Eugenol works synergistically with colistin against colistin-resistant Pseudomonas aeruginosa and Klebsiella pneumoniae isolates by enhancing membrane permeability.

Colistin is a potent antibiotic for the treatment of carbapenem-resistant Gram-negative bacteria and is considered a last-resort drug. Unfortunately, the incidence of colistin-resistant bacteria isolated from patients is continuously growing due to clinical reuse of colistin. In this study, we found that the combination of colistin and eugenol has a significant synergistic antibacterial effect and reverses the sensitivity of colistin-resistant Pseudomonas aeruginosa and Klebsiella pneumoniae against colistin, as confirmed by checkerboard and time-kill assays. Crystal violet staining and scanning electron microscopy revealed colistin and eugenol's synergistic antibiofilm action. Concerning the synergy mechanism, the results revealed that the combination of eugenol and colistin increases membrane permeability and causes considerable membrane damage, further inhibiting bacteria synergistically. Meanwhile, up to 500 µg/mL of eugenol is non-toxic to RAW 264.7 cells, and the colistin/eugenol combination is also efficacious in vivo, as demonstrated by the Galleria mellonella infection model. Our findings indicate that the colistin/eugenol combination is a viable treatment option for colistin-resistant P. aeruginosa and K. pneumoniae clinical infections. IMPORTANCE Colistin is used as a last resort for severe infections caused by multidrug-resistant Gram-negative bacteria, however, colistin resistance is increasing. As a result, we investigated the synergistic effect of eugenol/colistin combination, and the results revealed significant antibacterial and antibiofilm action. Eugenol may help clinical colistin-resistant Pseudomonas aeruginosa and Klebsiella pneumoniae recover their susceptibility. These findings suggest that combining eugenol and colistin may be a viable treatment option for colistin-resistant pathogen clinical infections.