MDPB has been extensively investigated as an antibacterial monomer and shown to be promising for dental applications including use in composite, primer and adhesive.7,8
In previous studies, each antibacterial resin contained only one type of antibacterial agent (MDPB). In the present study, dual antibacterial agents MDPB and NAg were combined for the first time in the primer. The results showed that MDPB indeed imparted a strong antibacterial function to a commercial SBMP primer, greatly reducing the microcosm biofilm viability, metabolic activity, CFU and lactic acid production. These results confirmed the previous studies reporting MDPB with strong antibacterial properties.7,8
Furthermore, the present study showed that the antibacterial potency of MDPB-containing primer can be further increased by the incorporation of a second antibacterial agent, NAg. The dual antibacterial agent method was supported by two benefits: (1) The use of dual antibacterial agents in the primer reduced the biofilm activity by more than half, compared to that using MDPB alone; (2) the use of dual antibacterial agents in the primer did not adversely affect the dentin shear bond strength and fibroblast cytotoxicity.
It is important to develop antibacterial primers in combating secondary caries. Caries is a dietary carbohydrate-modified bacterial infectious disease caused by acid production by biofilms.4–6
Therefore, antibacterial bonding agents are promising to inhibit biofilms and secondary caries at the tooth-restoration interface. Primer directly contacts the tooth structure at the interface, and hence can serve as a carrier to deliver antibacterial agents. There are often residual bacteria present in the prepared tooth cavity.18,51
With the increased interest in Minimal Intervention Dentistry and preservation of tooth structure,52
more carious tissues with active bacteria could remain in the prepared tooth cavity. For patients of certain ethnicity and poverty levels with a high incidence of untreated caries, the atraumatic restorative treatment (ART) method could be especially useful as it can be readily performed without requiring electricity and anesthesia.53,54
However, ART may not completely remove the carious tissues.53,54
Therefore, applying a strongly-antibacterial primer could be especially beneficial for these applications, as the un-cured primer with MDPB + NAg () could kill the bacteria remaining in the prepared tooth cavity.
In addition, once cured, the bonded interface could combat the future invading bacteria in vivo. While a complete sealing of the tooth-restoration interface is an important goal, it is often difficult to achieve. Polymerization shrinkage combined with chewing and wear stresses could create microcracks and form microgaps at the margins. Previous studies indeed showed the existence of microgaps at the tooth-restoration interfaces,55,56
which could harbor the invading bacteria to secrete acids and cause secondary caries. Therefore, it is important for the bonding agent in the cured state at the tooth-restoration interface to remain antibacterial. In the present study, when the primer contained MDPB and NAg, the cured primer/adhesive samples effectively inhibited the dental plaque microcosm biofilm growth. Therefore, the primer containing MDPB and NAg is promising to kill not only the residual bacteria in the prepared tooth cavity, but also the invading bacteria along the margins during service. While antibacterial primers have great potential to help inhibit biofilms and caries, to date, there have been only a few reports on them.8,14,33
The present study represents the first report in which MDPB and NAg were combined in the primer. The results indicate that the MDPB-NAg primer would contribute significantly to inhibiting biofilm growth, acid production, and secondary caries.
The use of dual antibacterial agents, and specifically, the combination of quaternary ammonium monomer with NAg, may be a promising approach for developing strong antibacterial biomaterials. Ag has been shown to possess antibacterial, antifungal, and antiviral functions,36,37
and has been known to be an effective antibacterial agent against a wide range of micro-organisms.57–59
Regarding its antimicrobial mechanism, it was suggested that the Ag ions could inactivate the vital enzymes of bacteria to cause the bacterial DNA to lose its replication ability, leading to cell death.57,59
Ag has several properties worth noting: Low toxicity and good biocompatibility with human cells;58
long-term antibacterial effect due to sustained silver ion release;60
and less bacterial resistance than antibiotics.61
An additional advantage for NAg is the small particle size and the associated high specific surface area, which yielded a strong antibacterial potency at a low NAg filler level in the resin. A low NAg filler level was advantageous because it did not adversely affect the physical properties of the primer. Furthermore, while the addition of NAg and MDPB to primer imparted a strong antibacterial activity, it is important that such addition does not compromise the biocompatibility of the bonding agent. This was indeed verified via the human gingival fibroblast cytotoxicity test, which showed not only that adding MDPB and NAg had no adverse effect on cytotoxicity compared to the commercial primer, but also that all the solutions with resin eluents had fibroblast viability similar to that using fibroblast medium without any resin eluents. Therefore, it is possible to use NAg in the MDPB resins to obtain additional strong antibacterial functions without compromising physical properties such as dentin bond strength and cytotoxicity.
MDPB has been shown to possess potent antibacterial activity against various oral bacteria including facultative and obligate anaerobe in coronal lesions, as well as bacterial species isolated from root caries such as actinomyces and Candida albicans
For application in composites, MDPB copolymerized with other monomers of the composite, yielding a strong antimicrobial effect against bacteria.7
For use in primer, MDPB imparted a potent antibacterial effect to a dentin primer, without compromising the dentin bond strength.62
In another study, the MDPB primer was applied to cavities in dog teeth infected with S. mutans
and exhibited in vivo antibacterial effects.35
The MDPB-containing bonding agent after curing also demonstrated a strong antibacterial effect against the growth of S. mutans
, without influencing the bond strength or curing characteristics.51
In addition, a composite restoration containing MDPB was shown to inhibit the progression of artificial secondary root caries lesions using extracted human teeth with Class V cavities.63
For all these various applications and restorations using MDPB, the antibacterial efficacy could be substantially increased via the addition of a small amount of NAg. Regarding antibacterial mechanisms, MDPB is covalently bonded with the polymer structure and immobilized in the resin, exerting contact-inhibition against adherent bacteria.7,8,16
Ag has long-term antibacterial effect due to the sustained silver ion release,60
which could not only kill bacteria on the surface, but also bacteria away from the surface. Therefore, the effects of releasing (NAg) and non-releasing (MDPB) antimicrobials could be synergistic and complimentary to each other, to enhance the resin’s antibacterial efficacy. The present study showed that incorporating 0.05% of NAg into the MDPB primer doubled the inhibition zone size, and cut the biofilm lactic acid and CFU by half, compared to those using MDPB alone. In addition, several other studies developed antibacterial dental materials using novel compositions.9,13,18
It is likely that the incorporation of NAg into these new antibacterial formulations will also greatly enhance the antibacterial potency, without compromising physical and mechanical properties. However, it should be noted that clinical conditions in vivo could significantly influence the antibacterial efficacy of the restoration. For example, fluctuating shear could occur due to saliva flow, drinking beverages, and tooth brushing. Fluctuating shear that occurs supragingivally could impact the efficacy of various antibacterial agents that might be released from materials placed in the oral cavity. Due to the hydrodynamic conditions in vivo, the antibacterial agents released from the material may be flushed away and the antibacterial effect may thus be limited. Another factor is that the fluids in vivo may dilute and decrease the local concentration of silver ions, and thereby adversely impact the antibacterial efficacy of silver-based materials in the oral cavity. Therefore, further studies are needed to investigate the antibacterial and anti-caries efficacy of the MDPB-NAg primer under in vivo conditions. Further studies should also investigate the promise of using the MDPB-NAg combination in various adhesive systems, composites, and glass ionomer cements to achieve strong antibacterial and caries-inhibiting capabilities.