Biosynthesis of 5-Methyltetrahydrofolate by Bacteria: Current Knowledge and Future Perspectives on MTHFR Polymorphisms

Folate (vitamin B9) is an essential micronutrient required for one-carbon metabolism, methylation reactions, and nucleotide biosynthesis. For decades, the food industry has relied on synthetic folic acid for supplementation and food fortification because of its chemical stability. However, folic acid is not a bioavailable form of folate and must undergo a multistep enzymatic reduction and methylation in vivo. This reaction, catalysed by 5,10-methylenetetrahydrofolate reductase (MTHFR), converts folic acid to 5-methyltetrahydrofolate (5-MTHF), which is the form of folate that circulates in plasma, crosses the blood-brain barrier, and is directly involved in remethylating homocysteine. Individuals carrying certain MTHFR polymorphisms may have reduced efficiency in folate metabolism, potentially contributing to the presence of unmetabolized folic acid (UMFA) in the circulation. The potential downstream effects of circulating UMFA remain debated, including its proposed association with the masking of vitamin B12 deficiency. These considerations have driven interest towards preformed 5-MTHF either through chemical synthesis or microbial fermentation. This review explores the fundamentals of folate metabolism, highlighting the critical function of the MTHFR enzyme and the clinical impacts associated with its genetic polymorphisms. Ultimately, it discusses whether naturally occurring or metabolically engineered bacteria capable of producing 5-MTHF could serve as a safe, natural alternative to conventional synthetic folic acid. Additionally, the review covers how microbially and chemically produced 5-MTHF compare in terms of bioavailability, shelf-life stability, and regulatory approval, while weighing both the supportive and sceptical arguments surrounding the evidence base. Although microbial 5-MTHF synthesis offers a more sustainable and stereospecific alternative to chemical synthesis, the current yield remains low (typically below 10 mg/L even in engineered strains) and requires further research and optimization to become industrially competitive; therefore, clinical superiority over folic acid is not consistently demonstrated across outcomes yet. Consequently, current evidence supports sustained investment to discover high folate-producing strains and enhance microbial productivity, while advising caution against overstating the clinical case for completely replacing standard folic acid fortification.