402 BACTERIOPHAGES 



members of these groups. Many types are lysed only l3y their 

 specific adaptations of Vi-phage II. 



Vi-phage typing is now recognized as the method of choice for 

 the epidemiological "fingerprinting" of the typhoid bacillus. 

 It has also established itself as the model for the development of 

 all subsequent schemes for the typing of bacteria by phage. 



3. Technical Considerations 



a. Isolation of Phages for Typing Purposes 



Phages may be isolated from sewage or fecal filtrates, from 

 sensitive bacterial strains contaminated with phage before or dur- 

 ing isolation (these are known as carrier strains and show phage- 

 nibbled colonies on plating), and from lysogenic strains of bac- 

 teria. Sewage usually yields virulent phages, but those obtained 

 from feces or carrier strains may be either virulent or temperate. 

 Lysogenic cultures normally produce the most specific phages, 

 but phages of high specificity are occasionally isolated from fecal 

 filtrates or carrier strains, and it is interesting to note that Vi- 

 phage II, the most useful of all typing phages, was originally 

 found in a carrier strain of Salmonella typhi (Craigie and Yen, 

 1938). It is often an advantage to include in a typing set a 

 phage which is specific only for the bacterial species under test 

 but which will lyse the great majority of strains of that species. 

 By ordinary typing standards, of course, such a phage would be 

 regarded as nonspecific, but, once it has been established that its 

 host range is effectively limited to the one bacterial species, it 

 provides a useful control for the identification of the latter if the 

 more specific typing phages do not produce lysis. Conn, 

 Botcher, and Randall (1945) used phages for the species identi- 

 fication of soil bacteria. A phage able to attack an entire bac- 

 terial group, such as the salmonellas, may also be usefully em- 

 ployed when it is necessary to confirm that a culture belongs to 

 that group (Cherry, Davis, Edwards, and Hogan, 1954; Wasser- 

 man and Saphra, 1955). It must be remembered, however, 

 that species or group specificity of host range is probably never 



