Abstract: The objective of this study was to investigate the survival of frozen Escherichia coli O157:H7 during thawing. The injury and death of four E. coli O157:H7 strains doe to freezing were compared and the effect of different thawing methods on the survival of frozen E. coli O157:H7 in nutrient-free phosphate buffer saline (PBS) solution was determined. The results showed that after being frozen at ?20 ℃ for 24, 48 or 72 h, cell injury and even death occurred in all the four strains and the severity was time dependent and varied among strains. After being frozen for 72 h, strain CICC21530 showed the highest injury rate (87.70%). When a mixture of four strains was frozen in PBS and then immediately thawed at 20, 30 or 37 ℃, cell death was aggravated with increasing thawing temperature. The colony counts of 72 h frozen E. coli cells were significantly reduced at 48 h of thawing at each temperature (P < 0.05). Additionally, to explore the effect of slow thawing on the survival of E. coli O157:H7, the frozen strains were thawed slowly at 4 ℃ for different durations (0, 2, 6, and 12 h), and then kept at 37 ℃ for 5, 10, and 30 min, respectively. The results showed that slow thawing for a longer time was more beneficial to bacterial survival. Frozen E. coli were thawed at 4 ℃ for 12 h/37 ℃ for 5–30 min and then cultured on sorbitol macconkey agar (SMAC). As a result, higher bacterial counts were found compared with they were cultured on tryptose soya agar (TSA), indicating that the injured cells existed. This study suggests that slow thawing is conductive to bacteria surviving freeze and more attention needs to paid to the detection and control of surviving bacteria, especially injured bacteria during the risk assessment of frozen food.

Key words: Escherichia coli O157:H7, freezing, thawing, survival, injury