Fig. 1

Laboratory evolution under a combination of two antibiotics. a The procedure of laboratory evolution. The laboratory evolution was performed in 8 × 8 wells in 96-well microplates with two antibiotic gradients. The dilution step was set at 2^0.25 fold. At a daily transfer, the bacterial cells were transferred from the well with the highest sum of log-transformed drug concentrations in which cells were able to grow (OD₆₀₀ > 0.03). b-d The change of MIC in laboratory evolution. The trajectories of MIC changes are plotted for b AMK + CP, c AMK + ENX, and d CP + ENX, respectively. The dots represent MICs at the end-point of laboratory evolution (33 days), and the lines show the changes of MIC during laboratory evolution. The red dots and lines correspond to the trajectories of evolution using combinatorial use of two antibiotics, while blue and green dots/lines show the results of laboratory evolution under addition of single antibiotics. For the single drug experiments, MICs to the counterpart drug were assumed to be identical to the parent strain in these figures. The data points have been slightly randomized by adding Gaussian noise to avoid overlapping of points