Purpose: A study was conducted by the independent Expert Laboratory Eurofins IPL Nord, to validate this new method, as part of the NF Validation approval process.
Methods: Samples were enriched at 30 ± 1 °C for 26 hours (food samples) or 22 hours (environmental surfaces) in the ready-to-use proprietary broth, the LPT broth. After incubation, samples were boiled for 5 ± 1 minutes before performing the assay. All presumptive positive samples were further confirmed after streaking on PALCAM or on a chromogenic agar according to Ottaviani Agosti. This new method was compared to the ISO 11290-1/A1 reference method, according to the ISO 16140 standard.
Results: A comparative study was performed on 349 products distributed over the 5 categories meat, dairy, vegetable, seafood and environmental samples of which 69% were naturally contaminated. The phage method detected 160 positive samples compared to 162 for the reference method. There was no statistical difference between the two methods using the Mc Nemar test at 5% level. The 50% detection limit was determined on 5 different products/strains combination tested at 4 contamination levels. Results were comparable between the two methods. In the inclusivity study, all the 51 Listeria spp strains tested were detected and in the inclusivity study, none of the 30 non-Listeria strains gave a positive result.
Significance: The LPT method provides a simple, convenient and reliable method for detection of Listeria species in food and environmental samples, providing a presumptive result for the presence of Listeria in less than 23 hours for environmental surfaces and 27 hours for food samples.