Purpose: The purpose of the present study was to validate performance of the modified amplification method for detection of Listeria spp. in inoculated food and environmental samples.
Methods: Three foods (hot dogs, queso fresco, and cantaloupe) and one environmental surface (stainless steel) were inoculated with Listeria spp. using conditions intended to simulate natural contamination. Inoculation levels were sufficiently low to produce fractional positive data sets (5 - 15 of 20 test portions positive). Replicate test portions were tested by the amplification method (after 16 h and 24 h of enrichment at 36°C) and either the FDA/BAM or USDA/MLG reference culture procedures for Listeria. Test results were analyzed using a probability of detection (POD) model at P < 0.05 to determine if the numbers of positive results produced by the amplification and reference methods were statistically different.
Results: For the four sample types in total, by the amplification method there were 42 and 45 positive results after 16 h and 24 h of enrichment, respectively. There were 39 positive results by the reference methods. In no case was the difference in the number of positive results obtained by the amplification and reference methods statistically significant by POD analysis.
Significance: The amplification method provides the user with a simple, fast, and effective tool for detection of Listeria spp. in food and environmental samples. The modified version of the assay offers improved ease of use and minimizes risk of contamination.