SUMMARY: Cultures of strain W of sometimes contained a phage, Wα, which could form plaques on strain W itself. Phage Wα was found to be a virulent mutant of a temperate phage, Wβ, with which strain W was lysogenic. Another non-lysogenizing mutant, Wγ, was also isolated which, unlike α, did not form plaques on strain W. The genetic determinants of α, β and γ are probably allelic. Neither phage γ nor phage β formed plaques on bacterial strains lysogenic with β, but these strains were not completely immune, even to the temperate phage. This finding shows that lysogenicity does not invariably confer complete immunity to the carried phage. The colonial appearance of a strain was altered after lysogenization in a way suggesting that the alteration was due to its incomplete immunity to the temperate phage produced during growth.


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