408 BACTERIOPHAGES 



b. Vi-Type Specificity in Salmonella typhi 

 The factors governing" the specificity of the Vi-types of Sal- 

 monella typhi have been clarified to some extent. Craigie (1942, 

 1946) isolated a temperate phage from type Dl of the organism 

 and found that he could convert type A into type Dl with this 

 phage, which he designated "the latent or gamma agent" of 

 type Dl. The significance of this observation does not seem 

 to have been realized, however, and the subject was not pursued. 

 Felix and Anderson (1951), Anderson (1951), and Anderson 

 and Felix (1953b) showed that the Vi-type specificity of many 

 types of S. typhi was partly controlled by temperate phages. 

 Such phages were isolated from a number of types, and lyso- 

 genization of type A with them produced specific types that 

 seemed similar in all respects to types found in nature. For 

 example, the phage already shown by Craigie to be carried by 

 type Dl, converted type A into type Dl ; that isolated from type 

 D6 transformed A into D6, that from 25 converted A into 25; 

 and so on. Thus, it became clear that Vi-type specificity in 

 -5". typhi, that is, the spectrum of sensitivity to the many adapta- 

 tions of Vi-phage II, was to some extent a resistance pattern with 

 which the cells were endowed when they carried particular pro- 

 phages. The phages concerned were designated "type-determi- 

 ning" phages. They were given small letter symbols when those of 

 the corresponding Vi-types were capitals, and a number with a 

 superscript prime sign when the type carrying them was desig- 

 nated numerically. For example, the determining phage carried 

 by type Dl is designated phage dl, and that carried by type 25 

 is phage 25'. The observations of Anderson and Felix were 

 confirmed by Ferguson, Juenker, and Ferguson (1955). 



Unless otherwise stated, the terms lysogenic and nonlysogenic 

 are used in the present discussion only in relation to the presence 

 or absence of type-determining phages. The determining 

 phages differ from Vi-phage II in not being specific for the Vi 

 form of Salmonella typhi; in fact, some of them will even attack 

 other Salmonella serotypes. They are also different from the Vi 

 phage in serological, physiological, and physical properties. 



