Sample preparation affects bacterial adhesion to dental polymers

Dental materials are often tested for bacterial adhesion by culturing the cariogenic Streptococcus mutans bacteria on samples prepared by means that may affect the surface chemistry of that material.  One factor that affects the degree to which bacteria adhere is the hydrophilicity of a surface; once attached, bacteria form colonies and generate biofilms.  S. mutans has been shown to adhere to hydrophobic surfaces, while hydrophilic and positively charged surfaces tend to reduce the level of attachment.  Objective:  To test the hypothesis that sample preparation methods affect surface chemistry and in turn bacterial adhesion.  Methods:  Thin (» 0.5 mm) polymer films of 1:1 (by mass)  BisGMA:TEGDMA were photopolymerized between glass slides separated by Teflon spacers.  Slides were either untreated or surface-treated with octadecyltrimethoxysilane to aid release; films of 49.5:49.5:1.0 (by mass) BisGMA:TEGDMA:methacryloyloxyethyltrimethylammonium chloride (METMA, a cationic monomer expected to have antibacterial properties) were prepared in the same manner.  Hydrophobicity of the films was evaluated by contact angle (CA) measurements; results were analyzed by One-Way ANOVA.  Bacterial colonization was tested by inoculating S. mutans on films for up to 24 h.  Samples were fixed, stained, and visualized using fluorescence microscopy.  Results:  CA measurements varied significantly among vertical groups (p<0.001, Table 1), suggesting that preparation with treated slides increased polymer surface hydrophobicity.  Bacterial morphology was diverse:  often, bacteria cultured on films prepared using surface-treated slides clustered into microcolonies, while bacteria cultured on films prepared using untreated slides appeared randomly scattered and individual.  Although there was no significant difference in hydrophobicity between the two materials, images revealed a small reduction in bacterial adhesion on METMA-containing films, suggesting antibacterial properties.  Conclusion:  Methods of preparation that affect surface chemistry of dental polymers may obscure actual bacteriostatic properties of that material.  Support: NIDCR/NIST Interagency Agreement Y1-DE-7005-01.

Table 1. Contact Angle of Polymer Films, Degrees (mean±std.dev.)