BtuJ1, a Surface-exposed B12-binding Protein in Bacteroidota, Functions as an Extracellular Vitamin Reservoir That Enhances Fitness

The acquisition of vitamin B₁₂ and related cobamides is a key determinant for the fitness of Bacteroidota in the gut. Depending on the species, this uptake process relies on one to four transport systems centred on conserved core outer membrane (OM) complexes composed of the TonB-dependent transporter BtuB and the surface-exposed lipoprotein BtuG. Additionally, the surface-exposed lipoprotein BtuH, although not tightly associated with the BtuBG complex, contributes to cobamide uptake