1 Division of Microbiology and Risk Assessment, National Food Institute, Technical University of Denmark2 National Food Institute, Technical University of Denmark3 Division of Food Microbiology, National Food Institute, Technical University of Denmark4 Danish Meat Research Institute5 Danish Meat Research Institute
We developed a 12-h Salmonella detection method, based on 8 h of preenrichment, followed by automated DNA extraction and a sensitive real-time PCR. The method was optimized to obtain the highest possible yield of cells and DNA. The growth of different Salmonella strains in various preenrichment media and the effects of adding growth-promoting and selective reagents were explored, taking into account their PCR compatibility. The effects of (i) analyzing larger volumes (1 to 5 ml) from preenriched samples and introducing wash steps prior to DNA extraction, (ii) regulating the amount of paramagnetic particles (increasing it from 60 to 90 mu l) in the DNA extraction, (iii) eluting the DNA in reduced volumes (25 or 50 mu l rather than 100 mu l), and (iv) increasing the PCR template volume (from 5 to 20 mu l) were investigated. After 8 h of preenrichment, buffered peptone water yielded the highest number of salmonellae. When analyzing minced meat samples, positive effects of increasing the initial sampling volume from 1 to 5 ml and increasing the amount of paramagnetic particles to 90 mu l were observed. However, washing the pellet and eluting the DNA in reduced volumes (25 and 50 mu l) had no positive effects and resulted in decreased reproducibility. Increasing the amount of PCR template DNA from 5 to 20 mu l improved the threshold cycle value by approximately 2. The improved 12-h PCR method was successfully compared to a reference culture method with 100 minced meat and poultry samples, with a relative accuracy of 99%, a relative sensitivity of 98%, and a relative specificity of 100%.
Applied and Environmental Microbiology, 2007, Vol 73, Issue 9, p. 3040-3048