Modeling the inactivation of Escherichia coli O157:H7 and generic Escherichia coli by supercritical carbon dioxide

In this study, supercritical carbon dioxide (SC-CO₂) was applied in the inactivation of pathogenic Escherichia coli (E. coli) O157:H7 and generic E. coli. For both strains suspended in physiological saline (PS), colony forming units per ml were reduced by 8 log orders within 15-30 min, in a treatment range of 80-150 bar and 35-45 °C. Any significant differences between the two E. coli strains during the inactivation by SC-CO₂ were not noticed. The microbial inactivation curve, which was established by the modified Gompertz model describing the survival rate with treatment time, was divided into three distinct stages. When using cells in PS, k_dm, λ and t₈ (the time for an 8-log reduction of cell counts) were 0 to 3 min^{- 1}, 8 to 16 min and 11 to 29 min, respectively. The temperature-dependency of the microbial inactivation was verified via the correlation of the logarithm of k_dm versus the inverse of temperature. We have observed lower inactivation rates in phosphate-buffered saline (PBS) than in PS, the lowered pH, and an increase of UV-absorbing substances in the cell suspension after SC-CO₂ treatment. Also, the deformation and collapse of the SC-CO₂-treated cells were revealed by scanning and transmission electron microscopy, and the deactivation of cellular enzymes occurred. These all suggest that the inactivation of E. coli O157:H7 and generic E. coli was possibly caused in a concerted manner by acidification, damage to cell membranes and subsequent leakage of cellular materials, and the inactivation of cellular enzymes.