Cooperative infection by host-modified lambda phage

Abstract

Host-modified λ which arises by phage growth in Escherichia coli strain C plates with an efficiency of 10 −3 to 10 −4 on E. coli strain K when infection is performed at a low multiplicity. The occasional infection which results arises from the presence in K populations of rare cells in which λ.C is capable of multiplying. When several λ.C phage particles infect a single K cell, a form of cooperative infection, analogous to multiplicity reactivation, occurs, such that at high multiplicities more than 10% of the infected cells yield progeny. The latent period and burst size are normal when λ.C infects strain K either singly or multiply. In both cases all the progeny that are formed in a single cycle of growth are of the unrestricted or λ.K type. The occurrence of cooperative infection is explained by assuming that the restriction present in host modification is applied independently to genetic structures smaller than the entire phage chromosome. Each susceptible site has a small chance of escaping restriction, and undamaged sites in separate phage chromosomes can complement each other to produce an infection. The quantitative dependence of the number of successful infections upon the multiplicity of infection suggests that approximately 10 independent phage sites are involved, each with approximately a 15% change of surviving.