Purpose: A study was conducted to validate the LMX for detection of Listeria monocytogenes in food. This study was conducted as part of the AOAC Research Institute approval process.
Methods: In the LMX method, samples are enriched for 26-30 h in LMX broth then an aliquot of the broth is transferred to the test strip, heat-treated for 5 min and cooled before testing. The assay gives a result for presence of L. monocytogenes within 80 minutes. Enriched samples were confirmed by streaking to Oxford agar, Chrom ID Ottaviani Agosti agar (OAA) and Chrom ID L. mono agar. Biochemical confirmation was performed by Vitek1 (AOAC method 991.13). The study included 6 foods (processed cheese, vanilla ice cream, frozen spinach, peanut butter, cooked shrimp and smoked cod). Twenty 25-g samples inoculated at a low level (target 0.2-2 CFU/25 g) and 5 x 25 g uninoculated samples were tested using the LMX assay and the appropriate reference method (AOAC 993.12 for cheese and ice cream) and the FDA BAM method for remaining foods.
Results: Results for the 6 foods showed that there was no significant difference between the LMX method and the reference methods for detection of L. monocytogenes using an unpaired chi-square test at 5% level. Confirmation using traditional and chromogenic agars gave equivalent results.
Significance: The LMX assay provides a presumptive result for the presence of L.monocytogenes, within 28 hours. The assay can detect the presence of the pathogenic species, L. monocytogenes, faster than traditional methods and other immunoassays.