Novel Cu(II) Schiff Base Complex Combination with Polymyxin B/Phenylalanine-Arginine β-Naphthylamide Against Various Bacterial Strains

Concerns on increasing trends of multidrug resistance (MDR) around the world have triggered the need to investigate and develop new therapeutic strategies and potent antibacterial drugs. Polymyxins, a class of polycationic antimicrobial peptides, have been regarded as the last-line therapy against Gram-negative bacteria due to limited new antibiotics and phenylalanine-arginine β-naphthylamide (PAβN), a peptidomimetic compound has been characterised as an efflux pump inhibitor (EPI) that have been investigated to overcome efflux-mediated multidrug resistance. In this work, the antibacterial activity of two Schiff base ligands derived from the condensation of S-benzyl dithiocarbazate with 4-carboxybenzaldehye (SB4CB) and 4-formyl-3-hydroxybenzoic acid (SBFH) their copper(II) complexes (Cu(SB4CB) 2 and Cu(SBFH) 2 ) were tested individually, and the most promising compound was tested in combination with polymyxin B (POLY) and PAβN against different bacteria, such as antibiotic-susceptible strains: Acinetobacter baumannii ATCC 19606, Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, and Staphylococcus aureus ATCC 35923; multidrug-resistant strains: A. baumannii ATCC BAA-1797, E. coli BAA-196, P. aeruginosa ATCC BAA-2108 and S. aureus ATCC 43300. Initial minimum inhibition concentration (MIC) results showed the Cu(II) complexes Cu(SB4CB) 2 and Cu(SBFH) 2 , demonstrated obvious antibacterial activity as compared to the ligand alone. Fractional inhibitory concentration (FIC) index showed improved MIC values with additivity and synergistic effect for Cu(SBFH) 2 in combination with POLY and PAβN. From the in silico molecular docking investigation, Cu(SBFH) 2 was shown to engage in hydrophobic interactions via its phenyl rings with surrounding hydrophobic residues in the binding pocket of S. aureus NorA, E. coli AcrB, P. aeruginosa MexB and A. baumannii AdeB efflux pumps. The phenyl rings of the ligand could also form π-π stacking with adjacent residues in the binding site of A. baumannii AdeB. Besides, hydrogen bonding and π-cation interactions were also observed via the carboxyl group, hydroxyl group and phenyl ring of SBFH moiety, respectively with nearby residues in the E. coli AcrB binding pocket. This study indicates that the combination strategy of Cu(SBFH) 2 with POLY and PAβN enhances therapeutic potential and sheds light on the binding pockets inside bacteria efflux pumps and the binding interactions of ligand in the binding site.