Detection without sample processing of yoghurt starters and DNA in milk products by PCR

The development of DNA-based methods (hybridization and PCR technology) for the specific and sensitive identification of microorganisms opened a new area in the field of food microbiology. However, suitable sample preparation procedures have at first to be developed for the single food matrix before these techniques can be applied in routine food analysis. We showed that yoghurt starters like S. thermophilus and L.delbrueckii ssp. can be identified using overnight cultures of the appropriate liquid media without DNA purification. For S. thermophilus it was demonstrated that different kinds of yoghurt (plain yoghurts or yoghurts containing fruits) could also be used in the PCR reaction mix to identify this starter without sample processing, provided that enough microorganisms were present (about 10(4)Cfu/mu l). In order to identify free DNA it could be shown for another milk product that 0.01 mu g DNA of a model plasmid pMG36enpr was detected in 1 ml of chocolate milk when the sample was added without any further purification of the nucleic acids. The use of different primer sets that addressed targets of increasing lengths (1.6-5.8 kb) made it possible to monitor the DNA contents of the product during, e.g. transformation experiments. As expected, the detection limits changed with primer design and decreased with increasing template lengths from 0.01 pg/mu l (1.6 kb) down to 1.0 ng/mu l (5.8 kb) of chocolate milk.