Purpose: To validate the method for detecting Listeria spp. in a variety of food matrices at 25 gram sample size.
Methods: Smoked salmon, peanut butter, ice cream, mixed salad, cooked shrimp, queso fresco, cantaloupe and ground pepper were inoculated with either L. monocytogenes, L. ivanovii, L. welshmeri, L. innocua, L. seeligeri or L. grayi at low levels (20 samples; 0.2-2 CFU/test portion), high levels (5 samples, 2-5 CFU/test portion) and 5 uninoculated controls. All test portions were enriched in LPT broth (225 ml) with the exception of cantaloupe, which was enriched by adding enough LPT broth to completely submerge the fruit. Enrichments were incubated 26 h at 30±1°C. Enrichments (0.5 ml) were analyzed by the assay and confirmed using the FDA-BAM or AOAC reference method, regardless of initial screen result. The Probability of Detection model (AOAC) and chi-squared analysis were applied to analyze the results.
Results: The method detected 14, 15, 18 , 20, 16, 9,12, and 8 Listeria positive samples compared to 15, 16, 15, 15, 13, 11, 10 and 10 positives out of the 20 low inoculated samples (0.2-2 CFU/test portion) in smoked salmon, peanut butter, ice cream, mixed salad, cooked shrimp, queso fresco, cantaloupe and ground pepper, respectively. POD and chi-squared analysis demonstrated that the method and FDA-BAM and AOAC reference methods were equivalent in detecting Listeria spp.
Significance: The VIDAS® UP Listeria method with single-step enrichment and next day automated detection was equivalent to the reference method for detecting Listeria spp. in 25 g test portions of the food matrices tested.