Fate of Field-Isolated Escherichia coli O157 in Ground Beef at Different Storage Temperatures

Abstract

The survival of six Escherichia coli O157 strains, including five strains recently isolated from beef carcasses and strain ATCC 43895, was evaluated at 0, 1, 7, and 14 days in ground beef held at −20, 1, 4, and 7°C. Only small losses in cell numbers occurred at −20 and 1°C; in general, cell numbers decreased during the first day of storage and then remained unchanged through day 14. At −20°C, statistically significant reductions in cell numbers were observed only for strains 55AC1 and 299AB3 due to greater losses in the first day. At 1°C, strain 131AC1 did not decrease in cell numbers during the first day of storage, but both this strain and strain 55AC1 experienced statistically significant reductions in viable cell numbers by day 14, primarily due to losses after day 7. At 4°C, after an initial loss of cell numbers for four strains, minor increases were observed for all six strains by day 14. The differences were statistically significant for strains 114AC1, 299AB3, and ATCC 43895, but were small enough to question whether they refect actual growth. When the inoculated ground beef was stored at 7°C for 14 days, growth of all six strains was statistically significant, with populations increasing between 0.9 and 1.5 log10 CFU/g. This study demonstrates that there are small differences in the abilities of various E. coli O157 strains to survive and sometimes grow in fresh ground beef at cold storage temperatures, but overall these differences do not appear to be meaningful. The differences cannot be attributed to recency of isolation, since strain ATCC 43895 behaved similarly to recently isolated strains. Storage temperatures of 4°C or below limited growth of E. coli O157 isolates, but did not have a noteworthy effect on survival.