Abstract: As a widely used non-thermal decontamination technology in the food processing industry, high hydrostatic pressure processing has a significant lethal effect on foodborne Escherichia coli O157:H7. However, effective methods for detecting viable cells after high hydrostatic pressure treatment are still lacking, and the inactivation mechanism of pressurized Escherichia coli O157:H7 needs to be confirmed. In this study, the living state of high hydrostatic pressuretreated Escherichia coli O157:H7 cells was evaluated by flow cytometric analysis and the conventional culture technique. The microstructure of pressurized bacterial cells was observed by scanning and transmission electron microscopy. The inactivation mechanism of high-pressure-treated cells was further explored. Results showed that double-staining flow cytometry was a rapid and effective method to sort Escherichia coli O157:H7 cells in different physiological states with high sensitivity and accuracy. Significant damage and sub-lethal injury of Escherichia coli O157:H7 cells were observed after treatment at 400 MPa, and the bacterial cells were seriously deformed with partial rupture of the cell membrane at this pressure level. When the pressure was increased up to 500 MPa, the bacterial cell wall and membrane were separated severely from each other with lots of radial filaments appearing in the light transparent area. It is worth noting that there is a large part of sub-lethally injured pathogenic cells surviving even after high hydrostatic pressure treatment at 500 MPa, which may be a potential risk factor for food-borne disease outbreaks.

Key words: Escherichia coli O157:H7, high hydrostatic pressure, flow cytometry, sub-lethal injury, electron microscope