Inhibition of Oral Streptococci Biofilm Growth on Hydroxyazobenzene-Coated Restorations

Objectives: The selective targeting of pathogens in the oral cavity without inducing dysbiosis is an ongoing challenge in the development of new antimicrobial dental technologies. This, in addition to a rise in antibiotic resistance, has driven interest in materials-based antibacterial strategies in dental care. Our work has developed tethered polymerized azobenzene surfaces that can selectively inhibit the oral pathogens Streptococcus mutans (dental caries) and Streptococcus oralis (endocarditis) without the use of a small molecule antibiotic.
Methods: Phenolic acrylic azobenzene monomers (OH-AAZOs) were synthesized and polymerized as a coating/bulk component of acrylic resin substrates. Curing rates were monitored via near Fourier-transformed infrared spectroscopy. Coating robustness was characterized via UV-Vis spectroscopy, and their biocompatibility was confirmed via cytotoxicity assay (ISO9993) using L929 mouse fibroblast cells.
Streptococci biofilms were grown in vitro on the surface of azobenzene-coated acrylic resins, removed via sonication, and plated to determine the number of colony forming units (CFUs). Biofilm growth and disruption was imaged via a Zeiss digital microscope.
Results: OH-AAZO coatings at surface concentrations of >5 µg/mm² completely inhibited Streptococci biofilm formation under sucrose dependent conditions (0 CFUs, n>3) relative to biofilms grown on uncoated substrates (10⁸ CFUs, n>3). OH-AAZO was also successful at killing bacteria in the surrounding media. To the best of our knowledge, the inhibitory effect of the coating is unique to Streptococci biofilms, and it requires both an azobenzene and a phenol group to inhibit Streptococci growth. The cytocompatibility of the acrylic resin was not impacted by the azopolymer coatings.
Conclusions: Preliminary results indicate that OH-AAZO surfaces can completely inhibit Streptococci biofilm formation on coated resins and in the surrounding media via a contact killing mechanism. Future work will involve elucidating the biofilm inhibition mechanism by OH-AAZO via specific bioassays in order to aid in the design of new azobenzene monomers with optimal antibacterial properties.