When bacteria lysogenized by certain temperate phages like λ and 2 are superinfected by distinguishable mutants of their prophage, the superinfecting phage appears in the progeny released after induction of vegetative phage growth (1, 2). Phage 22 differs, however, for although bacteria lysogenized with it adsorb superinfecting phage 22, no superinfecting genetic markers like (clear plaque) or (host range) appear in the phage obtained after induction, whether spontaneous or produced by u.v. irradiation. Superinfecting phage 22 is thus excluded and the wild-type prophage may be termed ‘excluding’ ( ). Non-excluding () mutants have now been isolated by taking advantage of the phenomenon of lysogenic conversion.

only forms somatic antigen 1 (01) if it is infected by phage 22 possessing the converting gene (3).


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