Determination of efflux activity in Enterococci by Hoechst accumulation assay and the role of zinc oxide nanoparticles in inhibition of this activity.

Background: Contribution of efflux pumps in development of antimicrobial resistance has been largely addressed in Gram negative and to a much lesser extent in Gram positive bacteria. Measuring accumulation of Hoechst (H) dye is known as a safe and rapid method for monitoring efflux activity in bacteria. Antimicrobial effects of metal nanoparticles have been attributed in part to inhibition of efflux pumps. This study aimed to first determine efflux activity in enterococci by Hoechst accumulation assay, and to second characterize the role of zinc oxide nanoparticles (ZnONPs) in inhibition of these pumps.

Results: Increased accumulation of Hoechst dye showed more potential of ZnONPs in efflux inhibition compared with CCCP. H33258 represented more suitability for accumulation studies in enterococci. Two to six-fold reduction in minimum inhibitory concentration (MIC) values of antimicrobial agents in the presence of ZnONPs was observed.

Conclusions: Efflux activity in enterococcal strains can be measured by H33258 accumulation assay. Application of ZnONPs as an efflux inhibitor, may rejuvenate the use of conventional antimicrobial agents against these bacteria.