SUMMARY: The effects of exogenous thiamin on the growth yield and vitamin B content of 18 strains of yeasts and a few strains of bacteria were examined. The addition of thiamin hardly affected the growth yield of the yeasts tested, except for two strains strain 4228 and IFO0751. In contrast, the vitamin B content of all the yeasts tested, except IFO 0189, decreased markedly in the presence of thiamin. In IFO 1265, the synthesis of vitamin B was maximally inhibited by the addition of thiamin (1·5 nmol ml) to the growth medium without affecting cell growth, whereas the amounts of cellular vitamin B increased in the presence of the thiamin antagonist, pyrithiamin or oxythiamin, at concentrations that did not affect growth. When [4′C]pyridoxine. HCl (0·5 μg ml) was added to the growth medium at least 54% of the added isotope was incorporated into the cells during 24 h incubation. In the presence of thiamin (15 nmol ml), at least 32% of the added isotope was incorporated. The metabolism of [4′C]pyridoxine. HCl to inactive forms having no vitamin B activity was not stimulated by the addition of thiamin. Thus, vitamin B synthesis in many yeasts was affected by thiamin, whereas, in bacteria, growth yield and vitamin B content were not affected by thiamin.


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