Introduction: Bacteria such as Streptococcus mutans metabolize sugar consumed by humans to organic acids. These acids demineralize teeth, causing caries. In dental practice, caries is treated by the removal of infected tissue and filling the cavity with dental composite restorative material (DCRM). Modification of DCRM to achieve microbiocidal materials is widely researched in the literature, due to the neglecting antibacterial activity of commercial materials.
This research aimed to characterize antibacterial activity against Staphylococcus aureus and Escherichia coli of common dental copolymers modified with previously synthesized monomers possessing quaternary ammonium groups (QAn+TMXDI, where n is the number of carbon atoms in the N-alkyl substituent: 8, 10, or 12) responsible for their antibacterial activity.

Methods: Two series of copolymers were prepared: 20 wt. % of QAn+TMXDI, 20 wt. % TEGDMA, 20 wt. % UDMA, and 40 wt. % Bis-GMA and 40 wt.% QAn+TMXDI, 20 wt. % TEGDMA, and 40 wt. % Bis-GMA. The bacterial adhesion was tested with disc-like samples, which were immersed in bacteria solution, washed with sterile water, and then, suspensions of bacteria remaining on the surface were prepared and spread on agar plates. Next, bacteria colonies were counted.

Results: Copolymers modified with 20 wt. % of QAn+TMXDI showed antibacterial activity comparable to a reference sample, except for a copolymer with QA8+TMXDI, which showed higher activity than other copolymers. In the case of copolymers with 40 wt. %, a significant increase in antibacterial activity was observed.

Conclusions: Microbiocidal properties of modified copolymers depended on the weight content of QAn+TMXDI and the length of the N-alkyl chain. The higher the QAn+TMXDI content, the higher the antibacterial activity. The N-alkyl substituent had the opposite effect, i.e. the higher the number of carbon atoms in the chain, the lower the antibacterial activity.