Validation of the VIDAS® UP Listeria (LPT) for the Detection of Listeria spp. in 125-g Samples in Various Food Matrices

Introduction:  The USDA-FSIS Listeria monocytogenes sampling plan has been modified to include a sample size of 125 g. Currently, rapid Listeria spp. detection methods are validated only for 25 g sample size. The VIDAS^® LPT is a phase ligand fluorescent immunoassay to be used on VIDAS^® family of instruments to detect Listeria in food and environmental samples. 

Purpose: To validate the Listeria method for the detection of Listeria spp. in 125 g test portions of a variety of food matrices.

Methods: Food matrices (125 g), deli-ham, queso fresco, deli turkey and ground beef, were inoculated with either L. monocytogenes ATCC 7644, 19114, 51776 or 13932 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) and incubated 24 h at 30 ± 1°C followed by a secondary for 22 h at 30 ± 1°C. Enrichments were analyzed by the Listeria assay and confirmed using the USDA-MLG or AOAC reference method, regardless of initial screen result.  The Probability of Detection model (AOAC) and chi-squared analysis were used to analyze data.

Results: The method detected 16, 11, 10 and 7 Listeria positive samples as compared to 15, 11, 12 and 7 out of the 20 low inoculated samples (0.2-2 CFU/test portion) in deli-ham, queso fresco, deli turkey and ground beef, respectively. POD and chi-squared analysis demonstrated that the method and USDA-MLG and AOAC reference methods were equivalent in detecting Listeria spp.

Significance: The VIDAS^® UP Listeria method was determined to be equivalent to the reference methods for the detection of Listeria spp. in 125 g test portions of the food matrices tested.