Quercetin 7-O-glutamate sensitizes Escherichia coli to vancomycin

We have reported previously that conjugation of glutamic acid at the 7-O position of quercetin could significantly enhance its biological activity. In this study, as a part of our ongoing efforts to investigate the therapeutic potential of this novel quercetin–glutamate conjugate (1), we evaluated the effect of its combination with vancomycin against Gram-negative Escherichia coli, which are not normally affected by vancomycin alone. Quercetin conjugate 1 was shown to potentiate the antibacterial activity of vancomycin against E. coli, which could be attributed to its membrane perturbation activity, but not to facilitation of the efflux pump-mediated penetration of vancomycin.