Limited multiplication of phages superinfecting lysogenic bacteria and its implication for the mechanism of immunity

Under normal plating conditions, weak virulent mutants of the temperate bacteriophage P2 are unable to form plaques on strains of Escherichia coli C made lysogenic for P2 or for any of its temperate mutants. By means of a special method described here, however, the multiplication of weak virulent mutants in lysogenic strains can be followed. The experiments show that (1) superinfection with a single weak virulent particle is sufficient to initiate replication of such a phage in a lysogenic host although with a low probability; no cooperative interaction between superinfecting particles in multiple superinfection is detectable; (2) the superinfecting phage requires, on the average, a longer time to complete a replication cycle in a lysogenic host than in a nonlysogenic host; (3) as a consequence of (2), the superinfected cells which continue to multiply, may also segregate out progeny that survive the superinfection; (4) the presence of superinfecting wild-type P2 in a lysogenic host interferes with the multiplication of a superinfecting weak virulent mutant; (5) as a rule, multiplication of weak virulent mutants is accompanied by multiplication of phage of the prophage type and also of cosuperinfecting, temperate phage type, if present. These results can be interpreted to mean that the “immunity substance,” postulated to be present in the lysogenic cells, is always in excess of the number of superinfecting particles used in the experiments, and that a superinfecting temperate phage, whose replication is blocked by the immunity of the host, can still direct the synthesis of more immunity substance.