The adhesion of the oral bacterium Streptococcus sanguis CH3 to various polymeric surfaces with surface free energies (-ys) ranging from 22 to 141 erg cm-2 was investigated. Suspensions containing nine different bacterial concentrations (2.5 x 107 to 2.5 x 109 cells per ml) were used. After adhesion for 1 h at 21°C and a standardized rinsing procedure, the number of attached bacteria per square centimeter (nb) was determined by scanning electron microscopy. The highest number of bacteria was consistently found on polytetrafluorethylene (YS = 22 erg cm2), and the lowest number was found on glass (-ys = 141 erg cm-2) at all bacterial concentrations tested. The overall negative correlation between nb and-ys was weak. However, the slope of the line showing this decrease, calculated from an assumed linear relationship between nb and ys, appeared to depend strongly on the bacterial concentration and increased with increasing numbers of bacteria in the suspension. Analysis of the data for each separate polymer showed that the numbers of attached cells on polyvinyl chloride and polypropylene were higher but that those on polycarbonate were lower than would be expected on basis of a linear relationship between nb and ys. Desorption experiments were performed by first allowing the bacteria to attach to substrata for 1 h, after which the substrata and attached bacteria were removed to bacterial suspensions containing 10-fold lower bacterial concentrations. During the second incubation period, bacteria continued to absorb onto the substrata with high surface free energy (-ys > 45 erg cm 2), whereas on lower-energy surfaces, significant desorption of the cells took place. From the results it was concluded that the surface free energy-ys is an important parameter for the adhesion of bacteria to solids, although it is not the only parameter operating. The surface free energy is presumably more directly related to the binding force rather than to the number of bacteria per surface area.

The kinetics of adhesion of Streptococcus sanguis CH3 from suspension to polymers with different surface free energies were studied by using three bacterial concentrations (2.5 x 107, 2.5 x 108, and 2.5 x 109 cells per ml-'). Substratum surface free energies (-ys) ranged from 18 to 120 erg cm-2. The kinetics of bacterial adhesion to these surfaces showed a typical two-step adhesion process, indicating an equilibrium in both steps. In the initial adhesion step (step 1), low equilibrium numbers of adhering bacteria were counted on substrata with surface free energies lower than 55 erg cm 2. A maximal number adhered on substrata with higher surface free energies. At the lowest bacterial concentration tested, the highest number of bacteria were found on substrata with a surface free energy around 55 erg cm 2. For each substratum, step 2 started after a characteristic time interval T*, being short (30 min) for ys < 50 and long (120 min) for ys > 50 erg cm-2. The relationship between the substratum surface free energy and the number of bacteria adhering at equilibrium after step 2 was similar to, although less distinct than, that during step 1 with a slight indication of a bioadhesive minimum around ys = 35 erg cm-2. The results are indicative of a two-step adhesion model, in which step 1 is controlled by macroscopic substratum properties.

The adhesion of 3 strains of oral streptococci from a buffered suspension onto 3 different solid substrata was studied. Representative strains of streptococci were selected on the basis of their surface free energy (γb), namely Streptococcus mitis L1 (γb= 37 mJ·m−2), Streptococcus sanguis CH3 (95 mJ·m−2) and Streptococcus mutans NS (117 mJ·m−2). Solid substrata were also selected on basis of their surface free energy (γs), and included polytetrafluorethylene (γs= 20 mJ·m−2), polymethylmethacrylate (53 mJ·m−2) and glass (109 mJ·m−2). Bacterial adhesion was measured as the number of bacteria adhering per cm2 at equilibrium. Equilibrium was usually obtained within 20 min. S. sanguis CH3, having an intermediate surface free energy did not show a clear preference for any of the 3 solids. S. mitis L1, however, the lowest surface free energy strain, adhered in highest numbers to the low energy solid PTFE, whereas the highest γb strain, S. mutans NS, adhered in highest numbers to the highest γs solid, glass. Calculation of the interfacial free energy of adhesion (ΔFadh) for each bacterial strain showed that this parameter was predictive of bacterial adhesion to solid substrata.