Bacterial NDH-1 is a key gut target of metformin mediating its antidiabetic effects

Metformin is the first-line therapy for type 2 diabetes, yet the mechanisms underlying its glucose-lowering effects remain incompletely understood. Increasing evidence implicates gut microbiota as an important mediator of metformin action, but the bacterial targets of the drug remain poorly defined. Here, we identify bacterial NADH oxidoreductase (NDH-1), the homolog of mitochondrial complex I, as a key target of metformin. Metformin inhibited bacterial growth at millimolar concentrations and, at subinhibitory concentrations, markedly impaired carbohydrate fermentation in Escherichia coli and other NDH-1-containing bacteria. Supplementation with nicotinic acid and Mg2+; restored fermentation, whereas pyruvate bypassed the metabolic defect, implicating impaired NAD+; regeneration and disruption of metabolism upstream of pyruvate formation. Genetic analysis further established NDH-1 dependence: metformin failed to inhibit fermentation in an E. coli delta-nuoF mutant lacking functional NDH-1 but retained activity in an NDH-2-deficient delta-ndh strain. Molecular docking predicted metformin binding within the NDH-1 quinone-binding region, proximal to conserved residues involved in quinone reduction. Consistent with this mechanism, Staphylococcus aureus, which lacks NDH-1, was not affected in carbohydrate fermentation, while several gut-associated probiotic and short-chain fatty acid-producing bacteria, including Lactobacillus, Bifidobacterium, and Akkermansia muciniphila, lacked detectable NDH-1 homologs, suggesting intrinsic resistance to this metabolic effect of metformin. These findings identify bacterial NDH-1 as a metabolic target of metformin and provide a mechanistic framework for its effects on the gut microbiota. Selective inhibition of NDH-1-dependent bacteria could alter microbial substrate utilization and ecological interactions in the intestine, potentially contributing to the microbiome-mediated component of metformin's antidiabetic effects.