Activity of the efflux pump inhibitor phenylalanine-arginine β-naphthylamide against the AdeFGH pump of Acinetobacter baumannii

AdeFGH is a multidrug resistance efflux pump of Acinetobacter baumannii belonging to the resistance-nodulation-cell division (RND) family. Analysis of expression of this pump established it as the most prevalent RND pump in clinical isolates of A. baumannii from Canadian hospitals. In this study, the activity of the efflux pump inhibitor (EPI) phenylalanine-arginine β-naphthylamide (PAβN) against the RND efflux pump AdeFGH of A. baumannii was analysed. The adeFGH operon was cloned under the control of the tac promoter and was introduced in single copy into a surrogate Pseudomonas aeruginosa strain that is devoid of its six native efflux pumps. Isopropyl-β-d-thiogalactopyranoside (IPTG) was used to induce expression of the pump and the sensitivity of strains expressing the adeFGH operon to PAβN was analysed in the surrogate strain and in clinical isolates of A. baumannii by measuring the minimum inhibitory concentrations of trimethoprim, chloramphenicol and clindamycin (substrates of AdeFGH) in the presence and absence of PAβN. PAβN was found to inhibit the ability of the AdeFGH pump to efflux all three antibiotics tested in the surrogate P. aeruginosa strain as well as in the A. baumannii strains. However, in A. baumannii strains the inhibition was more profound for clindamycin than for trimethoprim and chloramphenicol. In addition, these data also support the importance of a strain devoid of efflux pumps for screening of EPIs, since the inhibition of AdeFGH by PAβN was found to be more effective in the surrogate P. aeruginosa strain than in the clinical isolates of A. baumannii.