USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 401 



This capacity for modification of host range was wide enough to 

 enable Cragie and Yen, by progressive adaptation of the phage to 

 a collection of typhoid strains, to recognize 1 1 types of the ty- 

 phoid bacillus, and trials of the method soon showed that this sub- 

 division was epidemiologically reliable. The value of the scheme 

 was recognized by Felix (1943), and it was largely due to his 

 work that it was accepted for international use. Craigie and 

 Felix (1947) published suggestions for the standardization of the 

 method. As it was employed by an increasing number of 

 workers it was improved and extended; at present, 58 types and 

 subtypes of the typhoid bacillus are recognizable by the Vi- 

 phage typing scheme, and the flexibility of host range of Vi-phage 

 II is such that it is clear, on theoretical grounds alone, that many 

 more types could be defined. In this work the terms "type," 

 "Vi-type," and "Vi-phage type" all refer to the bacterium. 

 The typing phages, on the other hand, are simply prefixed by the 

 word "phage." The original suggestion of Craigie and Yen is 

 retained of designating the Vi-types and the corresponding typ- 

 ing adaptations of Vi-phage II by identical symbols. Thus, 

 type A is lysed by phage A, type C by phage C, and so on. Un- 

 fortunately, this system of designation has led to considerable con- 

 fusion among workers not directly concerned with phage typing. 

 However, the routine terminology has found such wide accept- 

 ance in the practical field that it is now too late to change it. 



Table XIX shows the Vi-phage typing scheme in an abbre- 

 viated and simplified form. Minor cross reactions are omitted 

 from this table. 



A number of features are apparent in Table XIX. Primarily, 

 type A is sensitive to all the typing preparations. For reasons 

 which will be apparent later, it is believed that A is the most 

 primitive type in this scheme and that phage A represents the 

 wild form of Vi-phage II. A number of types form associated 

 groups of which one member occupies a similar position rela- 

 tive to the remainder to that occupied by type A in relation to 

 the entire scheme. In some cases it has been possible to deter- 

 mine the reason for the relationship between some of the 



