CorB and MgpB (YegH) alleviate excess-magnesium stress and contribute to egg white tolerance and virulence in Salmonella

Maintaining proper cation homeostasis is essential for the survival of all living organisms. During infection, Salmonella must adapt to host-imposed magnesium (Mg2+) limitation. To compensate, Salmonella induces high-affinity Mg2+ transporters and synthesis of polyamines, organic cations that partially substitute for Mg2+. Once Mg2+ levels are restored and cells enter stationary phase, excess cations must be exported; failure to do so results in loss of viability. The CorC-domain protein PaeA exports excess polyamines, whereas MgpA and CorC reduce intracellular Mg2+ levels. Here, we identify two new CorC-domain proteins, MgpB (formerly YegH) and CorB, as mediators of Mg2+ homeostasis, showing that they also reduce intracellular Mg2+ accumulation and protect cells from endogenous excess-cation stress. Genetic analyses revealed the functional redundancy among these proteins in conferring resistance to exogenous high Mg2+, endogenous stress induced after Mg2+ starvation, and egg whites. Deletion of mgpB and/or corB conferred little to no phenotype. However, these mutations showed synergistic phenotypes in strains lacking MgpA. This synergistic relationship was also evident in mouse competition assays. Lack of MgpA, CorC, MgpB, and CorB leads to an increase in intracellular Mg2+ levels. Moreover, expression of mgpB and corB are increased in high Mg2+, all consistent with their role as putative Mg2+ exporters. Together, these findings identify MgpB and CorB as new mediators of Mg2+ homeostasis and reveal a multilayered system that enables Salmonella to finely balance intracellular cation levels during environmental and host-imposed stress.