Variability in DNA sequence of closely related nosocomial gentamicin-resistant plasmids

Endemic gentamicin-resistant plasmids derived from 12 species of gram-negative bacilli were analyzed by restriction-endonuclease digestion. These digests showed wide variations in digest patterns, but one or more common fragments were seen in all plasmids studied. Southern blot analysis using one plasmid as a probe revealed that most but not all of these plasmids shared extensive regions of homology. The variance observed in restriction-digest patterns could be accounted for either by introduction of unrelated DNA or by intramolecular events. We conclude that variation in restriction-digest patterns of endemic gentamicin-resistant plasmids is insufficient to establish that the plasmids are not closely related. Variation in molecular structure of closely related, endemic gentamicin-resistant plasmids may be more common than has previously been suspected.