Purpose: The goal of this study was to investigate this alternate Listeria method for the detection of Listeria spp. in 375-gram food samples and environmental sponges.
Methods: Seven 375-gram samples of hot dogs, deli turkey, RTE beef patties and brie cheese were inoculated with L. monocytogenes at a level of approximately 21 CFU/test portion. A single 375-gram sample of each product was tested unspiked. All test portions were enriched in LPT broth (1,125 ml) and incubated for 26 h at 35±1°C. For the environmental samples, stainless steel, ceramic and rubber were inoculated with Listeria spp. in a “4 × 4” area at around 105 cells/test square. Seven inoculated and one unspiked area were tested for each surface. After inoculation the surfaces were allowed to dry 20 ± 4 h at room temperature before testing. A sponge was then used to sample each surface and 100 ml of LPT broth was added to the individual sponges and incubated for 18 h at 35±1°C. Enrichments for both the food and environmental samples were analyzed by the ListeriaPCR method. All enrichments were confirmed using the USDA MLG reference method, regardless of initial screen result.
Results: The dPODCP as the difference between the alternate method presumptive result POD (PODAP) and the alternate method confirmed result POD (PODAC) values were calculated for each product and environmental surface. The 95% confidence interval of the dPODCP was also determined. For all products and surfaces, the calculated 95% confidence interval contained zero indicating that there was no significant difference between the alternate method and the USDA MLG reference method plating for this study.
Significance: These data demonstrate that the evaluated method is a suitable for detecting Listeria spp. in 375-gram food samples and environmental sponges. The method provides significant savings in terms of time and improved convenience when compared to reference methods and other commercially available PCR platforms.