Plasmid analysis of clinical isolates of Bacteroides fragilis group strains.

Cryptic plasmids were isolated from 26 of 52 Bacteroides fragilis group strains derived from severe infections. Four strains harboured two plasmids, one three plasmids and one five plasmids with different molecular weights. The same molecular weight plasmid (3.7 Md) was isolated from 17 of the 26 plasmid-containing B. fragilis group strains. No correlation was found between plasmid-harbouring and resistance against ten antibiotics and different heavy metal ions. No curing of the strains from the plasmids was achieved with ethidium bromide and acridine orange.