Binding site description of 2-substituted benzothiazoles as potential RND efflux pump inhibitors

The resistance-nodulation-cell division family (RND) efflux pumps exemplify a unique phenomenon with drug resistance in different gram negative bacterial strains as a single mechanism causing resistance against several different classes of antibiotics. In Escherichia coli AG102 and Acinetobacter baumannii SBMox2 strains the well characterized RND efflux pumps are AcrAB-TolC and the AdeABC respectively. Most of the antibiotics were found to be substrates for these pumps by increasing the expression of the efflux pump genes, leading to multidrug resistance (MDR) and the treatment failure and death caused by these gram negative bacterial infections or underlying diseases are common (Sun et al., 2014). Consequently, the need of searching new therapeutic solutions that suppress the activity of efflux pumps and restore the sensitivity of commonly used antibiotic is essential. RND efflux pumps, which are only found in Gram-negative bacteria, have a tripartite composition. RND type efflux pumps contain an inner membrane transporter protein (RND pump), an outer membrane protein (OMP) channel, and a periplasmic membrane fusion protein (MFP). They are allowed direct extrusion of various antibiotics from the cytosol or periplasmic space to the outside of the bacterial cell, and have been found to be associated extensively with clinically significant antibiotic resistance (Sun et al., 2014). Recent studies reported that RND type efflux pumps, which are named AcrAB-TolC in E. coli and AdeABC in A. baumannii, comprise a transporter protein (RND pump) AcrB in E. coli or AdeB in A. baumannii acting as a proton/drug antiporter, an outer membrane channel protein TolC in E. coli or AdeC in A. baumannii, and a periplasmic membrane fusion protein AcrA, which serves as a linker between TolC and AcrB in E. coli or AdeB, which serves as a linker between AdelC and AdeB in A. baumannii (Sun et al., 2014). The emergence of MDR strains of Gram-negative bacteria pathogens is a problem of ever increasing significance (Sun et al. 2014). Interestingly, these RND efflux pumps decrease the antibacterial activity of dissimilar antibiotic structures, which can be considered a MDR mechanism. Because of bacteria become insensitive to different classes of antibiotic therapy, new therapeutic approaches must be looked for, searching for new molecules to block efflux, to restore drug susceptibility to resistant clinical strains. The goal of this study is (i) to define the potential RND efflux pump inhibitor (EPI) activity of our previously synthesized BSN coded 2-substituted benzothiazoles by observing the reversal antibacterial activity of antibiotics particularly to chloramphenicol (CHL) and/or ciprofloxacin (CIP) in the AdeABC efflux pump overexpressor Acinetobacter baumannii SbMox2 and/or AcrAB-TolC efflux pump overexpressor E. coli AG102 clinical isolates, and (ii) to examine the structure activity relationships by describing the binding site features of these tested compounds and to analyze the active site protein-ligand interactions of RND efflux pump AdeABC in A. baumannii by generating pharmacophore hypothesis.