SUMMARY: Strains of Q1, lysogenized with type A phages, were superinfected with the heterologous free phages of the same group. This produced lysis (productive or vegetative development) and prophage change (either prophage substitution or double lysogenization) in a constant pattern. Prophage change was frequently detected when lysis was absent. Certain of the phages were aggressive, producing active lysis and prophage change in many of the heterologous lysogenic strains, others were intermediate and some were non-aggressive. In general, aggressive phages in prophage form conferred a good degree of immunity on the host bacterium, while non-aggressive phages did not: but to this rule there were exceptions. In most cases, immunity to lysis and immunity to prophage change ran an approximately parallel course, but again there were exceptions. Some strains, with certain superinfections, were immune to lysis but not to prophage change, while others showed greater resistance to prophage change than to lysis. The reaction to superinfection split the series into two groups. Superinfection of a lysogenic organism of either group with phage of the same group produced—if there was any prophage change—prophage substitution: superinfection of a lysogenic organism of one group with phage of the other group produced double lysogenization. Each group of phages had therefore its own site of attachment to the bacterial chromosome. Immunity appeared to be due neither to defective adsorption nor to steric interference, but to repressers with specific characters which varied from strain to strain.


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