Abolition of host cell restriction by high multiplicity of phage infection

When restricting host cells (Escherichia coli K12 or E. coli B) are infected first with a nonmodified lambda phage (called the helper phage) at high multiplicity and subsequently superinfected with a second nonmodified lambda vir phage (called the test phage), it can be shown that an increased number of infected cells produce a successful infection of phage which carry the virulent marker of the test phage. This multiplicity effect has been studied both in normal and in recombination defective strains, in the presence or the absence of chloramphenicol and mitomycin C and in circumstances where the gene expression of the nonmodified helper phage was prevented by immunity. The results indicate that the nonmodified helper-phage causes a temporary inactivation of the restricting system and that the multiplicity effect is not due to gene complementation or cooperation or to recombination. Furthermore this multiplicity effect can be observed in different hosts and with various nonmodified phages. When similar experiments were performed with modified helper phage no multiplicity effect was observed