Intracellular formate is a critical mediator of the antimicrobial fate of Salmonella Typhimurium Against meropenem and ciprofloxacin

Salmonella enterica serovar Typhimurium (STM) employs various strategies to endure antibiotic stress, but the contribution of central metabolites to resistance remains insufficiently characterized. In this work, we identify intracellular formate, produced by pyruvate-formate lyase (PflB), as a key determinant of STM susceptibility to meropenem and ciprofloxacin. Deletion of pflB disrupts cellular pH homeostasis, impairing efflux pump function, increasing reactive oxygen species, and causing membrane depolarization, collectively heightening sensitivity to antibiotics. Supplementing extracellular formate reverses these phenotypes. Mechanistically, pH imbalance activates the RpoE-csrB regulatory pathway, leading to reduced expression of efflux pump genes acrB and tolC, while extracellular formate engages the BarA/SirA two-component system to further influence the CsrA/csrB network. Disrupting the formate pool interferes with efflux-dependent resistance, revealing a previously unrecognized signalling function for intracellular metabolites. Overall, our results position formate as a metabolic driver of antibiotic outcomes in STM and highlight it as a potential target for mitigating resistance.