Magnesium induces iron starvation and metabolic rewiring to support the viability of cell envelope mutants and antibiotic-stressed cells

Abstract

Magnesium supplementation permits deletion of otherwise essential genes involved in cell envelope biogenesis in the Gram-positive model bacterium Bacillus subtilis. Yet, the specific underlying mechanism has remained elusive. To address this key knowledge gap, we made use of a mutant lacking ezrA and gpsB. Deletion of both of these genes involved in cell wall synthesis leads to severe growth inhibition which is ameliorated by magnesium addition. Our results indicate that, in the absence of magnesium, this mutant contains elevated levels of labile iron, is impaired in activating the oxidative stress response, and displays extreme sensitivity to iron and manganese intoxication. Intriguingly, we find that an ezrA single deletion, but not gpsB, exhibits heightened susceptibility to excess iron and manganese. This observation allowed us to investigate the source of toxicity and how EzrA may support metal homeostasis. Our data suggests that the major contributor of ROS is the electron transport system involved in cellular respiration. Both genetic and chemical means to reprogram the cells in favor of fermentation alleviate the metal toxicity in cells lacking ezrA. Collectively, our data shows that magnesium limits iron availability and redirects metabolism towards pathways that are preferred during iron scarcity. Consequently, these mechanisms result in reduced ROS production and oxidative stress mitigation. This explains why magnesium supplementation may render essential genes dispensable. In support of this model, we find that addition of magnesium helps cells to circumvent lysis typically caused by the treatment of an antibiotic that disrupts cell wall synthesis. Taken together, our results suggest that unmitigated oxidative stress fueled by labile iron is likely responsible for the detrimental effects of specific gene disruptions and certain antibiotic treatments. By reducing the pool of free iron and reprogramming cellular metabolism, magnesium mitigates oxidative damage and protects cells from ROS-mediated death.