Figure 2.

Effects of horizontal gene transfer and positive selection on plasmid stability. Theoretical scenario where antibiotics are used every day at a constant dose and where the rate of horizontal transmission (β) is allowed to change. (A) Comparison between plasmid dynamics for a non-conjugative plasmid (black line) and (blue line) when the rate of horizontal transmission is low (but non-zero) and the antibiotic is deployed at a dose corresponding to 12.5% the MIC of the plasmid-free cells. Note how if β = 5 × 10⁻¹¹ the plasmid-bearing population recovers after an initial decrease in frequency, but if β = 0 the plasmid-bearing population is driven to extinction. (B) By numerically simulating our evolutionary model under different rates of conjugation and intensities of positive selection we can characterize different plasmid stability regions.