The effect of environmental pH and fluoride from the substratum on the development of biofilms of selected oral bacteria.

The present study was initiated to answer the question, "Does fluoride from the substratum influence the accumulation of bacterial cells in an associated biofilm?" 'Fluoride-bound hydroxyapatite' (FHA) and 'fluoride-free hydroxyapatite' (HA) rods were prepared as test and control surfaces, respectively. Biofilms of S. mutans BM71, A. naeslundii genospecies 2 WVU627, and L. casei BM225 accumulated on the surfaces of rods in a semi-defined, mucin-based medium in a chemostat. Culture conditions were varied from pH 4.5 to 7.0 under carbon (glucose) limitation and excess, at a dilution rate of D = 0.1 h-1. Low environmental pH reduced both the numbers of cells on HA surfaces during the early phases (from 0 to 2 h) of accumulation and the final numbers of cells in mature biofilms (20 h). The initial adherence of cells was unaffected by surface fluoride under any of the conditions tested. Similarly, biofilm cells under carbon limitation and those under carbon excess at pH 7.0 were not affected by surface fluoride. However, at low environmental pH values, pH 5.5 for S. mutans and pH 6.0 for A. naeslundii under glucose excess, the accumulation of biofilm cells on the FHA surfaces was significantly reduced (p < 0.05-0.001). Biofilm cell number doubling times of S. mutans and A. naeslundii were increased on FHA relative to HA. Biofilms of L. casei were not significantly affected, even at pH 4.5 in glucose excess. The results confirmed that fluoride from the substratum affected fluoride-sensitive biofilm cells but only under conditions of glucose excess ad low pH.