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.