Cooked Beef Food Matrix Validation Study Using Real-time PCR BAX Method for Testing E. coli O157:H7

Introduction:  Humans can become infected with E. coli O157:H7 through the consumption of contaminated food and develop hemolytic uremic syndrome (HUS) which is a serious illness that can result in renal failure, seizures, paralysis and death. In 1994 FSIS declared E. coli O157:H7 an adulterant if found in raw beef food products and since then the BAX® System Real-Time PCR Assay has been validated for the detection of E. coli O157:H7 in raw beef products.

Purpose: The purpose of this study was to discover if the PCR Assay validated for E. coli O157:H7 could be used to detect E. coli O157:H7 cells at a concentration of 2 CFU/25g within cooked ground beef patties also subjected to freezing temperatures.

Methods: This study inoculated cooked ground beef patties with an estimated 2 CFU of E. coli O157:H7 per sample and then froze the samples to -15°C for a minimum of 24 h. The samples were then enriched with pre-warmed MP Media and incubated at 42°C. After the desired incubation time points of 8 h, 10 h and 24 h were met, 5ml aliquots were removed from the samples and tested per the PCR Assay method.

Results: This study found that the PCR Assay for E. coli O157:H7 can be used to detect E. coli O157:H7 in cooked ground beef. The results from this study also indicate the matrix required incubation for a time period of greater than 8 h or for a minimum of 10 h to obtain a positive result in 100% of the inoculated samples using the PCR Assay.

Significance: Cooked ground beef was previously thought to be protected from E. coli O157:H7 adulteration but could cause future outbreaks especially if the ground beef product is undercooked or if cross contamination occurs.