The effect of freezing on the survival of Escherichia coli O157:H7 on beef trimmings

The ability of three Escherichia coli O157:H7 strains to survive on beef trimmings stored at -18 degrees C overa 12 week period was determined. This was achieved by plating samples at weekly intervals on non-selective tryptic soy agar (TSA) and selective sorbitol McConkey agar (SMAC) to determine both total numbers and the degree of sub-lethal injury of this pathogen. Two freezing temperatures (-18 or -35 degrees C) to simulate slow or rapid freezing respectively, and two inoculation levels (10(3) cfu g(-1) or 10(4) cfu g(-1)) were used. Further, the ability of an immunoassay (VIP) to detect this pathogen at the end of the frozen storage period was assessed. Counts on TSA remained constant, with a few exceptions, for all treatments while those on SMAC decreased significantly (P less than or equal to 0.05) between 0.5 and 2 log cfu g(-1) over the study period. The VIP test confirmed the findings of the plating experiments. These findings indicate the presence of varying degrees of sub-lethal injury among strains of this pathogen during frozen storage. The inability to eliminate E. coli O157:H7 and the presence of sub-lethally injured strains which are still infectious but undetectable on selective media make freezing an unreliable method to assure the safety of beef trimmings. (C) 2000 Elsevier Science Ltd. All rights reserved.