Differences in Enterohemorrhagic Escherichia coli and Salmonella Prevalence in Raw Ground Beef and Trim

Introduction: Beef producers routinely screen for the presence of enterohemorrhagic Escherichia coli (EHEC) and Salmonella in their products.  While EHEC is found mainly on the surface of trim, Salmonella could be sequestered in the lymph nodes.  Therefore, we might expect to find differences in the prevalence of these pathogens in different classes of beef products.

Purpose: To determine if there is a significant difference in prevalence over multiple years between EHEC and Salmonella in ground beef and trim.

Methods: From two processing plants that continuously tested trim and raw ground beef during calendar years 2010-2012, 336,771 trim samples (375g, 60 surface pieces) and 75,249 ground beef samples (375g) were enriched for a minimum of 9h and then screened by a multiplex PCR targeting the stx and eae genes of E. coli and two Salmonella-specific genes.  Samples which were eae⁺ and stx⁺ (stx₁⁺ and/or stx₂⁺) were considered to be EHEC screen-positive. Samples sal₁⁺ and/or sal₂⁺ were considered to be Salmonella screen-positive.

Results: Over the three-year period, EHEC screen-positive sample prevalence was 8.15% and 11.38% in ground beef and trim, respectively, while the Salmonella screen-positive sample prevalence was 27.06% and 9.95% in ground beef and trim, respectively.  All results are significantly different (P≤ 0.05).

Significance: The differences in prevalence of EHEC and Salmonella in ground beef and trim are consistent with the observation that the former is found primarily on the surface of beef trim (because hide is the primary source) while the latter is found in lymph nodes which are dispersed throughout ground beef as well as on the hides.