Enhanced Adhesion of S. Mutans to Hydroxyapatite after Inoculation in Saliva

Streptococcus mutans cells form robust biofilms on human teeth and are strongly related to caries incidents. Hence, understanding the adhesion of S. mutans in the human oral cavity is of major interest for preventive dentistry. In this study, we report on AFM-based single cell force spectroscopy measurements of S. mutans cells to hydroxyapatite surfaces. We observe a significant increase of adhesion strength if S. mutans cells were exposed to human saliva before adhesion. In contrast, Staphylococcus carnosus cells are almost unaffected by the pre-treatment. Our results demonstrate that S. mutans cells are well-adapted to their natural environment, the oral cavity. This ability promotes the biofilm-forming capability of that species and hence the production of caries-provoking acids. In consequence, understanding the fundamentals of this mechanism may pave a way towards more effective caries-reducing techniques. Introduction It is known for decades that Streptococcus mutans is very closely related to the development of caries and other diseases in the oral cavity[Ham1980, Loe1986, Mit2003]. Furthermore, it can also enter the bloodstream through wounds in the oral cavity and travel from there through the body and even reach the coronary artery, where it can cause severe cardio-vascular diseases[Abr2009]. The main pathogenicity of this organism arises when the cell adheres to appropriate surfaces, since with this step, the formation of a biofilm is initiated. The process of caries formation is thereby influenced by substratum (e. g. enamel, fluoridated or not) and saliva[Sel2007,Tak2008,Fil2010,Los2013]. On exposure to saliva, a proteinaceous surface coating – called pellicle – is formed almost immediately on all solid substrates[Han2009].This conditioning layer changes the properties of the substrate. The nature of the chemical groups exposed at the surface mainly define the adhesion forces[Los2013]. Most studies focus on the adhesion of oral bacteria to salivary agglutinin (SAG) which is adsorbed to the oral pellicle on tooth surfaces [Ray1999, Yos2005, Mil2015]. Additionally , it has been shown that the characteristics of biofilm formation by S. mutans depend on many parameters like for example oxygen content or the presence of specific enzymes in the bacterial cell[Ahn2007, Ahn2008]. Next to other constituents, the salivary sucrose content increases adhesion to surfaces significantly and is also a key factor in the production of biofilms[Cos2013]. Furthermore, by using genetically modified S. mutans cells, the function of many proteins in adhesion processes and biofilm formation on SAG was identified[Ray1999, Yos2005, Mil2015]. Atomic force microscopy(AFM)-based force spectroscopy offers a unique tool to quantitatively …