Purpose: To validate the LMX method for the detection of L. monocytogenes in 125 g test portions of a variety of food matrices.
Methods: Food matrices (125 g), ground beef, deli ham and turkey and Mexican soft cheese – queso fresco), were inoculated with L. monocytogenes at low (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 (1:4 dilution) for 24 h at 30 ± 1°C followed by secondary enrichment in LPT broth for 22 h at 30 ± 1°C. Enrichments were analyzed by the LMX assay. All enrichments were culturally confirmed using the USDA (MLG Ch. 8.07) or AOAC (section F, b) 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 LMX method detected 7, 11, 16 and 11 L. monocytogenes positive samples compared to 7, 11, 15 and 8 by reference method out of the 20 low inoculated samples (0.2-2 CFU/test portion) in ground beef, queso fresco, deli ham and deli turkey, respectively. POD and chi-squared analysis demonstrated that the LMX method and USDA-FSIS and AOAC reference methods were not significantly different in detecting L. monocytogenes.
Significance: The LMX method was determined to be equivalent to the reference methods for the detection of L. monocytogenes in 125 g test portions of ground beef, deli ham and turkey and Mexican style soft-cheese (queso fresco).