Multicenter validation study of two blockcycler- and one capillary-based real-time PCR methods for the detection of Salmonella in milk powder

A collaborative study including 13 German laboratories was conducted to evaluate the performance of two non-patented real-time PCR methods for the detection of Salmonella in milk powder targeting the ttrC/ttrA- or the invA gene. The enrichment procedure and sample DNA preparation method prior to the real-time PCR was the same for both systems and the identical DNA extraction samples were analysed. The traditional cultural method according to EN ISO 6579:2002 for the detection of Salmonella in food was performed in each laboratory as the reference. The participants received twelve coded milk powder samples each of 25 g for the analysis. Four of them were Salmonella negative (level L0), four artificially contaminated with < 3 MPN/g Salmonella Typhimurium (level L1) and four artificially contaminated with 3.6 MPN/g S. Typhimurium (level L2) to the beginning of the experiment. Of the 13 laboratories 12 used various models of real-time PCR blockcyclers conducting both real-time PCR assays and three laboratories the Light Cycler 2.0 system (Roche Bioscience) conducting the ttr-based real-time PCR assay only. The relative accuracy for both real-time PCR assays performed on blockcyclers was for level L0 97.5%. For level L1 the relative accuracy was 94.1% and for level L2 it was 100% for both assays. The relative accuracy on the Light Cycler 2.0 system was 100% for all levels applied to the ttr-real-time PCR.