Infectious diseases e.g. gastric ulcer, caries and perodontitis, are caused by bacteria in a biofilm. Adhesion of bacteria to host ligands e.g. proteins, polypeptides and glycoproteins, is a key event in biofilm formation and colonization of surfaces such as mucosa and tooth tissues. Thus, host ligands could contribute to the susceptibility to infectious diseases. The general aim of this doctoral thesis was to study the effect of phosphorylated polypeptides and gp-340 in saliva and milk on oral bacterial adhesion and aggregation.
Statherin is a non-glycosylated, phosphorylated polypeptide in saliva. The polypeptide inhibits precipitation and crystal growth of calcium phosphate and mediates adhesion of microorganisms. By using a hybrid peptide construct, the domain for adhesion of Actinomyces isolated from human infections and from rodents was found to reside in the C-terminal end, and the adhesion was inhibitable. With alanine substitution the peptide recognition epitope in the C-terminal end was delineated to Q and TF, where QAATF was an optimal inhibitory peptide. In contrast, human commensal Actinomyces bound to the middle region in a non-inhibitable fashion. Gp-340 is another protein in saliva, and it is a large, multifunctional glycoprotein. Four novel size variants (I-IV) of salivary gp-340 were distinguished within individuals, and their glycoforms were characterized. All four size variants were identical in the N-terminal amino acid sequence and shared core carbohydrates. Low-glyco lung gp-340, high-glyco saliva gp-340, and size variants I-III aggregated bacteria differently. Human milk, which shares many traits with saliva, could inhibit adhesion of Streptococcus mutans to saliva-coated hydroxyapatite (s-HA), a model for teeth, in an individually varying fashion. Human milk caseins, lactoferrin, secretory IgA, and IgG inhibited the binding avidly. By using synthetic peptides the inhibitory epitope in b-casein was mapped to a C-terminal stretch of 30 amino acids. Inhibition by human milk, secretory IgA and the b-casein-derived inhibitory peptide was universal among a panel of mutans streptococci.
The main conclusions are: (i) statherin mediates differential binding of commensal versus infectious Actinomyces strains with small conformation-dependent binding epitopes, (ii) salivary gp-340 has individual polymorphisms that at least affect binding of bacteria, (iii) human milk inhibits S. mutans adhesion to s-HA in an individually varying fashion, and the C-terminal end of human milk β-casein is one inhibitory component. Together these results suggest that the studied host ligands can influence the composition of the oral biofilm. Statherin may protect the host from colonization of bacteria associated with infections. Gp-340 size variants may affect functions related to host innate immune defences such as interactions with a wide array of bacteria, and human milk may have a protective effect in infants from colonization of mutans streptococci.