Purpose: The purpose of this study was to compare the performance of LPT and SDIX by testing both naturally-contaminated environmental sponge samples collected from food processing facilities and a Listeria spp. pure culture inclusivity panel.
Methods: Five food processing facilities were enrolled for the study. Ten Zone 3 sites (e.g., drains, floors, walls) were identified in each facility. Two environmental sponge samples were collected from adjacent areas at each site, and were randomly assigned to either LPT or SDIX. Samples were enriched for 24 h, then screened, and culturally confirmed after an additional 24-h incubation period. False positive and false negative rates were calculated. The inclusivity panel consisted of 50 Listeria spp. pure cultures. Cultures were screened with each assay, and percentages of positives were calculated.
Results: In testing the naturally-contaminated environmental sponge samples, SDIX had a higher false positive rate (3.70%) and false negative rate (21.74%) than LPT (0.00% and 4.35%, respectively). LPT also identified 100.00% of the inclusivity panel cultures as positive, while SDIX only identified 96.00% as positive. The two cultures producing negative results were Listeria grayi cultures.
Significance: In this comparative study the LPT assay outperformed the SDIX assay when testing both a set of naturally-contaminated environmental sponge samples and an inclusivity panel of Listeria spp. pure cultures.