E. coli 0157:H7 is an important foodborne pathogen, responsible for several outbreaks of hemorrhagic colitis where improperly cooked hamburger meat was thought to be the vehicle. Various time/temperature combinations were used to determine the optimum conditions of heat shock which would result in the greatest number of survivors to a 55°C heat treatment. The optimum conditions were 42°C for 5 minutes and were used throughout the study.

Heat shock of aerobically grown cells resulted in an increase in the mean D value after a 55°C heat treatment by a factor of 2.1 over nonheat-shocked controls. Heat shock of anaerobically grown cells also resulted ina significant increase in mean D value over nonheat-shocked controls. Anaerobic growth itself resulted in an increase in the ability of the cells to survive the 55°C heat treatment when compared with aerobically grown cells. Both heat-shocked and anaerobically grown cells contained a protein corresponding to a sigma³² subunit of RNA polymerase which has been identified as the 71,000 Galton heat shock protein characteristic of E. coli cells.

Anaerobic plating resulted in a significant increase in the mean D values of both aerobically grown and anaerobically grown cells. The largest increase in mean D values was observed in aerobically grown non-heat-shocked cells, which increased by a factor of 2.3 when plated anaerobically instead of aerobically. The activities of catalase and superoxide dismutase in aerobically grown and anaerobically grown cells were studied to determine the reason why anaerobic plating enhanced recovery of cells. The activities of both enzymes were eliminated after heat treatment at 55°C for 20 minutes, regardless of whether the cells were heat-shocked or not.

The ability of heat shock and anaerobic growth to protect the cells from a subsequent heat treatment was tested by measuring the rate of release of cell materials during heating at 55°C. Heat-shocking and anaerobic growth resulted in even faster release of cell materials during heating than controls, suggesting that neither of these stresses protected the cells against the effects of heat.

The effect of heat shock on cell injury was studied. Heat shock of aerobically grown cells resulted in the greatest difference in log number of cells between cells plated in nonselective medium vs. selective medium. Thus, more cells were injured if heat-shocked than if not heat-shocked. Heat-shocking of anaerobically grown cells also resulted in more injured cells than non-heat-shocked controls.