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 tr immings stored at -18 degrees C overa 12 week period was determined. This w as achieved by plating samples at weekly intervals on non-selective tryptic soy agar (TSA) and selective sorbitol McConkey agar (SMAC) to determine bo th 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 free zing 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 t his pathogen at the end of the frozen storage period was assessed. Counts o n TSA remained constant, with a few exceptions, for all treatments while th ose 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 f indings 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 presenc e of sub-lethally injured strains which are still infectious but undetectab le on selective media make freezing an unreliable method to assure the safe ty of beef trimmings. (C) 2000 Elsevier Science Ltd. All rights reserved.