Simvastatin decreases the silver resistance of E. faecalis through compromising the entrapping function of extracellular polymeric substances against silver.

Enterococcus faecalis (E. faecalis) is a Gram-positive bacterium closely related to many refractory infections of human and shows the resistant ability against the antibacterial effects of silver. Simvastatin is a semisynthetic compound derived from lovastatin and a hydroxymethyl glutaryl coenzyme A(HMG-COA) reductase inhibitor showing certain inhibitive effects on bacteria. The main purpose of this study was to establish and characterize the Ag+/silver nanoparticles (AgNPs)-resistant E. faecalis, and further evaluate the function of extracellular polymeric substances (EPS) in the silver resistance and the effect of simvastatin on the silver-resistance of E. faecalis. The results showed that the established silver-resistant E. faecalis had strong resistance against both Ag+ and AgNPs and simvastatin could decrease the silver-resistance of both original and Ag+/AgNPs-resistant E. faecalis. The Transmission electron microscopy (TEM), High-angle annular dark-field (HAADF) and mapping images showed that the silver ions or particles aggregated and confined in the EPS on surface areas of the cell membrane when the silver-resistant E. faecalis were incubated with Ag+ or AgNPs. When the simvastatin was added, the silver element was not confined in the EPS and entered the bacteria. These findings may indicate that the silver resistance of E. faecalis was derived from the entrapping function of EPS, but simvastatin could compromise the function of EPS to decrease the silver resistant ability of E. faecalis.