Identification of three homology classes of small, cryptic plasmids in intestinal Bacteroides species

A systematic analysis of the plasmid content of intestinal Bacteroides spp. was made. Eight of fifteen clinical isolates and seven of nineteen normal rectal flora isolates examined contained small plasmids (less than 5 Mda). The majority of these small plasmids could be assigned to three homology classes by Southern hybridization. Further investigation demonstrated the presence of subclasses within one of the homologous classes. Class I plasmids were 1.8 Mda, Class III were 3.7 Mda, and Class II plasmids were of three different molecular weights: 2.6 Mda (IIA), 3.2 Mda (IIB), and 4.0 Mda (IIC). Among representative plasmids from each class there was remarkable sequence similarity based upon digestion patterns obtained using the restriction endonucleases, AluI and DdeI. In addition, similar polypeptide products were observed using purified Class I plasmids as template in an Escherichia coli in vitro coupled transcription-translation system. Small plasmids were found in seven of the ten recognized species of intestinal bacteroides, indicating that no species barrier exists for these plasmids. In addition, since plasmids from all three classes were found together in a single isolate, it was concluded that these plasmids are not incompatible.