USE OF PHAGES IN EPIDEMIOLOGICAL STUDIES 407 



and Fraser (1956) showed that it was possible, by the use of 

 suitable indicator strains in fluctuation tests, to measure the 

 incidence in the same stock of Vi-phage II of diff'erent particles, 

 some undergoing phenotypic modification and others host-range 

 mutation. It was suggested by Bertani and Weigle (1953) and 

 Weigle and Bertani (1953) that the phenomenon of host-induced 

 modification was probably due not to the presence in the parent 

 phage of a minority of particles able to multiply in any cell of 

 the new host, but to a heterogeneity amongst the host cells which 

 enabled a few organisms to support the multiplication of any 

 particle of the phage. 



The host-range flexibility of Vi-phage II was examined bv 

 Anderson and Felix (1952, 1953a, c), Anderson (1955a), and 

 Anderson and Fraser (1955, 1956). It was found that some 

 of the typing adaptations were host-induced modifications, 

 some were host-range mutants, and a third group were host- 

 range mutants with superimposed host-induced modification. 



Phage A appears to be the wild form of Vi-phage II and every 

 particle of phages which are pure host-induced modifications of 

 it reverts to phage A when grown in type A which, as Table 

 XIX shows, is fully sensitive to all the typing adaptations of Vi- 

 phage II. Host-range mutants without superadded host-in- 

 duced modification are unchanged by growth on type A. How- 

 ever, in a phage showing both varieties of modification, host- 

 induced change is abolished by growth in type A, and the host 

 range of the pure mutant is then expressed. It can thus be seen 

 that the following combinations aflfecting host range can occur in 

 Vi-phage II: wild phenotype, wild genotype (phage A) ; modi- 

 fied phenotype, wild genotype; wild phenotype, modified geno- 

 type; modified phenotype, modified genotype. As would be 

 expected, the widest host ranges are found in the last of these 

 four groups, because the respective phages have the extra range 

 of the host-induced modification added to that of the mutation. 

 A fuller discussion of this subject is given by Anderson and 

 Fraser (1955). 



