Validation Study of Assurance GDS MPX for Top STEC in Raw Beef Products

Introduction: Raw beef products have been analyzed for the presence of E.coli O157:H7 as defined in the United States Department of Agriculture (USDA) regulations for several years.  More recently, the USDA has proposed a new requirement to test certain raw beef products for a broader group of E. coli commonly referred to as “the top six Shiga toxigenic E.coli” or “Top STEC” as adulterants.  The USDA considers the following six STEC O serotypes, in addition to O157, to represent the greatest public health risk: O26, O45, O103, O111, O121 and O145.  The USDA will require routine testing of all these 7 serotypes for raw beef starting in March 2012.  Assurance GDSTop STEC MPX is an assay that allows simultaneous detection of E.coli O157:H7 and also the top 6 non-O157 STEC (Top 7 STEC) in a sample.

Purpose: Develop an assay to simultaneously detect E.coli O157:H7 and the top 6 non-O157 STEC to fulfill new USDA regulations.

Methods: Raw beef (beef trim, ground beef and finely textured beef) samples were inoculated with a 2-day cold stressed target organism.  375 g samples were analyzed with the Assurance GDS Top STEC MPX method for determination of the simultaneous presence of E.coli O157:H7 and the top 6 non-O157 STEC.  All samples were confirmed via IMS plating on a modified Rainbow agar and BVCC agar.

Results: All seven serotypes were analyzed among all three food types.  A total of 198 raw beef samples were inoculated with 0.5 – 0.7 CFU per sample to achieve partial recovery.  The enrichments were assayed with Assurance GDS Top STEC MPX and confirmed using IMS onto selective agar plates.  114 samples were positive by Assurance GDS MPX after 10 hours.  There was one sample which was negative by Assurance GDS Top STEC MPX which confirmed positive, for an overall detection rate of 99%.

Significance: The Assurance GDS Top STEC MPX assay is a reliable method for simultaneous detection of both E.coli O157:H7 and Top 6 non-O157 STEC in a common sample.