Abstract
K56-derived group A streptococcal strains permissive for the virulent phage A25 were lysogenized by bacteriophages isolated from a wide variety of serologically different Streptococcus pyogenes strains and examined for their ability to support the growth of A25. Of 25 artificial lysogens, 5 interfered with A25 as revealed by determining adsorption, efficiency of plating, yielder frequency, and relative burst size of the superinfecting phage. These interfering strains showed normal adsorption and penetration of A25 but could not replicate A25 DNA. Injected phage DNA was neither extensively broken down, cleaved to low molecular weight fragments, nor nicked, indicating the absence of phage restriction. Whereas one of the interfering prophages was unable to prevent its host from being killed by A25 infection, the remaining interfering lysogens continued to synthesize bacterial DNA and survived A25 infection. This observation, together with the finding that the various prophages mediated different degrees of interference and further evidence presented in an accompanying paper, suggests that we encountered at least three different interference mechanisms.
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