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SUMMARY: Seven analogues of vitamin B12 were examined for their influence upon the growth of Ochromonas malhamensis, Euglena gracilis, Lactobacillus leichmanniiATCC 4797 and Escherichia coli 113–3. The analogues were active in promoting the growth of E. gracilis and L. leichmannii, but were essentially inactive for O. malhamensis. For E. coli three of the analogues were active and four inactive. In O. malhamensis and in E. coli, the ‘inactive’ analogues antagonized the growth- promoting action of cyanocobalamin, apparently by satinrating the cells’ mechanism for ‘binding’ the vitamin and so preventing its uptake.
Tests were also carried out with Ochromonas malhamensis and Escherichia coli on twelve derivatives of benziminazole. None of these proved inhibitory towards E. coli, at least up to 100 µg./ml. culture medium. Eight of the compounds were inhibitory towards O. malhamensis; the effects were relatively small, however, and were possibly unrelated to the metabolism of vitamin B12.