The Na+ transport in gram-positive bacteria defect in the Mrp antiporter complex measured with 23Na nuclear magnetic resonance

23Na nuclear magnetic resonance (NMR) has previously been used to monitor Na+ translocation across membranes in gram-negative bacteria and in various other organelles and liposomes using a membrane-impermeable shift reagent to resolve the signals resulting from internal and external Na+. In this work, the 23Na NMR method was adapted for measurements of internal Na+ concentration in the gram-positive bacterium Bacillus subtilis, with the aim of assessing the Na+ translocation activity of the Mrp (multiple resistance and pH) antiporter complex, a member of the cation proton antiporter-3 (CPA-3) family. The sodium-sensitive growth phenotype observed in a B. subtilis strain with the gene encoding MrpA deleted could indeed be correlated to the inability of this strain to maintain a lower internal Na+ concentration than an external one.