The double strain inoculation test as used for the assay of nutritional factors capable of enhancing survivorship in mouse salmonellosis has been modified and improved. The modification has been based on the use of genetically marked strains of S. typhimurium, and substitutes for survivorship a bacteriological datum, a certain minimum population size of the virulent salmonellae in the mouse spleen at a certain time. The analysis permitting this substitution and the economies achieved by the modification has been presented.

In the course of development of the new assay evidence was obtained that the nutrient entity or entities responsible for enhanced survivorship, as measured by the bacteriological correlate, are not stored in the mouse. A deficiency state was achieved 24 hours after supplying the basal synthetic diet. Similarly, positive effect of active nutrients was evident as soon as 24 hours after feeding.

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