Inactivation of Staphylococcus aureus and Escherichia coli by the synergistic action of high hydrostatic pressure and dissolved CO2

This study focused on the synergistic inactivation effects of combined treatment of HHP and dissolved CO2 on microorganisms. The aim was to reduce the treatment pressure of the traditional HHP technology and make it more economically feasible. The combined treatment showed a strong bactericidal effect on Staphylococcus aureus and Escherichia coli in liquid culture, which usually have high levels of barotolerance under pressure alone. To identify the influence of CO2, a new setup to dissolve, retain and measure the concentration of CO2 was constructed. The results demonstrated that an inactivation rate of more than 8 log units was obtained for E. coli both at 300 MPa with 1.2 NL/L CO2 and at 250 MPa with 3.2 NL/L CO2, while only 2.2 and 1.8 log reductions were observed at 300 MPa and 250 MPa, respectively, for the HHP treatments alone. For S. aureus, the inactivation rate of more than 7 log units was found at 350 MPa with 3.8 NL/L CO2, while only a 0.9 log reduction was achieved at this pressure in the absence of CO2. The SEM photographs showed seriously deformed cells after the synergistic treatments. In contrast, the cells treated with individual HHP maintained a relatively smooth surface with invaginations. Propidium iodide staining and fluorescence observation was performed after pressure treatments. The results demonstrated that the combination of CO2 with HHP also promoted pressure induced cell membrane permeabilization greatly. It was deduced that the enrichment of CO2 on the cell surface and its penetration into the cells at high pressure accounted for the membrane damage and cell death.