Growth of many microbial species was inhibited by pyrithiamine, the pyridine analog of thiamine. Growth of many other species was not influenced. In a series of bacteria, yeasts, and molds, it was found that inhibition of growth occurred only in those in which growth was stimulated by thiamine or its component pyrimidine and thiazole portions. The amount of pyrithiamine required for inhibition was correlated with the type of thiamine requirements of various species. The least amount was needed to inhibit organisms which required intact thiamine. Those which could use the pyrimidine and thiazole portions were not so readily inhibited. In the case of the former organisms, half maximal inhibition was produced by as little as 0.03 γ per cc. In all instances, the inhibition was overcome by sufficient amounts of thiamine. The synthesis of thiamine by insusceptible species was studied, and it was concluded that formation of thiamine or other antagonistic substance did not provide an adequate explanation of the resistance of these species to the action of pyrithiamine.

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