Plasmid profiles of multiple drug resistant local strains of Staphylococcus aureus.

In an attempt to determine the molecular basis for the degree of resistance among strains of S. aureus to Beta-lactam antibiotics, 50 clinical isolates of coagulase-positive strains of S. aureus were obtained. The strains and a control strain, Oxford NCTC 6571, were screened for the presence of Beta-lactamase, R-plasmid, and sensitivity to six Beta-lactam antibiotics. The molecular weight of the isolated R-plasmid DNA was determined by agarose gel electrophoresis. Forty-five of the 50 strains of S. aureus exhibited single or multiple drug-resistance. Plasmid DNA isolated from 13 of the resistant strains of S. aureus had the same molecular weight of 21 kb or 13.63 MDa, while one of the 13 strains and 3 others had R-plasmids with a higher molecular weight.