Evaluation of the IQ-Check Kits for Detection of Shiga Toxin-producing E. coli and Salmonella in Ground Beef and Comparison to the USDA Microbiology Laboratory Guidebook Methods

Introduction: Shiga toxin-producing E. coli (STEC) O157:H7 and non-O157 STEC serogroups O26, O45, O103, O111, O121, and O145 are important foodborne pathogens declared as adulterants by the USDA Food Safety and Food Inspection Service (FSIS). Bio-Rad developed real-time PCR kits (iQ-Check) to screen for STEC and Salmonella, and the FSIS Microbiology Laboratory Guidebook (MLG) method utilizes the BAX System kits (DuPont). Both screening tests target Shiga toxin (stx1 and stx2), intimin (eae) and O-group-specific genes. The Bio-Rad and MLG methods utilized SEB medium and mTSB, respectively, for enrichment of both STEC and Salmonella simultaneously.

Purpose: To compare the MLG and Bio-Rad methods for detection of STEC and both STEC and Salmonella simultaneously in ground beef after a short enrichment period, followed by isolation of colonies from selective agars and confirmation. 

Methods: Beef samples were artificially contaminated with the STEC strains (4-19 CFU), and STEC (4-16 CFU) together with Salmonella Typhimurium (10-15 CFU), kept at 4°C for 72 h, enriched for 12 and 18 h, and then tested with iQ-Check kits and with BAX System kits as described in the MLG. For Salmonella isolation, 0.1 ml of enrichments that had been co-inoculated was transferred to RVS broth, incubated for 24 h at 42°C, and then plated onto XLT-4 agar. STEC isolation was carried out using the MLG protocol. Presumptive colonies were confirmed using the real-time PCR kits.  

Results: All of the samples were positive by PCR after 12 and 18 h of enrichment, and no notable differences were found between the two methods. Confirmed STEC and Salmonella colonies were obtained from every enrichment.  

Significance: This study demonstrated that the iQ-Check kit results compared well with the MLG methods, and detection of both STEC and Salmonella in beef using the same enrichment medium is possible after a relatively short enrichment period.