USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 409 



Moreover, they affect the sensitivity of their host cells to Vi 

 phages other than Vi-phage II and, more significantly, to non- 

 Vi phages. Bearing in mind the fact that all Vi- types will 

 absorb all Vi-phage II adaptations, irrespective of their speci- 

 ficity, it has become clear as the result of this work that the Vi 

 antigen of the typhoid bacillus provides the Vi-typing phages 

 only with a common receptor of access to the organism. Once 

 this function is carried out it appears to play no further role in the 

 control of Vi-type specificity, which is determined by the presence 

 of prophages, or of other factors hitherto unidentified, elsewhere 

 in the bacterial soma. Anderson and Felix (1 953b) found that the 

 specificity of a lysogenic Vi-type was determined by two factors : 

 the identity of the nonlysogenic "ancestral" type; and that of 

 the determining phage with which it was lysogenized. It was 

 observed, moreover, that by the choice of suitable host strains of 

 S. typhi to be lysogenized, many new types could be produced 

 with the few determining phages isolated. It was also found, 

 as would be expected, that several naturally occurring lysogeni- 

 cally determined types that differed from each other carried the 

 same phage, and it was apparent that the nonlysogenic host cell 

 and the determining phage each played a precise part in the 

 control of Vi-type specificity. Like most Salmonella species, 

 S. typhi carries many temperate phages, but relatively few have 

 type-determining properties. 



Of the Vi-types shown in Table XIX, types Dl, D4, D6, 

 F2, T, 25, and 26 have yielded determining phages. These 

 phages fall into three serological groups and those most closely 

 related to each other serologically show strong similarities in 

 type-determining function and other properties. The phages 

 carried by types Dl and D4 are indistinguishable from each 

 other. Table XX, which is modified from Anderson and Felix 

 (1953b), shows .he symbols and serological groups recognized 

 at present. Phages b3, 28', and k, the first two of which con- 

 stitute a separate serological group, have only limited type-deter- 

 mining properties, but are included for the sake of complete- 

 ness, since they were isolated at the same time as the remainder 

 of the phages shown. 



