Indole phytochemical camalexin as a promising scaffold for AcrB efflux pump inhibitors against Escherichia coli

Escherichia coli is amongst the most frequent causative agent of nosocomial infections and the overexpression of the efflux pump gene acrB plays a major role in its resistance to various antibiotics. In this study, we evaluated two indole phytochemicals, camalexin and brassinin, as potential AcrB efflux pump inhibitors. Among these two phytochemicals, camalexin increased the accumulation of ethidium in acrB proficient E.coli with no membrane permeabilization effect observed, indicating a direct interaction of camalexin with the pump. Camalexin also showed up to 64-fold MIC reduction for drugs in the acrB proficient strain. Brassinin was less effective, showing up to 4-fold MIC reduction for the same drugs. Camalexin did not potentiate drugs in the AcrB inactive strain D407N. Plate dilution assays in E. coli acrB variants further corroborated the effect of camalexin in diminishing pump activity. Blind docking results suggested that camalexin and brassinin may enter mainly via CH3, one of the channels present in AcrB, and camalexin showed a more stable binding mode than brassinin in the distal binding pocket of AcrB. Camalexin, therefore, holds potential as a scaffold for further development as a potent AcrB inhibitor to tackle antimicrobial resistance in the gram-negative bacterium E. coli.