Figure 1

E. coli ampicillin survival curves. All E. coli are isogenic, recently derived from a single colony. Triangles show survival rates for cells previously grown in the absence of ampicillin. Resistance rates were ramped higher by prior exposure to ampicillin. Cells surviving selection of 1 μg/ml ampicillin (circles) showed improved survival on both 1 μg/ml and 2.5 μg/ml ampicillin. Cells surviving 2.5 μg/ml ampicillin (squares), showed even more dramatic improvement, with an approximately three log increase in survival on 2.5 μg/ml ampicillin. Nevertheless, reversion rates were extremely high, with for example over 95% of cells from 2.5 μg/ml ampicillin not surviving dispersion and immediate re-plating on another LB agar plate with the same antibiotic concentration, 2.5 μg/ml ampicillin. Survival rates for previously unexposed E. coli at ampicillin concentrations of 5 μg/ml and higher were extremely low, below 1/10⁶.