Summary: The medium redox potential (Eh) influenced the folate-limited growth of Lactobacillus casei; the growth response was maximal at an Eh of +120 mV (pH 6·35). At raised Eh serum folate would support less growth than pteroyl-glutamic acid, and the response to N5-methyl tetrahydrofolic acid was intermediate between them. Pteroylglutamic acid was not destroyed during 24 h incubation at 37 °C in medium with Eh values between +40 and +440 mV. Destruction of N5-methyl tetrahydrofolic acid occurred within 24 h when the medium Eh was greater than +125 mV. Folate was taken up rapidly by L. casei with an Eh optimum at + 270 mV.
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