Synergistic Actions of Benzyl Isothiocyanate with Ethylenediaminetetraacetic Acid and Efflux Pump Inhibitor Phenylalanine-Arginine β-Naphthylamide Against Multidrug-Resistant Escherichia coli

The aim of this study was to assess the efficacy of benzyl isothiocyanate (BITC) in combination with efflux inhibitors and metal chelators against multidrug-resistant Escherichia coli. In vitro synergism between testing molecules was observed based on the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC), fractional inhibitory concentration index (FICI), bactericidal kinetics, and growth inhibition assay. BITC alone exhibited moderate antibacterial activity against E. coli strains with MIC and MBC values of 0.625–1.25 μM and 1.25–2.5 μM, respectively. In contrast, double and triple combinations of BITC, ethylenediaminetetraacetic acid (EDTA), and phenylalanine-arginine β-naphthylamide (PAβN) resulted in synergistic activities with FICI values between 0.18 and 0.5, whereas combination of BITC with carbonyl cyanide m-chlorophenyl hydrazone or 2, 2′-dipyridyl revealed additive or indifference effect with FICI values of 0.75–1.5 and 1–1.5, respectively. Results of bactericidal kinetics and growth inhibition assays also supported the synergistic effects of EDTA and PAβN with BITC against E. coli strains. Our data demonstrate the possible use of adjuvant agents, such as the chelating agent EDTA and the efflux inhibitor PAβN to improve the antibacterial potential of isothiocyanate and may help to develop an alternative strategy for reducing the occurrence of multidrug resistance.