Figure 2.

A simple model describes the population dynamics of a cooperative antibiotic resistance plasmid in the β-lactam antibiotic ampicillin. (A) Growth rates of resistant (blue) and sensitive (red) bacteria as a function of antibiotic concentration. Free of the metabolic cost associated with resistance, sensitive cells grow faster than resistant cells (γ_S>γ_R) at antibiotic concentrations below the MIC of the sensitive bacteria. Above the MIC, sensitive cells die at a rate of γ_D. (B) The population dynamics within a single competition cycle (1 day). During the lag phase (t<t_lag), neither cell type divides nor dies, but the antibiotic is constantly hydrolyzed by resistant cells. After the lag phase, each sub-population grows at a rate that depends on the extracellular antibiotic concentration. At time τ_b, the extracellular antibiotic concentration drops below the MIC of the sensitive cells. Cell growth ceases when the total population density reaches saturation. Inset: the time trace of the resistant fraction within a single day. (C) The model gives rise to difference equations that resemble experimental data (Figures 1C, 3A, and B). (D) The equilibrium-resistant fraction predicted by our model as a function of the antibiotic concentration and the initial cell density. According to the model, coexistence between resistant and sensitive cells is possible at antibiotic concentrations above the MIC of sensitive cells.