Evaluation of the Performance of an Alternative Rapid Molecular Detection Assay Based on Loop-mediated Isothermal Amplification (LAMP), Compared to a Reference Official Mexican Method (NOM 210), in Artificially Contaminated Alkaline-treated Corn Meal (Nixtamal)

Introduction:  Listeria monocytogenes, is a widely recognized pathogen associated with foods, leading to illness and death of vulnerable populations including pregnant women, immunocompromised adults and children under 5 year of age. L. monocytogenes surveillance and its monitoring in raw ingredients and environment is pivotal to prevent contamination and its detection is often a requirement for product release.

Purpose: To evaluate the performance of a LAMP based rapid method for the detection of Listeria monocytogenes in alkaline treated corn meal compared with Official Mexican Method (NOM210) that is based on ISO11290.

Methods: Five different lots of nixtamal meal were used. For each lot 25 g portions were inoculated with 1 to 5 CFU/portion of L. monocytogenes (ATCC 19115) and analyzed with the LAMP-based method (n=30 samples/lot with additional n=10 samples/lot of uninoculated samples) or with the NOM210 method (n=2 samples/lot with additional n=2 samples/lot of uninoculated samples). All samples were enriched with 225mL of Demi Fraser broth for 24h at 35°C. Samples analyzed with the NOM210 method were further confirmed through biochemical tests. Analysis of results was done using an unpaired design.

Results: Inoculated samples from all lots resulted in positive detection of L. monocytogenes by both LAMP-based and NOM210 methods. For all analyzed samples (N=220), no positive or negative deviations were determined and the LAMP-based method was found to be equivalent to the NOM210.

Significance: The alternative LAMP-based molecular detection assay can offer a specific and rapid approach for the detection of L. monocytogenes that can be implemented as part of the microbiological quality test for release test of nixtamal meal.