Growth rate and biofilm thickness of Streptococcus sobrinus and Streptococcus mutans on hydroxapatite

Bacteria in biofilm and planktonic bacteria exhibit different properties. The objective of the present study was to compare the growth rates of Streptococcus sobrinus and Streptococcus mutans on different types of biofilm with their planktonic growth rate. Our experimental model consisted of hydroxyapatite beads coated with human saliva (sHA). Glucans or fructans were synthesized in situ on sHA by immobilized cell-free glucosyltransferase or fructosyltransferase isolated from oral bacteria. S. sobrinus or S. mutans was then adsorbed onto the glucan- or fructan-coated sHA and incubated for different time intervals. The depth of the developing biofilm was measured. Our results show that growth rates of S. sobrinus and S. mutans on both fructan- and glucan-coated sHA were similar during a 23 h period. In addition, the profile was similar to the growth profile of the same planktonic bacteria. The resemblance in growth rates between planktonic and biofilm bacteria may be attributed to the thin and non-dense biofilm formed in the initial stages of the biofilm formation. The thin biofilm coat, reaching a maximal depth of 11 μm, has only imposed limited diffusion restrictions, thus not affecting the growth of the bacteria in the biofilm. Our study shows that growth of bacteria on surfaces may resemble their growth in suspension if the bacteria are not embedded in a thick dense biofilm.