USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 399 



feet on other phages. Burnet (1934b) confirmed these observa- 

 tions and Gough and Burnet (1934) demonstrated that the speci- 

 ficity of this phage-inactivating agent was determined by a poly- 

 saccharide. Levine and Frisch (1935, 1936) established, by 

 means of phage sensitivity, differences between certain strains of 

 S. cholerae suis which were confirmed by antigenic analysis of the 

 bacteria. These findings led to the diflferentiation of the 6i and 

 62 somatic antigens in the G group of salmonellas. Sievers 

 (1943) first identified the somatic antigens oi S. koln because of 

 the organism's sensitivity to Sonnenschein's phage for S. para- 

 typhi B. 



2. The Vi-Phage Typing Scheme of Craigie and Yen 



In 1934 Felix and Pitt announced their discovery of the Vi 

 antigen of Salmonella typhi. This antigen is present in the great 

 majority of freshly isolated strains of the typhoid bacillus. Its 

 presence renders the organism inagglutinable by sera containing 

 antibodies against only the somatic (or O) antigen of S. typhi. 

 The Vi form of the organism is more virulent for mice than is the 

 non-Vi form, and sera containing Vi and O antibodies offer a 

 more powerful protection to mice against Vi-positive typhoid 

 strains than do sera containing the O antibody alone. Phages 

 specific for the Vi form of S. typhi were isolated independently by 

 three groups of workers (Graigie and Brandon, 1936; Sertic and 

 Boulgakov, 1936a; Scholtens, 1936). These phages were in- 

 active on and were not adsorbed by the non-Vi (or O) form of 

 the organism. Two years later, Graigie and Yen (1938) pub- 

 lished the Vi-phage typing scheme for S. typhi. These workers 

 had isolated four Vi phages which they designated by the Roman 

 numerals I to IV. They had originally intended to devise a 

 typing scheme for the typhoid bacillus based on the diflferential 

 patterns of lysis produced by these phages on different strains 

 of the organism. The peculiar behavior of one of the phages, 

 Vi-phage II, however, attracted their special attention. They 

 found that this phage possessed the unusual property of acquiring 

 a high specificity for the last strain on which it had been grown 



