Thermal tolerance characteristics of non-O157 Shiga-toxigenic Escherichia coli in meat systems

In the United States, the Shiga toxigenic Escherichia coli (STEC) serogroups O26, O45, O103, O111, O121, O145 and O157, are considered as adulterants in raw, nonintact beef. This study focused on the behavior of non-O157 STEC in comparison to established pathogens STEC O157:H7 and Salmonella, when exposed to a thermal process. We first compared thermal tolerance of ≥3 strains from each STEC serogroup in a beef-derived broth, and calculated comparative D-values at 54.6, 58 or 63.6°C. D58°C - values for O157 STEC strains were significantly higher than that for at least one strain in each of the non-O157 STEC serogroups (P<0.05), except for serogroup O103. The z-values for non-O157 STEC strains were comparable to those for O157:H7 strains ( P≥0.05), and ranged from 4.10 to 5.21°C. Next we compared thermal tolerance (D55°C) of the same set of strains in ground beef of varying fat levels and frankfurter batter. In addition, thermal tolerance of pathogen cocktails of Salmonella, STEC O157, non-O157 and two potential surrogates was determined. There were significant differences in D55°C across all STEC strains in 7% fat ground beef ( P<0.05), but no non-O157 strain had D55°C outside the range observed for O157 STEC. In addition, the effect of fat on thermal tolerance was strain dependent. D55°C for pathogen cocktails was not significantly different when measured in 7, 15 and 27% fat ground beef (P≥0.05). We then compared the effects of heat shock (HS) on thermal tolerance (D55°C) of cocktails of STEC O157, non-O157, or Salmonella in brined and un-brined lean (7% fat) ground beef. Brine consisted of 5%NaCl + 2.5%sodium tripolyphosphate in distilled water to mimic beef moisture-enhanced to 110% initial weight. Overall HS significantly increased D55°C, regardless of pathogen ( P<0.05). After a 30 min HS, D55°C increased by 18.21, 9.17, and 6.17 min, for O157 and non-O157 STEC, and Salmonella, respectively. The presence of brine significantly increased D55°C, regardless of pathogen (P<0.05). Overall, results suggest that thermal processes in beef validated against E. coli O157:H7 have adequate lethality against non-O157 STEC, and that thermal processes that target Salmonella may not be adequate against STEC in some situations.