# Antimicrobials in Dentistry
## 2 Antimicrobials Used in Dentistry
### Eugenyl Methacrylate in Dental Adhesive Systems  
Dental adhesives play an important role in restorative dentistry. Enabling polymerisable monomers with antibacterial properties in dental adhesives is of value since the interface between the restorative and tooth tissue is susceptible to the ingress of microbial species. EgMA is a compatible comonomer with methacrylate monomers present in dental adhesives; thus, post curing immobilises the antibacterial eugenol moieties in the polymer backbone without the inhibitory effect characteristic of the phenol group. The antibacterial property of the dental adhesive is expected to help eliminate residual bacteria, which often remains in dentinal tubules, and lower the risk of reinfection and secondary caries. The efficacy of EgMA as a comonomer in single-component self-etch systems and the adhesive and catalyst components of a total-etch system were reported by [PERSON] et al. [103]. The curing ability of a bonding system is a major factor that governs the formation of a strong and durable bond with the dentinal substrate, and the degree of cure is influenced by any compositional modifications to the system. [PERSON] et al. [106] showed that despite the significantly lower initial degree of cure of the modified adhesives than their corresponding control, they were increased when measured at 24 h post curing, with no significant differences [106]. As with the EgMA-modified resin composites, the initial reduction in the degree of conversion was attributed to the bi-functional nature of acrylic and allylic double bonds in the EgMA moiety that led to the formation of crosslinked structures with unreacted allylic bonds. An additional advantage of the EgMA formulations in adhesives was attributed to the significantly lower polymerisation exotherm, which would decrease the potential of thermal damage to the host tissue. EgMA in bonding formulations significantly increased the contact angle of the polar and dispersive liquids tested and decreased surface free energy indicating the higher hydrophobicity of the bonding surfaces. Although the effect of surface free energy of substrates on bacterial adhesion has been critically discussed in the literature with no clear consensus, our previous findings for polymers and resin composites incorporating EgMA reported a greater hydrophobic character of these materials, presented a lower surface free energy, and ultimately impacted the total interaction energy with the bacteria, as higher accessibility to the eugenyl moieties was responsible for improved bacteriostatic activity [100,104]. The chemical composition of adhesive resins and their net hydrophilicity influence the water sorption, solubility and water diffusion of these polymers. The hydrophobic nature of EgMA substituted methacrylates and its ability to form slightly crosslinked structures of the modified adhesives resulted in a significant reduction in their water sorption and solubility in comparison to bonding systems containing acidic, highly polar functional groups in newer formulations of bonding agents. The reduction in solubility indicates the limited extraction of any unreacted monomers into the surrounding environment, potentially increasing the stability and durability of the resin-dentine bonds [105]. The incorporation of EgMA into self-etch and total etch dental adhesives showed potent antibacterial activity, before and after light polymerisation. The EgMA-modified adhesives produced significantly greater inhibition zones than their corresponding controls, indicating that the eugenyl residue maintained the bactericidal effect of eugenol and the sensitivity of _P. acnes., S. mutans_ and _E. faecalis_ to this monomer within their formulations. Moreover, the inclusion EgMA reduced biofilm viability, evidenced by a decrease in the number of CFUs of the total species, live biofilm volume and the percentage of live bacteria (Figure 5).