Pyruvate-formate lyase-derived formate regulates the response of Salmonella Typhimurium to 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 and is associated with impaired efflux pump function, increased reactive oxygen species, and membrane depolarization, collectively correlating with heightened antibiotic sensitivity. 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. Overall, our results position formate as a metabolic driver of antibiotic outcomes in STM and highlight it as a potential target for mitigating resistance.