Verapamil increases susceptibility of colistin-resistant Acinetobacter baumannii to colistin

•The combination of verapamil and colistin showed synergistic efficacy against colistin-resistant Acinetobacter baumannii both in vitro and in vivo.•Provide new possible approaches and strategies to combat resistance to colistin.•Enhance the antibacterial activity of colistin and reduce the amount of colistin to reduce its side effects.•A new exploration of the antibacterial effects of verapamil, which has been commonly used in the past as an anti-arrhythmic and anti-hypertensive drug. Acinetobacter baumannii, which is predominantly responsible for hospital-acquired infections, presents a tremendous clinical challenge due to its increasing antibiotic resistance to colistin (COL), a last-line antibiotic. As a result, the combination of antimicrobial and non-antimicrobial agents is emerging as a more popular treatment approach against infections caused by COL-resistant A. baumannii. This study administered COL and verapamil (VER), that is an antihypertensive and antiarrhythmic agent. We found that the susceptibility of A. baumannii to COL was restored both in vitro and in vivo. Scanning electron microscope and Crystal violet staining showed inhibition of the VER/COL combination on bacterial biofilm formation. Cytotoxicity assay and haemolysis test were used to confirm in vitro safety evaluation. Further experiments using propidium iodide staining revealed that the VER/COL combination improved the therapeutic efficacy of COL by modifying the permeability of bacterial membranes. As demonstrated by reactive oxygen species experiments, the drug combination caused the accumulation of bacterial reactive oxygen species and their eventual death. Additionally, VER/COL treatment significantly reduced the efflux of Rhodamine 123 (Rh123). For the first time, this study identifies the anti-hypertensive drug VER as a COL potentiator against A. baumannii, providing a potential treatment approach against A. baumannii infections and improving patient outcomes. [Display omitted]