Abstract
The inactivation of Escherichia coli O157:H7 (E. coli) in physiological saline and lotus roots by high hydrostatic pressure (HHP) in combination with CO2 or N2 was studied. Changes in the morphology, cellular structure, and membrane permeability of the cells in physiological saline after treatments were investigated using scanning electron microscopy, transmission electron microscopy, and flow cytometry, respectively. It was shown that after HHP treatments at 150-550 MPa, CO2-packed E. coli cells had higher inactivation than the N2-packed and vacuum-packed cells, and no significant difference was observed in the latter two groups. Further, both the morphology and intracellular structure of CO2-packed E.coli cells were strongly destroyed by high hydrostatic pressure. However, serious damage to the intracellular structures occurred in only the N2-packed E. coli cells. During HHP treatments, the presence of CO2 caused more disruptions in the membrane of E. coli cells than in the N2-packed and vacuum-packed cells. These results indicate that the combined treatment of HHP and CO2 had a strong synergistic bactericidal effect, whereas N2 did not have synergistic effects with HHP. Although these two combined treatments had different effects on the inactivation of E. coli cells, the inactivation mechanisms might be similar. During both treatments, E. coli cells were inactivated by cell damage induced to the cellular structure through the membrane components and the extracellular morphology, unlike the independent HHP treatment.