Phosphate and osmotic adaptation: a major role for phosphate in charge balance and metabolic responses in Escherichia coli
Adaptation of Escherichia coli to osmotic upshift requires rapid accumulation of intracellular solutes to restore turgor and maintain cellular homeostasis. While compatible solutes are well-established contributors to this process, they do not fully account for the early events following osmotic stress. Here, we demonstrate that inorganic phosphate and phosphorylated metabolites play a major and previously underappreciated role in osmoadaptation. Following osmotic upshift under conditions where accumulation of compatible solutes is restricted, E. coli exhibits a substantial increase in intracellular phosphate after a short lag. This increase accounts for a significant fraction of the charge balance required during rapid uptake of K+ and NH4+, the latter supporting glutamate synthesis as a principal counterion. Concomitantly, nucleotide pools display complex, multiphasic dynamics, including a transient decrease in adenylate energy charge whose duration correlates with stress magnitude. In addition, levels of pyrophosphate and key glycolytic intermediates, including dihydroxyacetone phosphate and 1,3-bisphosphoglycerate, increase markedly, indicating redistribution of phosphate into central metabolic pathways. These findings support a model in which phosphate uptake and metabolic redistribution contribute both to intracellular charge balance and to dynamic metabolic reorganisation during osmotic stress. By linking ion transport with central metabolism, this work expands current models of bacterial osmoadaptation and identifies phosphate flux as a key component of the early stress response. IMPORTANCEBacterial survival in fluctuating environments depends on rapid adaptation to osmotic stress. While compatible solutes are central to this process, their contribution does not fully account for early events in Escherichia coli following osmotic upshift. This work demonstrates that inorganic phosphate uptake and redistribution into nucleotide and glycolytic pools contribute substantially to balance the large positive charge entering the cell as it takes up K+ and NH4+ during osmotic upshift. These findings expand current models of bacterial osmoregulation by identifying phosphate flux as a central integrator of ion homeostasis and metabolic adaptation.