On salivary film formation and bacterial retention to solids. A methodological and experimental in vitro study

A parallel-plate flow cell system was developed to assess individual and combined parameters relevant to oral microbial retention. In this system, internal reflection germanium prisms represented oral solid surfaces including tooth surfaces. The system used, allowed adsorbed protein films to be analyzed in situ by multiple non-destructive surface analytical techniques, without provoking surface associated biofilms. Using a range of biochemical and biophysical techniques, germanium prisms of medium critical surface tension were noted to accurately model film formation at tooth-saliva interfaces. A range of clinically relevant temperatures (22 and 37 degrees C), rinse shear rates (1 and 32 ml/min) and critical surface tensions (high, medium and low) were systematically tested in the flow-cell system for their influence, if any, on salivary microbial retention from whole human saliva. Comparisons of the interfacial organization of adsorbed material were made between human parotid (HPS) and submandibular-sublingual (HSMSL) salivas. The influence of these salivary secretions was also tested on the retention capacities of Streptococcus sanguis and Streptococcus salivarius. Microbial retention was found to be significantly dependent on initial critical surface tensions and rinse flow rates, but not to be affected by tested temperatures. When compared to prisms of low and high critical surface tension, medium critical surface tension prisms retained the highest numbers of microorganisms. No statistically verified morphological selectivity was observed in the retained microbial populations. When the interfacial organizations of HPS and HSMSL were evaluated at low- and medium energy surfaces in the absence and presence the streptococci, comparatively thinner and denser films with pronounced bacterial retention was noted for HPS on prisms of low critical surface tension. In the presence of saliva or salivary fractions, no difference in retention capacities was observed between the tested bacterial strains. The data suggest that in the presence of saliva, physico-chemical and mechanical factors have a major influence on the short term microbial retention to solid surfaces. The amount of salivary proteins being initially retained to a material also seems to be more directly related to the critical surface tension quality of the solid than to the type of bacterial strain present in suspension. The empirical critical surface tension of a material is thus a useful predictor of both microbial retention capacities and primary interactions between saliva and solid surfaces.