SUMMARY: The properties of the latent phages present in twelve of the thirty recognized Vi-phage types of are described. The isolation and characterization of these phages presented great technical difficulties which have been partly overcome.

Nine of the latent phages were shown to control Vi-phage type specificity in the lysogenic strains that carry them. The remaining three phages have not so far been demonstrated to have type-transforming properties.

The latent phages are O phages, as indicated by their lytic action on the Vi-negative strain O 901 and on . By cross-neutralization tests the twelve phages are subdivided into four serological groups, one of which shows a considerable degree of antigenic community with an anti-O phage known to attack many salmonella species belonging to different O groups. No serological relationship was found between any of the latent phages and Vi-phage II.

The Vi-phage type of lysogenic strains of is an expression of a resistance pattern dependent on the carried phages. These phages seem to confer resistance against unadapted Vi-phage II and simultaneously to induce phenotypic modifications in it to yield specifically adapted Vi-phage preparations.

Artificially prepared Vi-phage types of the typhoid bacillus, produced by the treatment of susceptible strains with type-determining phages, appear to be as stable as the corresponding Vi-types found in naturally occurring lysogenic strains. The fact that world-wide experience with the Vi-phage typing technique over many years has proved its epidemiological reliability suggests that the chances of the typhoid bacillus encountering type-changing phages under natural conditions are remote.

It is shown that the recently proposed method of typing bacteria by identifying the phages they carry is unsuitable for application to .


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