Synergistic mechanism of Ag + –Zn 2+ in anti-bacterial activity against Enterococcus faecalis and its application against dentin infection

Background Ag + and Zn 2+ have already been used in combinations to obtain both enhanced antibacterial effect and low cytotoxicity. Despite this, it is still unclear how the Zn 2+ co-works with Ag + in the synergistic antibacterial activity. The main purposes of this study were to investigate the co-work pattern and optimum ratio between Ag + and Zn 2+ in their synergistic antibacterial activity against E. faecalis , the possible mechanisms behind this synergy and the primary application of optimum Ag + –Zn 2+ co-work pattern against the E. faecalis biofilm on dentin. A serial of Ag + –Zn 2+ atomic combination ratios were tested on both planktonic and biofilm-resident E. faecalis on dentin, their antibacterial efficiency was calculated and optimum ratio determined. And the cytotoxicity of various Ag + –Zn 2+ atomic ratios was tested on MC3T3-E1 Cells. The role of Zn 2+ in Ag + –Zn 2+ co-work was evaluated using a Zn 2+ pretreatment study and membrane potential—permeability measurement. Results The results showed that the synergistically promoted antibacterial effect of Ag + –Zn 2+ combinations was Zn 2+ amount-dependent with the 1:9 and 1:12 Ag + –Zn 2+ atomic ratios showing the most powerful ability against both planktonic and biofilm-resident E. faecalis. This co-work could likely be attributed to the depolarization of E. faecalis cell membrane by the addition of Zn 2+ . The cytotoxicity of the Ag + –Zn 2+ atomic ratios of 1:9 and 1:12 was much lower than 2% chlorhexidine. Conclusions The Ag + –Zn 2+ atomic ratios of 1:9 and 1:12 demonstrated similar strong ability against E. faecalis biofilm on dentin but much lower cytotoxicity than 2% chlorhexidine. New medications containing optimum Ag + –Zn 2+ atomic ratios higher than 1:6, such as 1:9 or 1:12, could be developed against E. faecalis infection in root canals of teeth or any other parts of human body.