P1-27 Recovery of Escherichia coli O157:H7 from Ground Beef after Enrichment in Three Different Media Types Using the BAX® Real-time PCR Assay

Monday, July 23, 2012
Exhibit Hall (Rhode Island Convention Center)
Christina Ferrato, Provincial Laboratory for Public Health, Alberta, Calgary, Canada
Linda Chui, Provincial Laboratory for Public Health, Alberta, Edmonton, Canada
Marie Louie, Provincial Laboratory for Public Health (Alberta, Calgary, Canada
Introduction: The Provincial Laboratory for Public Health, Alberta, works with local health inspectors to investigate foodborne illness investigations. It is essential to identify Escherichia coli O157:H7 from food samples with high background contamination.

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 (= 0.0012; Fischer’s Exact Test); however mTSB-n and BAX MP were more likely to yield a positive result (< 0.0001; = 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.