349 An Analysis of the Vi-Phage Typing Scheme for

The seventy-two currently recognized Vi-phage types of Salmonella typhi have been examined in order to determine, where possible, their structural formulae. The genetic nature of the changes in host range of the corresponding typing adaptations of the Vi-phage I1 has also been in-ves tigated. Thirty-seven temperate phages have been isolated from the type strains and type-determining ability has been demonstrated in all but five of these phages. Several of the determining phages were found in more than one type strain and it is probable that no more than 16 different determining phages are represented in our collection. Because there are fewer phage reactions to be eliminated in cultures already possessing a restricted sensitivity spectrum to the typing adaptations of Vi-phage 11, some of the determining phages ( ' semi-determining phages') display their full effect only in such cultures. One determining phage, dl-C, can co-operate with others, especially phages ds and d9, in multiply lysogenized cultures to produce an effect which is more than a summation of the known type-determining powers of the individual phages concerned. Moreover, the type-determining effects of phages dl-C, d8, and d9 in singly or doubly lysogenized cultures are influenced by the temperature of incubation. As has been shown by earlier workers, the changes in host-range of Vi-phage I1 that produce the Vi-typing phages may be phenotypic (host-induced) or genotypic (mutational) in nature, or a combination of these two types of variation. Amongst the current typing phages are represented eighteen different phenotypic modifications and forty-three different host-range mutants. Many of these mutations seem to be related to each other but their chromosomal distribution has not been mapped