The spread of plasmids in model populations of Escherichia coli K12.

Comparison of R100 with its derepressed derivative R100-1 showed that the capacity to repress tra function does not significantly affect the spread by retransfer of R100. F′lac was used to investigate the contributions of growth and transfer to spread of a plasmid through a recipient population. Ability to transfer F′lac was lost rapidly when donor cultures entered stationary phase, but aggregate-forming ability was lost much more slowly. Comparison of F′lactra⁺ with F′lactraH88, which is unable to retransfer from recipients, showed the importance of retransfer. We used a mathematical model to calculate the amount of retransfer needed to explain the rate of increase of F′lac progeny. This showed that the lag between a cell receiving F′lac and being able to retransfer it was a less important constraint on this rate of increase than the inherent rate of plasmid transfer by established donors.