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The Effect of Lithium and Fluoride Upon In Vitro Plaque and Bacterial Growth

Oral Health Research Institute, Indiana University School ofDentistry, 410 Beauty Avenue, Indianapolis, Indiana 46202 J Dent Res 58(4) 1428, April 1979 Various trace elements are receiving increasing attention with regard to their potential roles in determining susceptibility to dental caries. One such element is lithium. Gallagher and Cutress (Arch Oral Biol 22:555, 1977) have recently noted that 1000 ppm lithium in the growth medium of Streptococcus mutans significantly inhibited acid production but did not hinder growth of the organism. In addition, epidemiologic studies (Lossee, Caries Res 3:23, 1969) have shown that highly mineralized water from a low caries area in Ohio contains 0.035 ppm lithium (as well as 1.5 ppm F). In contrast, Herbison and Handelman (J Dent Res 50:1605, 1971) have observed that a similar concentration of lithium alone and in combination with fluoride produced no significant reduction in the acid solubility of synthetic hydroxyapatite by Streptococcus mutans. Little information exists, however, concerning the influence of the combination of fluoride and lithium upon bacterial growth and dental plaque formation. Thus, this study tested combinations of lithium and fluoride with regard to bacterial growth, acid production and in vitro plaque formation. An in vitro glass-slide technique was utilized to assess adherent plaque formation using Streptococcus mutans 6715 (NIH) as the test organism. Jordan's complex medium (J Dent Res 39:116, 1960) containing 5% sucrose was used as the control. The test compounds consisted of lithium chloride and sodium fluoride. The concentrations tested included lithium at 50, 100, and 200 ppm; fluoride at 25 ppm; and combinations of 25 ppm fluoride with 50, 100 and 200 ppm lithium. Conventional glass microscopic slides (2 X 7.5 cm), ten to a test solution, were incubated in test tubes (3 X 20 cm) containing 50 ml of the appropriate test medium. The tubes were inoculated with 0.5 ml of an 18-hour culture of Streptococcus mutans 6715 (NIH) and incubated for 18-hours at 370C. At the end of this period, the slides were removed and rinsed three times in distilled water to remove any non-adherent plaque plus any medium residue. The slides were dried, weighed, the plaque removed from the glass slides and the slides re-weighed to obtain the plaque dry weight. The terminal pH of the test solution was also measured with a glass-combination electrode at the end of the 18-hour incubation period. Bacterial growth was assessed by measuring the absorbance (turbidity) of the test solutions containing Streptococcus mutans 6715 at 660 nm at various time intervals. Jordan's complex medium containing 0.25% glucose was used in the test solutions as well as the control. The same concentrations of the test compounds previously mentioned were tested. Test tubes (1 X 12.5 cm) containing 10 ml of the appropriate test medium were inocu-