A pH-Sensitive Antimicrobial Agent

Objectives: In oral environments, pH may be reduced when biofilm forms. A smart antimicrobial agent that increases antibacterial efficacy in acidic condition will generate the minimal interference to the oral microbes, and prevent damage to tooth and restorations. The objectives of this research are to synthesize and characterize a quaternary pyridinium salt (QPS) that has the needed pH-sensitive antimicrobial capability, and to understand the mechanism behind the auto-activation of its antimicrobial activity.
Methods: The QPS compound was synthesized in our laboratory. It was further purified using column chromatography, and characterized by Nuclear Magnetic Resonance (NMR) spectroscopy and UV-vis spectroscopy. Cyclic Voltammetry (CV) was applied to monitor the electrochemical properties. The antibacterial activity was analyzed by THB agar plating and growth curve plotting at three pH values using broth dilution method (all assays were conducted in triplicate in the same assay plate, and repeated 3 times from bacteria prepared individually on different days). The switching mechanism of antibacterial activity via pH was probed by monitoring the compound at varying pH via ¹H NMR spectroscopy, UV-vis spectroscopy together with CV measurements.
Results: Bacteria tests indicated a 10-fold enhancement in minimum inhibitory concentration (MIC) of QPS against S. mutans when environmental pH was changed from 6.8 to 4.6. Specifically, the MIC was approx. 10 ug/mL and 1 ug/mL, respectively. The growth curve experiment confirmed this trend. This QPS adopts a cation-radical transition from acidic to neutral or basic environment as indicated by UV-vis and ¹H NMR.
Conclusions: We have introduced a novel antibacterial agent which is “activated” when the environment becomes acidic, as occurs during accumulation of biofilms that may cause tooth decay and secondary caries. This QPS compound provides us new opportunities in designing and developing resin composites and adhesives that may prevent secondary caries without potential drug resistance.