Mutual Exclusion Between Related Phages

The term mutual exclusion is used to designate an extreme form of interference which often occurs when two phage particles attack the same host cell. The cell liberates numerous particles of one of the parental types and not a single particle of the other parental type (Delbruck and Luria, 1942; Dulbriick, 1945). Mutual exclusion is the rule when the two infecting particles are very dissimilar. However, when they are similar and infect the cell simultaneously, both may multiply. Under these conditions genetic recombination between the two parental types may also take place. Recently French et al. (1951) discovered that infection of an Escherichia coli, strain B, cell with a particle of the related large phages T2, T4, and T6, and of the unrelated phage T6, within two minutes produces a profound modification in the bacterium. This modification is manifested in the fact that T2 particles super-infecting the cell two minutes or later after the first infection are broken down. In the experiments of Lesley et at. (1951) the superinfecting phage was labeled with P32, and its breakdown was tested for by determination of trichloracetic