Isolation and Characterization of Several Cryptic Plasmids from Clinical Isolates of Bacteroides fragilis

Cryptic plasmids were isolated from three strains of Bacteroides fragilis by dyebuoyant density-equilibrium centrifugation and were characterized by density velocity sedimentation and electron microscopy. Two of the strains carried many copies of a plasmid with a molecular weight of approximately 4 × 106 daltons. One strain carried many copies of a plasmid with a molecular weight of 2.7 × 106 daltons and one or a few copies of a plasmid with a molecular weight of 1.6 × 107 daltons. A fourth strain similarly studied contained no plasmid DNA. The presence of the three populations was not associated with resistance of these strains of B. fragilis to a number of commonly used antibiotics.