Colistin, a last-resort antibiotic, faces limitations due to resistance. This study reveals that valnemulin (Val), an FDA-approved drug, synergistically enhances colistin's activity against colistin-resistant and -susceptible gram-negative pathogens *in vitro* and *in vivo*. The combination disrupts bacterial membranes, dissipates proton motive force, reduces ATP levels, and suppresses motility, leading to bacterial death. These findings suggest Val's potential as a colistin adjuvant to combat multidrug-resistant bacterial infections.
