Escherichia coli serotype O157:H7 cells were grown at 30°C for 6 h and subjected to a heat stress, or heat shock, at 42°C for 5 min. Heat-shocked and nonheat-shocked controls were heat treated at 55°C for up to 60 min. The number of injured cells was significantly higher in heat-shocked cells than in controls, and the rate of release of cell components was higher in heat-shocked cells. Anaerobic plating resulted in higher recovery of injured cells, when compared with aerobic plating, regardless of whether the cells were heat shocked or not. In addition, heat shocking resulted in lower catalase and superoxide dismutase activities when compared with controls. It also resulted in greater survivability after exposure to hydrogen peroxide, suggesting that heat shocking somehow enables the cells to survive exposure to toxic substances in addition to heat. The heat-shock response, coupled with anaerobic conditions, increased the ability of E. coli O157:H7 cells to recover after a heat treatment. Thus, heat shock did not afford protection to the cells against injury, but rather enhanced their ability to recover during storage.
1Present address: Dept of Microbiology, Immunology & Preventive Medicine, Iowa State University, 382 Science I, Ames, IA 50011.