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 ATC C 43894 in Trypticase soy broth (TSB) and ground beef slurry (GBS). Cu ltures were grown to stationary phase, stored for 24 h at 4 to 6 degre es C, and then heat shocked to simulate consumer mishandling of meat d uring the summer. Control or heat-shocked ATCC 43894 cells were then t ransferred to prewarmed TSB (54, 58, and 62 degrees C) or GBS (58 degr ees C) and refrigerated TSB and GBS that were subsequently heated to a nd held at 58 degrees C (TSB and GBS) and 62 degrees C (TSB only). Hea t shock increased D values by 37, 68, and 50% in 54, 58, and 62 degree s C prewarmed TSB, respectively, but had no significant effect on the D value in 58 degrees C GBS. Immediate plating of heated samples yield ed greater cell recovery than if samples were held on ice prior to pla ting. Heat shock did not lead to significant increases in D values whe n cells were transferred to 4 degrees C TSB and GBS that were heated t o the test temperature. This study showed that for E. coli O157:H7 ATC C 43894 the heat-shock effect was lost upon subsequent chilling and re warming and overshadowed by the protective effects of ground beef cons tituents. The results do not support the hypothesis that short-term te mperature abuse will significantly increase the heat resistance of E. coli O157:H7 in ground beef.