A Magnesium Tug-of-war Impedes Bacterial Antibiotic Resistance

Bacteria spontaneously generate ribosome variants that provide antibiotic resistance. However, such naturally occurring variants may also carry some physiological costs. We used modeling and quantitative experiments to investigate a spontaneously arising antibiotic-resistant L22 ribosomal protein variant (L22*) in Bacillus subtilis and determine its fitness cost. Specifically, elastic network modeling of ribosome conformational dynamics revealed that, compared to wild-type, L22* ribosomes associate more tightly with structural magnesium ions. Concurrently, quantitative single-cell experiments indicate that the L22* ribosome variant dominates a tug-of-war with ATP over a shared pool of magnesium ions, leading to reduced ATP levels. Consequently, growth of the L22* variant exhibits a higher dependence on extracellular magnesium availability, revealing a physiological cost. These data show that the association of magnesium ions with ribosomes regulates ATP activity. We postulate that such ion-based modulation may suppress antibiotic-resistant ribosome variants in bacteria that disequilibrate competition for magnesium ions.