Certain non-proliferating bacteria, suspended in saline, are inhibited by folic acid as judged by the ability of the cells to form visible colonies on a nutrient agar subsequent to exposure to folic acid. On the other hand, some yeasts (three strains) are insensitive to folic acid. In Escherichia coli ATCC 9637, folic acid toxicity can be prevented by a variety of metabolites. Although a purine-requiring strain of E. coli is resistant to folic acid, and purines prevent folic acid toxicity in another strain of E. coli, experiments with C14-formate failed to demonstrate an unequivocal effect of folic acid on de novo purine biosynthesis.
Folic acid and the synthesis of methionine by extracts of Escherichia coli