Strength of selection pressure is an important parameter contributing to the complexity of antibiotic resistance evolution. Escherichia coli str. K-12 substr. MG1655

We evolved 88 isogenic Escherichia coli populations against 22 antibiotics for three weeks. For every drug, two populations were evolved under strong selection and two populations were evolved under mild selection. By quantifying evolved populations’ resistances against all 22 drugs, we constructed two separate cross-resistance networks for strongly and mildly selected populations. Subsequently, we sequenced representative colonies isolated from evolved populations for revealing the genetic basis for novel phenotypes.