LYSOGENY 333 



the presence of a prophage. For example, if lysogenic E. coli K12(A) is infected 

 with phage PI, responses to infections are similar to those found with non- 

 lysogenic E. coli Kr2: a certain fraction of the bacteria lyses and produces 

 phage, whereas another fraction survives and becomes lysogenic. The latter 

 clones are then doubly lysogenic and carry both prophages A and PI. 



However, it is sometimes observed that a lysogenic cell does not react to 

 phage infection in the same way as a nonlysogenic ceU of the same strain. 

 In such cases, the lysogenic bacterium is often unable to ensure the multi- 

 pHcation of a phage which is able to develop on the nonlysogenic one. This 

 is the case with many strains of Salmonella typhi in which the presence of a 

 variety of prophages (type-determining phages) prohibits multiplication of 

 a phage active on the nonlysogenic derivative (Anderson and Felix, 1953). 

 Similarly, strain Sh of Shigella dysenteriae is sensitive to infection with the 

 phages of the T series. Lysogenic Sh(P2) fails to support the complete de- 

 velopment of T2, T4, T5, and T6. But the infected bacteria are killed with- 

 out lysis or phage production (Bertani, 1953a). Very remarkable is the ob- 

 servation by Benzer (1955) that the presence of prophage X'm. E. coli K12(A) 

 prevents the multiplication of r^ mutants of T2, T4, and T6, but not of any 

 other mutants of the same phages. As for immmiity, this phenomenon reflects 

 a specific block in the multiplication of the infecting particles. 



Besides preventing phage multiplication, the presence of a prophage may 

 also induce phenotypic modifications of unrelated phages which multiply in 

 lysogenic bacteria. An example of such modifications is the "adaptation" of 

 the Vi phages of Salmonella (Craigie and Felix, 1947) analyzed by Anderson 

 and Felix (1953), Vi phages II have a common host called A. When the 

 original phage type is plated on different Vi positive strains, it forms only a 

 small number of plaques. From the plaques formed on any given strain can be 

 isolated an "adapted phage," which multiplies normally on the same strain 

 and forms only rare plaques on the others. Some of these "adapted phages" 

 are host-range mutants but most of them revert to the original nonadapted 

 Vi phage II upon a single transfer on strain A. The modification resulting in 

 "adaptation" of the Vi phages is thus, in most cases, a phenotypic change 

 induced by the host. Analysis of these cases has shown that Vi positive 

 strains which induce such phenotypic changes are lysogenic and differ by 

 the prophage they carry. These prophages appear to be mirelated to Vi 

 phage II. In such strains, therefore, the presence of a prophage not only 

 decreases the sensitivity of a given Vi positive strain to a Vi phage grown on 

 another strain, but it also determines the capacity of this strain to alter in a 

 specific way the properties of those phages which succeeded in multiplying 

 (Anderson and Fraser, 1955). 



Numerous examples of such interference of lysogeny with the multiplica- 

 tion of unrelated phages have now been described in many species. This kind 



