Purpose: This study evaluated inclusivity, exclusivity, and effectiveness of a PCR assay for screening L. monocytogenes in foods (frankfurters, bagged spinach, queso fresco, and cooked shrimp) and from environmental surfaces (stainless steel, plastic, and concrete).
Methods: Inclusivity (n = 53 strains) testing was performed at ~105 CFU/ml, while exclusivity testing (n = 61 strains) was performed at ~108 CFU/ml. For method effectiveness, foods were inoculated with Listeria at target levels likely to yield fractional positive results then evaluated using the appropriate culture-based method with twenty fractionally inoculated, five high level inoculated, and five uninoculated samples per matrix, per method. Alternative enrichments were performed in Oxoid 24 LEB Complete media for food and environmental matrices and in FoodChek Actero™ media for environmental matrices. One food type, queso fresco, and one environmental sample type, stainless steel, were tested in an external independent laboratory. PCR testing was conducted from both reference method and alternative method enrichments.
Results: Inclusivity and exclusivity of the assay were evaluated and shown to be 100% consistent with expected results. For effectiveness testing, 900 PCR tests were performed across all matrix/media/enrichment/time conditions with a > 99% concordance to culture confirmation. Statistical analysis using the AOAC POD model indicated PCR and culture results were indistinguishable.
Significance: These data suggest that the PCR method is an acceptable alternative to the reference methods when tested from the reference culture enrichments, or using either of the two proprietary enrichment media for more rapid target detection.