Mutual exclusion between an infecting phage and a carried phage

When bacteria are simultaneously or consecutively infected by two dissimilar phages, any one bacterium yields, upon lysis, phage particles of one or the other of the parental types, never of both types (Delbrück and Luria, 1942; Delbrück, 1945). This phenomenon has been called mutual exclusion; the mechanism responsible for it is at present unknown. Recent work by Lwoff and his collaborators has opened for quantitative studies a new field of phage research: lysogenesis. Lwoff and Gutmann (1950) have shown that in lysogenic strains of bacteria the ability to yield phage is carried by the bacteria through many generations. The phage is presumed to be carried intracellularly in a noninfective form called prophage. The conversion from prophage into infective phage can be initiated simultaneously in all the bacteria of a culture by irradiation with a small dose of ultraviolet light (Lwoff, Siminovitch, and Kjelgaard, 1950), or by exposure to certain compounds containing sulfhydryl groups (Lwoff and Siminovitch, 1951). About 60 minutes after applying the inducing treatment, each bacterium lyses, liberating many phage particles. We were interested in testing whether mutual exclusion would occur between carried phage, on its way to maturity by virtue of the inducing treatment, and a different phage introduced from the outside at various times during the maturation period. If the external phage can exclude the carried phage, then any mechanism of exclusion operating by erecting a block at the invasion stage of the bacterium by the phage would be ruled out. A system which is suitable for such studies consists of the Escherichia coli, strain K12, its carried phage λ, and one of the phages of the T series. Strain K12 is sensitive to the seven phages of the T series, but T5 was employed in this study because of a combination of two advantageous features. First, it registers on strain K12 with the same efficiency of plating as on strain B, its normal host strain. Second, its long latent period gives greater freedom for a number of manipulations. A disadvantage of phage T5 is its very low rate of adsorption on strain K12, even lower than on strain B. This complicates the techniques in various ways and necessitates additional controls