Purpose: To evaluate the performance of real-time PCR assays, the FDA-BAM reference method, and traditional culture method to detect two separate significant human pathogens, Escherichia coli O157:H7 and Listeria monocytogenes, in artificially inoculated samples of Whey Protein Concentrate 34 (WPC34).
Methods: WPC34 powder was artificially inoculated with either a L. monocytogenes or E. coli O157:H7 strain to obtain fractional recovery, after being held at room temperature for 2 weeks to stress the organisms. L. monocytogenes-spiked 125 g portions (n = 20) were enriched in 1125 ml pre-warmed (30°C) Demi-Fraser (DF) Broth, homogenized and incubated (30°C) for 22 - 26 h. A secondary enrichment in pre-warmed (35°C) MOPS-BLEB was incubated (35°C) for 18 - 24 h. E. coli O157:H7-spiked portions of 375 g (n = 20) were enriched in 1500 ml pre-warmed (42°C) modified Tryptic Soy Broth (mTSB), homogenized and incubated (42°C) for 18 - 24 h. A set of reference samples (n = 20) were also enriched according to the FDA-BAM method. After incubation, all samples were processed with real-time PCR testing and the appropriate culture method.
Results: Of the L. monocytogenes-spiked samples, the real-time PCR assay for Genus Listeria and L. monocytogenes returned 5/20 presumptive positive results from MOPS-BLEB. Of the E. coli O157:H7-spiked samples, the real-time PCR assay returned 5/20 presumptive positive results after a BHI regrowth. Reference method samples returned 13/20 presumptive positive results after a BHI regrowth. All culture results matched PCR results.
Significance: The results of this study demonstrate the DuPontTM BAX® System can rapidly, accurately and reliability detect E. coli O157:H7 in 375 g and Listeria monocytogenes in 125 g samples of WPC34.