Purpose: This study evaluated the BAX® Q7 Real-Time PCR Assay for Escherichia coliO157:H7 after enrichment in three different enrichment media.
Methods: A panel of 12 human clinical Escherichia coli O157:H7 isolates (inclusivity) and 16 non-E. coli O157:H7 isolates (exclusivity) were tested against mTSB-CCV (cefixime, cefsulodin, and vancomycin), mTSB-n (novobiocin), and BAX MP commercial broth at a concentration of 100 CFU/250ml. To determine sensitivities, four E. coli O157:H7 isolates were spiked into raw ground beef at 100, 10, and 1 CFU/25g and enriched in each media. All results were confirmed by culture using the Health Canada reference culture method (MFLP-80), and extent of background flora was estimated.
Results: The BAX® assay detected 100% (25/25) of the samples in the inclusivity panel and none in the exclusivity panel. A 100% positivity rate was observed when samples were inoculated at 100 and 10 CFU/25g but dropped to 63.5% (94/148) when the inoculum was 1 CFU/25g. A low false negative rate was seen due to a single sample inoculated at 1 CFU/25g. Enrichment in mTSB-CCV significantly reduced background growth within the matrix (P = 0.0012; Fischer’s Exact Test); however mTSB-n and BAX MP were more likely to yield a positive result (P < 0.0001; P = 0.0294) when background flora in the matrix was high.
Significance: The BAX® assay demonstrated a high degree of specificity towards Escherichia coli O157:H7. Although media containing CCV was more likely to reduce the background flora in the ground beef matrix after enrichment, use of mTSB-n or BAX MP media may lead to improved detection of E. coli O157:H7 in foods with high background contamination.