Bacterial plasmids are circular double-stranded DNA molecules that are physi- cally separate from the bacterial chromosome. They are replicated and stably inherited in the extrachromosomal (autonomous) state. The plasmids of entero- bacteria can be divided into two distinct groups according to their size: (i) small plasmids with MW of less than 10 Mdal, and (ii) large plasmids with MW ranging from 50-100 Mdal. These two groups differ strikingly in their copy numbers per cell (multiplicity). Whereas most small plasmids are multicopy plasmids (20-100 copies per cell), large plasmids are...