Plant phenolic compounds as ethidium bromide efflux inhibitors in Mycobacterium smegmatis

Abstract

One-third of the world's population is infected with the dormant tuberculosis bacillus, and there have been no new antimycobacterial compounds with new modes of action for over 30 years. Extensively drug-resistant tuberculosis is resistant to first- and second-line drugs, which can have severe side effects, and requires the breakthrough of new antituberculotics and resistance-modifying agents. Efflux pumps can cause multidrug resistance and have recently evoked much interest as promising new targets in antimicrobial therapy. The study was performed to set up an ethidium bromide (EtBr) efflux assay in Mycobacterium smegmatis mc(2)155 for testing plant natural compounds as mycobacterial efflux pump inhibitors (EPIs). After determining the MICs of the putative EPIs, they were tested for synergistic effects with EtBr prior to the efflux assay. We established an EtBr efflux assay in M. smegmatis mc(2)155. The isoflavone biochanin A exhibited efflux pump inhibiting activity comparable to that of verapamil. The flavone luteolin and the stilbene resveratrol were less active. A new assay was established to observe the EtBr efflux in M. smegmatis and was applied to evaluate plant phenolic compounds. Our results highlighted that the isoflavonoid biochanin A exhibited better EPI activities than other flavonoids in mycobacteria.