Figure 1

Evolution of antibiotic resistance. (A) Immediately after pouring the agar layers form two distinct wedges, as shown with the dye malachite green as a visual indicator. (B) After 16 hr of incubation the two layers have intermixed to create a smooth gradient of the dye. (C) A top-down schematic, showing the site of inoculation and chemical gradient. (D) Changes in minimum inhibitory concentration following antibiotic exposure, minimum of three biological replicates. Speed-selected E. coli MG1655 cells were passed through SAGE plates containing the listed antibiotics at approximately 5x the initial MIC value. Those which evolved resistance were then passed through a separate plate containing 25x the initial MIC. Individual tests are shown in Table S1. Lineages derived from the first passage were named e.g. Poly B 1, Poly B 2, etc., and lineages built by passing evolved strains through the 25x plates were correspondingly named Poly B 1-1, Poly B 1–2, etc. *Bacteria were exposed to plates containing 10x and 50x the initial MIC of ciprofloxacin, and 20x and 100x the initial MIC of rifampicin, respectively. ^‡Resistance was greater than 256x the initial MIC. Cells were uniformly resistant to 8192 mg/L of streptomycin, and poor solubility limited testing at higher drug concentrations.