The amounts and types of nutrients in the environment influence the development and final bacterial and chemical composition of biofilms. In oligotrophic environments, organisms respond to nutrient stress by alterations in their cell morphology and cell surfaces, which enhance adherence. Little is known of the responses to stress by bacteria in the animal oral cavity. The environment in the oral cavity is less extreme, and saliva provides a constant source of nutrients. Catabolic cooperation among oral bacteria allow carbon and nitrogen from salivary glycoproteins to be utilized. Modification of growth environments of oral bacteria can influence their cell surfaces and adhesion. Studies in experimental animals have shown that feeding either glucose or sucrose diets or fasting has little effect on the initial stages of development of oral biofilms. However, diet can influence the proportions of different bacterial species later in biofilm development. Studies of competition among populations in communities of oral bacteria in vitro and in vivo have shown the significance of carbon limitation and excess and changes in environmental pH. Relatively few studies have been made of the role of a nitrogen metabolism in bacterial competition in biofilms. In keeping with biofilms in nature, oral biofilms provide a sequestered habitat, where organisms are protected from removal by saliva and where interactions among cells generate a biofilm environment, distinct from that of saliva. Oral biofilms are an essential component in the etiologies of caries and periodontal disease, and understanding the biology of oral biofilms has aided and will continue to aid in the prevention and treatment of these diseases.
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.
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