Summary: All adaptations of Vi-phage II to the known Vi-types of have been grown on Vi-type A. In confirmation of earlier work these adaptations were found to be phenotypic or genotypic modifications of the original Vi-phage II, or showed a combination of both changes. Most adaptations to non-lysogenic types are changes of phenotype only, whereas those to lysogenic types are usually genotypically stable, although they may have a superimposed phenotypic modification. The fact that Vi-types that are exactly similar to naturally occurring types can be made artificially by the lysogenization of non-lysogenic types with type-determining phages makes it possible to assign structural formulae to most of the known naturally occurring Vi-types. The formulae consist of the symbol of the non-lysogenic precursor of a lysogenic type followed by that of the type-determining temperate phage in parenthesis. Thus, type F2 is type F1 carrying phage f2 and the formula of type F2 is therefore F1(f2). Using these formulae it is possible to predict the host range of Vi-phage II adapted to each lysogenic Vi-type. The phenotypic and genotypic changes concerned in the many adaptations of Vi-phage II are independent of one another, and by taking advantage of this independence it is possible to adduce experimental proof of the correctness of the proposed structural formulae for the lysogenically determined Vi-types. In studying the Vi-types it is useful to regroup those that are lysogenic according to the non-lysogenic precursor types common to certain of them.


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