Caries and periodontal disease: Two diseases, one biofilm

Dental plaque, a natural oral biofilm is involved in the aetiology of dental caries and periodontal disease. Despite decades of research, the microbiology, aetiology and pathogenesis of these diseases remain controversial. A number of factors interplay in these diseases, the indigenous microbes that inhabit the oral cavity, diet, host susceptibility and time. The ?Non-Specific Plaque Hypothesis? (NSPH) was proposed where the overall mass of plaque interacted with the host and caused disease. An alternative view was the ?Specific Plaque Hypothesis? (SPH) where, among the diverse microbial community, a limited subset of specific bacteria were associated with disease. In recent years, the ?Ecological Plaque Hypothesis? (EPH) has been proposed that it be recognised that the oral ecology as a whole contributes to the aetiology of dental caries and periodontal diseases, with shifts in the composition of microbial communities being of particular importance.

Structural Aspects of Salivary Glycoproteins

The protective functions of saliva are attributed, in part, to its serous and mucous glycoproteins. We have studied, as representative molecules, the proline-rich glycoprotein (PRG) from human parotid saliva and the high (MGI ) and low (MG2) molecular weight mucins from submandibular-sublingual saliva. PRG (38.9 kDa) contains 40% carbohydrate consisting of 6 triantennary N-linked units and a single peptide chain of 231 amino acids, 75% of which = PRO+GLY+GLN. PRG's secondary structure is comprised of 70% random coil (naked regions) and 30% β-turns (glycosylated domains). MGI (>103 kDa) contains 15% protein (several disulfide linked subunits), 78% carbohydrate (290 units of 4-16 residues), 7% sulfate, and small amounts of covalently linked fatty acids. MG2 (200-250 kDa) contains 30% protein (single peptide chain), 68% carbohydrate (170 units of 2-7 residues), and 2% sulfate. The major carbohydrate units of MG2 are: NeuAcα2, 3Galβ1, 3GalNAc, Galβ1, 3GalNAc, and Fucα1, 2Galβ1, 3GalNAc. MG1 contains hydrophobic domains, as evidenced by its ability to bind fluorescent hydrophobic probes; MG2 does not. Collectively, the biochemical and biophysical comparisons between MGI and MG2 indicate that these two mucins are structurally different. Several functional properties of MG1, MG2, and PRG have been examined, including their presence in two-hour in vivo enamel pellicle, binding to synthetic hydroxyapatite, lubricating properties, and interactions with oral streptococci. The data presented suggest that these glycoproteins may have multiple functions which are predicated, in part, on their carbohydrate units. The potential significance of the structure-function relationships of these glycoproteins to the oral ecology is discussed.