Sublethal injury and recovery of Escherichia coli O157:H7 by high pressure carbon dioxide

A study was carried out to investigate possible sublethal injury caused by high pressure carbon dioxide (HPCD) to Escherichia coli O157:H7 suspended in phosphate-buffered saline (PBS, pH 7.00). The treatment pressure was 5 MPa, treatment temperatures were 25–37 °C and treatment time was 5–70 min. Only 1.21 log₁₀ cycles reduction was obtained in the nonselective medium, while a higher reduction of 5.18 log₁₀ cycles was obtained in the selective medium after treatment at 5 MPa and 25 °C for 50 min, indicating HPCD could induce sublethal injury of E. coli cells and these cells could not recover in the selective medium. The proportion of the sublethally injured E. coli cells in the survivors increased dramatically up to 72.4% with increasing the treatment time from 20 min to 30 min by HPCD at 5 MPa and 25 °C, and this proportion increased gradually with increasing the treatment time, up to 100% at 5 MPa and 25 °C for 60 min. The effect of recovery media, inhibitors of specific metabolic processes, Mg and Ca cations on the recovery of the sublethally injured cells was also investigated. When PBS, peptone water, tryptic soy broth (TSB) and carrot juice were used as recovery media, the maximum repair rate of the sublethally injured cells was found in TSB and the minimum was in PBS. The addition of inhibitors such as sodium azide, chloramphenicol, rifampicin and penicillin G showed that repair of most sublethally injured cells required energy, protein and RNA synthesis, but was not dependent on peptidoglycan synthesis. Moreover, Mg and Ca cations were also needed in the repair.