Changes in heat resistance of Escherichia coli O157:H7 following heat shock

This study examined the effects of a heat shock at 45 degrees C for 30 min on the subsequent heat resistance of Escherichia coli O157:H7 ATCC 43894 in Trypticase soy broth (TSB) and ground beef slurry (GBS). Cultures were grown to stationary phase, stored for 24 h at 4 to 6 degrees C, and then heat shocked to simulate consumer mishandling of meat during the summer. Control or heat-shocked ATCC 43894 cells were then transferred to prewarmed TSB (54, 58, and 62 degrees C) or GBS (58 degrees C) and refrigerated TSB and GBS that were subsequently heated to and held at 58 degrees C (TSB and GBS) and 62 degrees C (TSB only). Heat shock increased D values by 37, 68, and 50% in 54, 58, and 62 degrees C prewarmed TSB, respectively, but had no significant effect on the D value in 58 degrees C GBS. Immediate plating of heated samples yielded greater cell recovery than if samples were held on ice prior to plating. Heat shock did not lead to significant increases in D values when cells were transferred to 4 degrees C TSB and GBS that were heated to the test temperature. This study showed that for E. coli O157:H7 ATCC 43894 the heat-shock effect was lost upon subsequent chilling and rewarming and overshadowed by the protective effects of ground beef constituents. The results do not support the hypothesis that short-term temperature abuse will significantly increase the heat resistance of E. coli O157:H7 in ground beef.