon-PL by measuring the content of cytoplasmic bgalactosidase, proteins and DNA. In addition, SEM and TEM were carried out to assess the mechanism. These results show that epsilon-PL has the ability to decrease the content of large molecules, cellular soluble proteins and nucleic acids associated with increasing the content of cytoplasmic beta-galactosidase in supernatant by causing damage to the cell membranes. Consequently, the use of epsilon-PL as a natural antimicrobial agent should eventually become an appealing method in the field of food preservation.