P1-119 Comparative Survival of Shiga-toxin Producing Escherichia coli in Ground Beef with Different Fat Levels under Conditions Resembling Refrigeration, Partial Cooking and Digestion

Monday, July 23, 2012
Exhibit Hall (Rhode Island Convention Center)
Kyriaki Chatzikyriakidou, University of Wisconsin-Madison, Madison, WI
Steve Ingham, Wisconsin Department of Agriculture, Madison, WI
Barbara Ingham, University of Wisconsin-Madison, Madison, WI
Introduction: In the fall of 2011, the USDA declared six non-O157 Shiga toxin-producing serogroups of Escherichia coli(STEC) as legal adulterants in non-intact raw ground beef due to the threat that these organisms present to public health. The need to understand the behavior of foodborne non-O157 STEC exposed to the human gastric environment is crucial to understanding their role in public health.

Purpose:   This study compared survival of non-O157 STEC (6 strains from serogroups O26, O45, O103, O111, O121 and O145) and the known heat-acid-tolerant O157:H7 strain ATCC 43895 in 15% and 27% fat ground beef during sequential refrigeration, partial cooking, and incubation in a simulated gastric environment.

Methods: Inoculated ground beef (25 g) was refrigerated (4°C) for 16-17 h and then heated 30 min at 54.4°C to mimic partial cooking. After cooling in ice 10 min, the inoculated beef was combined with pre-warmed (37 °C) Synthetic Gastric Fluid (pH 2.0; 70 ml) and held at 37°C. Surviving cells were enumerated before and after heating and after 0.5, 1, 2, 3, 4, and 6 h of incubation in SGF. Overall changes in population (D-log CFU/g) were calculated for each strain separately at the 6-h sampling time, relative to the level before heating.

Results:   STEC populations decreased by 1.5 – 3.0 log CFU/g or by 0.8 – 3.5 log CFU/g during cooking in 15% and 27% fat ground beef, respectively, and by 1.4 – 5.7 log CFU/g during subsequent 6-h incubation in SGF in both 15% and 27% fat ground beef. In 27% fat ground beef there were very significant (P <0.01) inter-strain differences in survival. One strain each from serogroups O26, O103, and O111, and the O157:H7 reference strain all survived significantly better than other strains tested.  There was no statistical difference in D-log CFU/g when results for 15% and 27% fat ground beef were compared, except for one strain of serogroup O26 which exhibited significantly lower D-log CFU/g values in 15% ground beef.

Significance:   Several non-O157 STEC strains were shown to survive as well as O157:H7 in ground beef when exposed to sequential refrigeration, partial cooking, and synthetic gastric fluid. Fat level, 15% or 27%, did not affect STEC survival.