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

The emergence of multidrug-resistant Salmonella enterica serovar Typhimurium (STM) poses a critical threat to global health. While classical antimicrobial resistance (AMR) mechanisms are well-characterized, alternative metabolites and metabolic pathways also contribute to resistance. This study highlights the role of intracellular formate, maintained by pyruvate-formate lyase (PflB), in modulating susceptibility of STM to meropenem and ciprofloxacin. Loss of pflB disrupts intracellular pH (ipH), impairing efflux pump activity, increasing reactive oxygen species (ROS), and causing membrane depolarization. These effects enhance antibiotic susceptibility but are reversed by extracellular formate supplementation. Mechanistically, disrupted ipH activates the RpoE– csrB axis, leading to downregulation of efflux pump genes acrB and tolC . Additionally, the BarA/SirA two-component system can also modulate the pathway. Our findings uncover a non-canonical role of formate in regulating efflux-mediated AMR, emphasizing its potential as a metabolic target in combating resistant STM strains.