Fig. S1.

Selection of CdiA-CT^EC869–resistant mutants. (A) Cultures of logarithmic growth-phase E. coli cells were UV-irradiated and cocultured at a 1:1 ratio with CDI^EC869 inhibitor cells for 3 h. After three cycles of selection, target-cell clones from independent experiments were isolated and tested for CDI resistance. Complementation analysis was used to identify the gene(s) responsible for the CDI^R phenotype. The CDI^R mutant from pool 3 was labeled with GFP and transduced with a cosmid library of E. coli genomic DNA. Individual transductants were screened for CDI^S in competition cocultures with CDI^EC869 inhibitor cells in microtiter plates. Complementation to the CDI^S phenotype results in low GFP fluorescence in competition well. (B) Pools of UV-irradiated target cells were mixed 1:1 with CDI^EC869 inhibitor cells and cocultured for 3 h. CDI^R mutants were selected with iterative cycles of competition coculture. Mutant pools containing CDI^R target cells were identified by determining the competitive index for round of enrichment.