Detection and preliminary characterization of extrachromosomal DNA in clinical isolates of Staphylococcus aureus and Staphylococcus epidermidis.

Isolates of Staphylococcus aureus and Staphylococcus epidermidis frequently harboured extrachromosomal DNA. From the 20 Staphylococcus aureus strains examined 85% contained extrachromosomal DNA and 65% carried more than one plasmid DNA species. The results obtained with the 19 Staphylococcus epidermidis strains analyzed were similar, 84% of the strains had extrachromosomal DNA and 68% contained several DNA species. The agarose gel electrophoresis of the DNA obtained from cleared lysates of the staphylococci proved to be efficient for the detection of plasmids and the determination of their molecular weights. The gel method appeared therefore to be suitable for the establishment of the role of plasmids in the phage typability of staphylococci. Using a Staphylococcus epidermidis strain as an example it was shown by electron microscopy that the extrachromosomal DNA demonstrated by the agarose gel electrophoresis represented covalently closed circular plasmid DNA. This plasmid DNA could be sufficiently labeled with 3H-thymidine to undertake studies on the relationship and molecular characterization of staphylococcal plasmids.