Thymidine and Vitamin B 12

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Abstract
Growth of Streptococcus faecalis R, Lactobacillus leichmanii 313, and Leuconostoc citrovorum 8081 on standard media to which various combinations of crystalline vit. B12(B12), commercial liver extract, thymidine, thymine, and crystalline folic acid were added, was studied turbidimetrically to evaluate the presumed relationship of B12 as a coenzyme for the synthesis of thymidine from thymine, or of thymidine as an agent in the synthesis of B12. In S. faecalis there was no growth response to B12 in 40 hrs. In 16 hrs. thymine and thymidine replaced folic acid. L. citrovorum grew with liver extract and thymidine, but response to thymidine was delayed 21-40 hrs. B12 failed to replace liver or thymidine, even in the presence of thymine. L. leichmanii responded equally well to thymidine whether thymine was present or absent; response to B12 equaled the response to thymidine only when folic acid was present, although some growth occurred when folic acid was absent; thymine only partially replaced folic acid; and liver extract was effective, apparently because of its B12 content.