| Summary: | The inactivation of <i>Escherichia coli</i> O157:H7 (<i>E. coli</i>) in physiological saline and lotus roots by high hydrostatic pressure (HHP) in combination with CO<sub>2</sub> or N<sub>2</sub> 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, CO<sub>2</sub>-packed <i>E. coli</i> cells had higher inactivation than the N<sub>2</sub>-packed and vacuum-packed cells, and no significant difference was observed in the latter two groups. Further, both the morphology and intracellular structure of CO<sub>2</sub>-packed <i>E.coli</i> cells were strongly destroyed by high hydrostatic pressure. However, serious damage to the intracellular structures occurred in only the N<sub>2</sub>-packed <i>E. coli</i> cells. During HHP treatments, the presence of CO<sub>2</sub> caused more disruptions in the membrane of <i>E. coli</i> cells than in the N<sub>2</sub>-packed and vacuum-packed cells. These results indicate that the combined treatment of HHP and CO<sub>2</sub> had a strong synergistic bactericidal effect, whereas N<sub>2</sub> did not have synergistic effects with HHP. Although these two combined treatments had different effects on the inactivation of <i>E. coli</i> cells, the inactivation mechanisms might be similar. During both treatments, <i>E. coli</i> cells were inactivated by cell damage induced to the cellular structure through the membrane components and the extracellular morphology, unlike the independent HHP treatment.
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