Fast Detection of Listeria monocytogenes in Deli Meat and Dairy Products

Introduction: The ability of L. monocytogenes to proliferate in various foods at refrigeration temperatures and survive even after deep freezing makes the occurrence of this foodborne pathogen in ready-to-eat (RTE) foods of particular concern. It is especially threatening to the deli meat and dairy industries if fast and reliable detection methods are not applied. Since L. monocytogenes in RTE food can be present at low concentration with sub-lethal injury during food processing, an enrichment step is crucial to resuscitate injured cells and allow sufficient growth for detection.

Purpose: The objective of this study was to validate a sensitive and rapid method for L. monocytogenes detection in deli meat and dairy products.

Methods: Deli meat (cold smoked turkey and cured ham) and dairy product (double chocolate ice cream and pasteurized milk) samples were artificially contaminated with sub-lethally heat-stressed L. monocytogenes of different serotypes and stabilized for 48–72 hours at 2–8°C or for 14 days frozen. Samples were enriched in Actero™ Listeria Enrichment Media, then processed with the DuPont™ BAX® System Real-Time PCR Assay for L. monocytogenes.

Results: A total of 240 artificially contaminated food samples were examined to evaluate performance of the candidate method in comparison with the appropriate USDA-FSIS or US FDA reference method. Additionally, efficacy of the method was evaluated with four other ice cream flavors (vanilla, strawberry, caramel with pecan and cookie dough) as well as with 375 g samples of double chocolate ice cream. Optimization studies resulted in significant reduction in the enrichment phase with incubations of 18-22 hours for dairy products and 24 hours for deli meats. Method comparison studies demonstrated no false positive results or false negative results with the candidate method.

Significance: According to the AOAC International Probability of Detection statistical model, the candidate method was equivalent to the reference methods.