Injury and recovery of Escherichia coli ATCC25922 cells treated by high hydrostatic pressure at 400-600 MPa

Escherichia coli cells were inactivated by high hydrostatic pressure (HHP) at 400-600 MPa and their recovery under various conditions was evaluated by colony counting and flow cytometer (FCM) analyses. The lag time in colony formation and an improved recovery of cells under less oxidative conditions (pyruvate addition to the medium and incubation in anaerobic conditions) were observed for HHP treated cells, which indicated that a significant portion of cells were injured and recovered during incubation after HHP treatment. The lag time for colony formation varied, which suggested a wave of resuscitation and recovered cells may multiply before other injured cells complete resuscitation. The recovery process was monitored by FCM: The FCM profile of cells stained using propidium iodide and SYTO9 indicated that while the majority of cells died just after HHP treatment, the staining pattern of possibly injured cells displayed a specific spectrum that gradually became consistent with that of the dead cell population and a living cell population simultaneously appeared. Pyruvate addition to the medium not only enhanced viability of HHP treated cells, but also reduced the lethal effect of HHP. These observations suggested that the degree of damage by HHP may differ cell-by-cell, and oxidative stress may continue after HHP treatment. Pyruvate addition to the recovery medium enhanced viability of E. coli cells inactivated by HHP treatment in tomato juice as well.