Characterisation of cryptic plasmids in clinical isolates of Bacteroides fragilis

A screening method for plasmids in the fragilis group of Bacteroides spp. was developed, taking account of the lysozyme resistance of these species; 26 strains, 24 of them B. fragilis, were investigated by this method. Eleven strains contained plasmids and up to three different plasmids were found in individual strains. The plasmids belonged to five different size classes of mol. wt (10(6] 2.8, 3.5, 3.6, 4.2 and 19. Plasmids of equal size showed homology; no homology was found between plasmids of different sizes. Plasmids of equal size showed identical restriction patterns with 17 restriction endonucleases. Restriction maps were constructed for the five classes of plasmid.