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

Bacteria in biofilm and planktonic bacteria exhibit different properties. T he objective of the present study was to compare the growth rates of Strept ococcus sobrinus and Streptococcus mutans on different types of biofilm wit h their planktonic growth rate. Our experimental model consisted of hydroxy apatite beads coated with human saliva (sHA). Glucans or fructans were synt hesized in situ on sHA by immobilized cell-free glucosyltransferase or fruc tosyltransferase isolated from oral bacteria. S. sobrinus or S. mutans was then adsorbed onto the glucan- or fructan-coated sHA and incubated for diff erent time intervals. The depth of the developing biofilm was measured. Our results show that growth rates of S. sobrinus and S. mutans on both fructa n- and glucan-coated sHA were similar during a 23 h period. In addition, th e profile was similar to the growth profile of the same planktonic bacteria . The resemblance in growth rates between planktonic and biofilm bacteria m ay be attributed to the thin and non-dense biofilm formed in the initial st ages of the biofilm formation. The thin biofilm coat, reaching a maximal de pth of 11 mum, has only imposed limited diffusion restrictions, thus not af fecting the growth of the bacteria in the biofilm. Our study shows that gro wth of bacteria on surfaces may resemble their growth in suspension if the bacteria are not embedded in a thick dense biofilm.