Growth Factor Requirements of the Root Nodule Bacteria

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Abstract
Rhizobium trifolii synthesizes all the organic substances essential for its growth from the simple ingredients of a properly reduced carbohydrate mineral-salts medium. Although various tissue and microbial extracts contain a heat-stable substance (or substances) stimulative to growth, these are not required for successful continued transfer of the organism in a synthetic medium. Continuous transfer of R. trifolii in a synthetic medium is dependent on a factor synthesized by the growing culture which is ordinarily transferred in sufficient amount in the inoculum to stimulate growth initiation. This factor can be separated from cultures of Rhizobium by ultrafiltration. It is readily destroyed by heat. Small inocula, washed free of metabolic products of the culture from which they were taken, produce little or no growth in the absence of the Rhizobium factor. In the presence of certain specific amounts of thiamin or flavin, either vitamin alone is capable of replacing to some extent the stimulative material synthesized by R. trifolii. However, certain combinations of both vitamins are more active than either one used singly. R. trifolii synthesizes appreciable amts. of both vitamin B1 and riboflavin. These vitamins are present in culture autolysates or filtrates as complexes which are readily destroyed by heat, resulting in decreased activity. Further evidence is presented which indicates that the activity of the metabolic products of R. trifolii, in stimulating growth initiation of the same organism, is due, at least in part, to the presence of thiamin and flavin in those products.