In the recent era of minimal invasive dentistry (MID), the choice of restorative materials is based on their bioactive functions that provide therapeutic effects [1]. With the present concept of minimal invasion, it is expected that more of the saved, affected hard tissue will possibly harbour more residual bacteria [2]. Thus the ability to eliminate these bacteria would be advantageous to prevent microleakage and secondary caries. Hence, one of the bioactive functions proposed for restorative materials is their anti-bacterial activity that may play a major role in restorative treatment of a caries tooth.
The longevity and success of a restoration depends upon a good marginal seal. Poor marginal adaptation is the most common reason for failure of adhesive resin based restorations [3, 4]. Advanced analytical techniques to examine the adhesive resin-dentin interfacial region have revealed a number of potentially deleterious phenomena that could interfere with successful dentin bonding [5–7]. Lack of marginal adaptation that eventually leads to microleakage is caused by polymerisation shrinkage of the composite resin that results in gap formation and bacterial invasion into the interface leading to post- operative pain, marginal discolouration and secondary caries. Bioactivity toward the pulp-dentin complex and prevention of secondary caries were rated as the key to success and future of restorative dentistry and restorative materials over the next 20 years based on the Delphy survey report by Rainer Seemann et al (2014) [8].
Adhesive materials have decreased antimicrobial activity when compared to amalgam and zinc oxide [9]. Composite resin surface favours more plaque accumulation than any other restorative material. Several techniques have been employed in order to increase the antimicrobial activity of these materials and to inhibit biofilm formation on composite resin, mainly by incorporating slow release antibiotics and biocides [10]. However, such attempts proved short term due to solubility over a period of time, leading to void formation in the composite resin and unfavourable mechanical behaviour of the restoration. Further, such modifications provided antibacterial effect only on the surface of the composite restoration and not at the resin dentin interface where failure occurs commonly.
Literature shows various attempts at incorporation of antibacterial components such as fluorides, antibiotics, methacryloxydodecyl pyidinium bromide (MDPB), methacryloxy ethyl cetyl di-methyl ammonium chloride (DMAEC) in dentin adhesives [11]. However, fluorides, antibiotics and inorganic agents dispersed in the matrix phase; hence it is difficult to strictly control the release kinetics [12]. Also, adhesive bonding may be compromised due to the constant release of these agents. To overcome this, polymerisable cationic monomers such as quaternary ammonium monomer (MDPB), (DMAE-CB) that can be covalently bound within the polymer matrix, were incorporated in the dental adhesive systems [12]. Since MDPB can polymerise and can be immobilised in polymer, the bonding interface is considered to be stably maintained in contrast to soluble anti-bacterial agents incorporated in bonding agents. They also showed that MDPB exerts contact inhibition on the growth of S. mutans at the bonding interface leading to prevention of secondary caries. Therefore, attempts of functionalizing adhesive system with antibacterial activity was made for proper biological seal without compromising bonding.
Our study is one such attempt to incorporate Nisin, a polypeptide bacteriocin to fifth generation bonding agent. Nisin, a ribosomally synthesized and post-translationally modified lantibiotic, produced by Lactococcus lactis subsp [13, 14]. Lactis, is a food preservative, approved by FDA as GRAS (generally regarded as safe) that is incorporated in the binder solutions of acrylic polymer and vinyl acetate co-polymer in food packaging [15]. It has a relatively broad spectrum of anti-microbial activity against various lactic acid bacteria and other gram positive bacteria.
Since Nisin has yet to be tried in restorative dentistry, this study has been designed as a preliminary in-vitro evaluation of the antibacterial activity of fifth generation bonding agent incorporated with Nisin, against S. mutans. Confocal laser scanning microscopy (CLSM) in conjunction with fluorescent indicators SYTO-9 and propidium iodide were used for the membrane integrity test.