A diverse collection of 52 E. coli strains were treated with MMC to induce native prophages. Supernatants were separately collected and filtered before plating onto three recipient strains. Single lysogens were isolated from individual plaques and confirmed by whole genome sequencing. For each recipient strain background, all single lysogens were tested for defense against all phages used in that background. An iterative deletion-based strategy was used to identify the anti-phage defense systems responsible. See also Table S1.
