Optimization of BAX® System Enrichment Media for Listeria monocytogenes and Genus Listeria Assays

Introduction: Food legislation in many countries rigidly enforces limits for Listeria monocytogenes and other species in genus Listeria. The DuPont™ BAX® System includes a range of PCR assays for detecting genus Listeria and L. monocytogenes in food and environmental samples. As part of its commitment to customers, DuPont strives to continually improve its products to satisfy the evolving needs of the industry and exceed customer expectations.

Purpose: The purpose of this study was to reduce the enrichment times used with the system’s method for detecting genus Listeria and L. monocytogenes. Variations of standard enrichment media and their selected components were examined for optimal outcome.

Methods: A variety of standard enrichment media were compared to the approved proprietary media for recovering heat stressed Listeria cells by pure culture growth and plating. The top performing standard media was then evaluated to determine if the addition or alteration of media components could further improve performance with our PCR Assay for L. monocytogenes24E. Components evaluated included pH buffering reagent, nutrients, energy and carbon source, salts, antimicrobials and other supplements.

Results: The optimized formulation returns more true positive calls than the certified proprietary media by the system’s PCR Assay for L. monocytogenes 24E on ham (P = 0.001 by z-test), smoked salmon (P = 0.028 by z-test), raw shrimp (P < 0.001 by z-test), hotdog (P < 0.001 by z-test) and cheese (P < 0.001 by z-test) at 22 hours post-enrichment that has not been validated for the certified proprietary media.

Significance: These findings suggest it is possible to further reduce the enrichment times of the system for detecting Listeria and L. monocytogenes, while continuing to offer the same accuracy, reliability and ease of use as the original protocols.