Detection of Shiga Toxin-producing Escherichia coli in 25 Gram Samples of All-purpose Flour Using the BAX® System

Introduction:  Flour is a low moisture ingredient not commonly associated with foodborne outbreaks. Although pathogens such as Shiga toxin-producing Escherichia coli (STEC) in raw flour are usually rendered harmless by further processing to create baked goods and other finished products, many consumers will often eat raw dough and cake batters that potentially contain these pathogenic organisms. In 2016, flour and flour-containing mixes emerged as a “new” potential carrier for pathogens after infecting 63 people with STEC.

Purpose:  In response to last year’s multi-state STEC outbreak, this study was designed to evaluate the BAX® System for detecting STEC in flour.

Methods:  Forty 25-g samples of unbleached all-purpose flour were inoculated with a low level of E. coli O121 and held at room temperature for 2 weeks to acclimate the target cells. For the alternative method, twenty samples were homogenized with 225 mL of pre-warmed (42°C) mTSB+2mg/L Novobiocin and incubated at 42°C for 22 hours. Samples were tested using real-time PCR both directly from the primary enrichment and after a BHI regrowth, and all results were confirmed using the appropriate FDA-BAM procedure. For the reference method, twenty samples were enriched and plated according to the FDA-BAM method for Diarrheagenic E. coli.

Results:  The real-time PCR assays detected 20 positive samples containing stx, eae and the O121 serogroup with 95% confirmation. The reference method resulted in 19 positives by culture.

Significance:  The results of this study demonstrate that the BAX® System can detect STEC from 25g samples of flour enriched in mTSB+2mg/L novobiocin with no significant statistical difference to the reference culture method.