Abstract
Dental caries remains a global health challenge, driven by bacterial acid production and characterised by a dynamic balance between demineralisation and remineralisation. Root caries presents significant challenges in dental practice due to its complexity and the need for careful restorative management. Current caries management strategies provide some benefit but face limitations, such as drug resistance, side effects, and undesirable aesthetics. Consequently, this has prompted the exploration of biologically inspired therapies, such as peptide-based approaches, including antimicrobial peptides and mineralising peptides. Synthetic antimicrobial peptides can be engineered to selectively target specific microorganisms, such as Streptococcus mutans, or the oral environment. Mineralising peptides, derived from proteins or developed synthetically, mimic natural processes to promote enamel and dentine repair. More recently, dual-action peptides have emerged, combining antimicrobial and mineralising properties to achieve simultaneous microbial control and tissue restoration. This thesis aimed to develop a novel targeted dual-action peptide for caries management, and to investigate its selective antibacterial activity against S. mutans, as well as its remineralising effects on human root dentine carious lesions.
To establish a solid methodological and analytical foundation for the experimental design of the mineralisation part of the study, this thesis includes two comprehensive reviews in the field of caries mineralisation research. The first review highlighted the need for standardisation to improve reproducibility and comparability across in vitro caries mineralisation studies and provided a structured overview of the commonly used substrates, methods for lesion formation, and mineralisation models to study tooth de-/re-mineralisation. In addition, advanced analytical techniques for cariology research were evaluated, such as nanoindentation, micro-/nano-computed tomography, and electron microscopy. These methods provide high-resolution and complementary insights into the chemical, mechanical and structural changes in caries lesions, thereby guiding the selection of appropriate and robust methodologies for caries research. These findings directly informed the design of the experimental model and the choice of assessment techniques used in this thesis.
The laboratory studies of this thesis focused on the design and evaluation of a novel dual-action peptide, GA-C16G2. The first study developed the peptide, synthesised by conjugating gallic acid to the selectively targeted antimicrobial peptide C16G2. Characterisation confirmed that GA-C16G2 retained structural similarity to its parent peptide and exhibited favourable stability and biocompatibility, as demonstrated by spectroscopic and chromatographic analyses, as well as cytotoxicity testing. The second study investigated the antimicrobial performance. Results demonstrated that the peptide possessed potent and selective activity against S. mutans while sparing commensal species, and that it exhibited faster bactericidal activity than the parent peptide. GA-C16G2 significantly inhibited biofilm formation and induced morphological disruption of bacteria, as visualised by electron microscopy. In the third study, the mineralising effects of GA-C16G2 were evaluated in demineralised root dentine under pH cycling conditions. GA-C16G2 promoted substantial mineral recovery and mechanical reinforcement, achieving higher mineral density, as revealed by micro-computed tomography, and enhanced hardness and elastic modulus, as measured by nanoindentation, compared with C16G2 or the control. The morphological characteristics of the dentine cross-section were examined using scanning electron microscopy, while its chemical composition was analysed using energy-dispersive spectroscopy and Fourier transform infrared spectroscopy.
Overall, this thesis establishes GA-C16G2 as a stable, biocompatible, and dual-functional peptide with both selective antimicrobial activity and potent mineralising effects. By combining targeted antimicrobial activity with the ability to restore mineral integrity, GA-C16G2 demonstrates potential as a therapeutic candidate for the prevention and management of root caries.