Phase-variable restriction of SpSL1 by SpnIII in a wild-type population. Plaque assays of a wt strain and spnDP1004III-locked mutants. The wt DP1004 strain, harboring 85% and 12% spnDP1004IIIA- and spnDP1004IIIB-positive cells, respectively, was tested for infection by SpnIIIA-methylated phage (A) and SpnIIIB-methylated phage (B) and compared to infection of the spnDP1004IIIA-locked and spnDP1004IIIB-locked strains. (A) Equally efficient SpnIIIA-methylated phage infection of a wt host with a predominance of SpnIIIA cells and of an spnDP1004III-locked mutant. (B) Plaque generation of SpnIIIB-methylated phage. Unrestricted plaque formation in spnDP1004IIIB cells yielded SpnIIIB-methylated phage (data of phage methylation status not shown), while infection of the wt containing 85% spnDP1004IIIA cells (gray bar in panel B) yielded fewer plaques (two-tailed t test, P < 0.001), and all phages obtained were SpnIIIA methylated (data not shown).