Resistance plasmids isolated from Streptococcus agalactiae (group B) and S. faecalis (group D) have been compared in regard to resistance markers, molecular weight, and DNA-DNA homology. Three of them (pIP501, pIP612, and pIP613) have been found to confer identical (or very similar) resistance patterns (erythromycin, lincomycin, and streptogramin B, respectively) and to have similar molecular weights (19.8 x 106, 22.7 x 106, and 17.6 x 106, respectively) and a high level of DNA-DNA homology in hybridization experiments (90 to 100%). These results are compatible with the view that these plasmids may derive from one common ancestor, and/or that they can be transferred between unrelated Streptococcus strains belonging to the same or different groups.Strains of Streptococcus resistant to erythromycin (Em), lincomycin (Lm), and tetracycline (Tc) have recently appeared with increased frequency mainly in serological groups A (6, 7, 21, 24), B (8), and D (9, 18,20, 22,25,27 Anitibiotics and reagents. These have been previously described (13).Technique for genetic and physical studies. Techniques of cure, isolation of plasmid DNA, and determination of molecular weight (MW) were as previously described (13), with the following modifications: (i) BHI medium was supplemented with DLthreonine (20 mM) (1), and (ii) the bacteria were washed, frozen, and then thawed slowly in order to make the cell walls fragile before lysozyme treatment.
