Mechanisms of Fluoride Inhibition of Ammonia Production by Oral Bacteria

Objective: Urease and arginine deiminase (ADS) systems are important for ammonia production and alkalinization of dental plaque. The objective of this study was to determine mechanisms of fluoride inhibition of the ADS of Streptococcus rattus FA-1 and ureolysis by Actinomyces naeslundii ATCC12104. Methods: Suspension cultures were grown to early stationary phase in TY (tryptone-yeast-extract) medium with excess arginine for S. rattus and TYglucose for A. naeslundii. TYsucrose was used for initial growth of biofilms on glass microscope slides. Ammonia was assayed enzymatically. Results: Fluoride inhibition of ammonia production from arginine by intact cells of S. rattus in suspensions was highly dependent on pH with no inhibition at a pH value of 6 but nearly complete inhibition by 1 mM fluoride at pH 5. The inhibition appeared to be due entirely to fluoride-induced acidification of the cytoplasm and inhibition of arginine transport enzymes. The organic weak acids, indomethacin and caprate, showed similar patterns of inhibition. Fluoride inhibition of ureolysis by A naeslundii was also very pH dependent with little inhibition at pH 6 but nearly complete inhibition at pH 4 by 0.05 mM NaF for suspensions or 0.5 mM for biofilms. Inhibition involved direct fluoride inhibition of urease enzyme and indirect inhibition due to cytoplasmic acidification. At pH 5, fluoride and caprate were potent inhibitors in suspensions (90% inhibition by 0.5 mM), while indomethacin was marginal, even though it acted to increase proton permeability of cells. At pH 4, fluoride and both organic acids were potent inhibitors. Conclusions: Fluoride and organic weak acids inhibit ammonia production by oral bacteria at low pH values mainly through cytoplasmic acidification, but fluoride can also directly inhibit urease enzyme. Supported by NIH awards R01-DE06127 and PO1-DE11549.