USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 



405 



typical typhoid phage typing plates are shown in Figure 13. 

 For routine purposes the phages corresponding to rare Vi- types 

 are pooled so that the test may be completed on one plate. 

 Full details of the media, techniques, and methods of reading 



Figure 13. Reactions of S. typhi to its typing phages. Left, 

 right, Vi-t ype J. 



Vi-tv 



pe 



and interpretation are given by Anderson and Williams 



(1956). 



4. Theoretical Aspects of Vi-Phage Typing 



a. The Adaptation of Vi-Phage II 



All the adapted typing preparations of Vi-phage II are 

 neutralized by an antiserum against phage A, which was the 

 form in which the phage was first isolated, and there is no doubt 

 that all are modifications of the one phage. The demonstration 

 of the variability of host range of Vi-phage II stimulated con- 

 siderable interest in the possible mechanism of adaptation, and 

 also aroused curiosity concerning the nature of the differences 

 between the various Vi-types of the typhoid bacillus. Craigie 

 and Yen (1938) believed that a specific host-range mutant of Vi- 

 phage II existed for each Vi-type of Salmonella typhi, and suggested 

 that the process of phage "adaptation" consisted in the selection 



