Hydrophilicity of dentin bonding systems influences in vitro Streptococcus mutans biofilm formation
Introduction
Resin-based composites are increasingly used because of their excellent esthetic properties and improved mechanical characteristics [1], [2]. The evolution of polymer matrices and filler particles composition enhanced the performances of these materials [3], [4], [5] along with more reliable bonds to dental hard tissues. Nevertheless, the main reason for failure of resin composite restorations is still secondary caries occurring at the interface between the composite material/dentin bonding systems (DBS) and enamel and dentin [6], [7], [8], [9], [10], [11].
As the development of such lesions is mainly related to the effect of acids and enzymes produced by bacteria colonizing the interface, the interactions between the surface of resin composites, DBS and the overlying biofilm greatly influence the lifespan of the adhesive restorations. A relevant problem is that resin composite and DBS surfaces are more heavily colonized by oral biofilms than surfaces of other restorative materials (such as amalgam, glass-ionomer cements and ceramics), as well as sound enamel surfaces [12], [13], [14].
Dentin bonding systems are blends of hydrophilic and hydrophobic monomers with the ability to couple the hydrophobic resin composite materials to hydrophilic surfaces such as dentin or enamel. They may be classified as etch-and-rinse or self-etching adhesives. These adhesives may be further classified as multi-step (i.e. three-step etch-and-rinse and two-step self-etching) or simplified by combining the number of steps required for the clinical application (i.e. two-step etch-and rinse and one-step self-etching) [15], [16], [17]. Since simplified formulations involve mixing of nonsolvated adhesives with solvated primers (i.e. two-step etch-and-rinse) or with self-etching primers (i.e. one-step self-etch), DBS simplification strongly increases the hydrophilicity of the mixture and of the bond [18]. It has been previously shown that the chemical composition of DBS influences bacterial colonization. The results suggest that variations in the chemical structure of the monomers, solvents or application techniques can extensively influence biofilm formation even when a DBS does not contain any specific antibacterial formulation [19], [20], [21], [22], [23], [24], [25].
Previous studies investigated the effect of commercially available DBS on cariogenic bacteria colonization. Pinheiro et al. [26] studied Streptococcus mutans biofilm formation on the surface of different DBS. The authors concluded that different materials produce diverse colonization levels in relation to their variable chemical composition, solvent and application technique. Comparison between etch-and-rinse and self-etching adhesives showed lower S. mutans colonization for the latter, when compared to the etch-and-rinse approach [27], [28]. Additionally, discrepancies in bacterial growth induced by different one-step self-etching adhesives and self-etching primers have been reported [29].
Experimental resins that cover a wide range of hydrophilicity and other properties of contemporary DBS have been formulated (R1–R5); these resins rank from very hydrophobic to very hydrophilic, as manifested by their solubility parameters [17]. Previous studies have evaluated their tensile strength, modulus of elasticity, degree of conversion, influence of solvent content, water sorption and solubility [17], [30], [31], [32], [33], [34], [35], [36]. However, no data is available regarding the influence of the hydrophilicity of these experimental resin blends on surface biofilm formation. Since S. mutans is considered to be one of the most important microorganisms responsible for primary and secondary caries [37], [38], [39], [40], the aim of the present study was to investigate the influence of experimental DBS chemical composition and hydrophilicity on S. mutans colonization in vitro. The null hypothesis tested was that hydrophilicity of DBS does not affect S. mutans biofilm formation.
Section snippets
Specimen preparation
All reagents and multi-well plates used in the present study were purchased from Sigma–Aldrich (St. Louis, MO) unless otherwise specified.
Five light-curing versions of neat experimental resin blends with increasing hydrophilicity were investigated (R1, R2, R3, R4 and R5). Their compositions are listed in Table 1. All blends included 0.25% camphorquinone and 1% 2-ethyl-dimethyl-4-aminobenzoate as the photoinitiator and accelerator, respectively. Resin blend R1 and R2 are similar to nonsolvated
Results
The means and standard errors of the MTT assay on the 5 experimental resin blends are reported in Fig. 1. Resin blends R2 and R3 showed the highest S. mutans colonization. Resin blends R1 and R4 showed similar intermediate biofilm formation while R5 surfaces were significantly less colonized than the other resin blends, although its composition differed from R3 only for the presence of an acidic phosphate monomer Bis[2-(methacryloyloxy)ethyl]phosphate (p < 0.001). Similarly, R4 showed a
Discussion
Reduction of biofilm formation on the surface of resin-based materials is considered an important target to reduce secondary caries risk and improve the longevity of restorations. Previous studies investigating biofilm formation [12] were performed using the agar diffusion test. In contrast, the method employed in the present study directly evaluated adherent biomass in experimental conditions that simulated in vivo clinical conditions. The results of the present study warrant rejection of the
Conflict of interest
The authors report no conflict of interest.
Acknowledgements
The authors wish to thank Mr. Aurelio Valmori for photographical assistance. The study was funded by grants from MIUR (Italy): FIRB RBAP1095CR to Breschi L (P.I.), PRIN 2009SAN9K5 to Breschi L (P.I.) and R21 DE 091213 (P.I. FRT).
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