Formate as electron carrier in the gut acetogen Blautia luti

Introduction: The gut microbiome is a highly complex community, which is involved in the digestion of nutrients but also affects the development of diseases as well as the human well-being. Gut acetogens such as Blautia strains are often connected to the human well-being^[1]. However, only little is known about the physiology of different Blautia species. Bacteria of this genus are often classified as acetogenic bacteria, but the use of a functional Wood-Ljungdahl pathway (WLP) has only been proven and characterized for a few strains. To shed light on the physiology of Blautia species, we characterized the WLP of the gut acetogen Blautia luti and its connection to its heterotrophic metabolism.

Goals: To unravel the role of the WLP in the physiology of human gut acetogens.

Materials & methods: Growth experiments, fermentation and metabolite analysis, enzyme assays, genome analysis.

Results: Bacteria of the genus Blautia can be divided into FDH-containing and FDH-lacking strains. While FDH-containing species such as B. schinkii can produce acetate from H₂ + CO₂ via the WLP, FDH-lacking strains such as B. luti cannot. However, analysis of the genome sequence revealed that all genes encoding the WLP with exception of the FDH encoding gene are present in the genome of these strains. Interestingly, these bacteria performed acetogenesis from CO + formate, showing an unusual but functional WLP. In addition, B. luti produced formate during heterotrophic fermentation by pyruvate formate lyase; in the presence of CO or H₂, formate is reduced to acetate by the WLP.

Summary: Many bacteria of the genus Blautia utilize an unusual WLP lacking a FDH. For these gut acetogens, formate plays an essential role as intraspecies electron carrier. Beyond that, formate might also be commonly used as interspecies electron carrier in the human gut.

[1] Liu, X., Mao, B., Gu, J., Cui, S., Wang, G., Zhao, J., Zhang, H., Chen, W. (2021) Gut Microbes 13: e1875796