The development of DNA-based methods (hybridization and PCR technology) for
the specific and sensitive identification of microorganisms opened a new a
rea 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 y
oghurt starters like S. thermophilus and L.delbrueckii ssp. can be identifi
ed using overnight cultures of the appropriate liquid media without DNA pur
ification. For S. thermophilus it was demonstrated that different kinds of
yoghurt (plain yoghurts or yoghurts containing fruits) could also be used i
n 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 chocolat
e milk when the sample was added without any further purification of the nu
cleic acids. The use of different primer sets that addressed targets of inc
reasing lengths (1.6-5.8 kb) made it possible to monitor the DNA contents o
f the product during, e.g. transformation experiments. As expected, the det
ection limits changed with primer design and decreased with increasing temp
late lengths from 0.01 pg/mu l (1.6 kb) down to 1.0 ng/mu l (5.8 kb) of cho
colate milk.