Effect of RND-efflux pumps inhibitor on the synergy of different antibiotics combinations against carbapenem-resistant Pseudomonas aeruginosa

Introduction: The high-level antimicrobial resistance, particularly carbapenem resistance, in Pseudomonas aeruginosa is a global health challenge. The combination of antibiotics and synergy effects is beneficial in control of drug-resistant P. aeruginosa. The synergic interaction of antimicrobial agents is af-fected by the mechanisms of antimicrobial resistance. The aim of the current study was to evaluate the effect of efflux pump inhibition on the synergy of antibiotics against carbapenem-resistant P. aeruginosa. Methods: The antibiotics’ minimum inhibitory concentration (MIC) was determined by the microbroth dilu-tion method. The synergy effect of antibiotics was determined using the checkerboard assay with-out and with Resistance-Nodulation- Division (RND) efflux pump inhibitor phenylalanine-arginine beta-naphthylamide (PAβN). Results: The highest levels of synergistic effects were found between cefepime/tobramycin and meropenem/tobramycin combinations in 35.3% of isolates. After adding PAβN, the most frequent synergistic effects were observed between the meropenem/ciprofloxacin and cefepime/ciprofloxacin combinations, found in 64.7% of isolates. The adding PAβN led to an increase in the synergy of all combinations except tobramycin/colistin. The highest effect of PAβN on the synergy effects of antibiotics combination was observed in meropenem/ciprofloxacin, cefepime/ciprofloxacin, and ciprofloxacin/colistin (an increase of 41.2%).  Conclusion: RND efflux pump inhibition has a noticeable effect on the results of synergy tests of some antimi-crobial agent combinations. Given the drug- and strain-dependent effects of PAβN on synergy re-sults, the effects of efflux pump inhibitors should be studied on different combinations of drugs and a large population of bacterial strains.