Researchers at Goethe University Frankfurt have discovered a surprising role for formic acid in the human gut: The small molecule acts as a kind of “taxi” for electrons – both within bacteria and, likely, also between different microorganisms. The gut bacterium Blautia luti produces formic acid as part of a metabolic trick that allows it to respond flexibly to what is available in the gut. In addition to carbohydrates, the bacterium can also metabolize toxic carbon monoxide derived from the body’s own hemoglobin degradation.