In vitrostudies on long‐term stability of R plasmids inEscherichia coliK12

Abstract
The maintenance and stability in anE. coliK12 host of environmental isolates of R plasmids encoding gentamicin resistance during multiple passages in antibiotic‐free and antibiotic‐containing broth were investigated in regard to the conferred resistance phenotypes and the respectiveEcoRI digestion patterns.Only two plasmids belonging to theIncM group maintained stable endonuclease digestion patterns over a 15‐month period in both of the media applied. Other members of this group revealed a considerable variability in theirEcoRI digestion patterns, but a stability in their resistance determinants. TwoIncM plasmids and anIncK plasmid exhibited partial resistance loss under nonselective conditions. The complete segregation of resistance determinants fromIncOF plasmids resulted in a stably maintained cryptic core plasmid.