Uptake of fluoride and its inhibitory effects in oral microorganisms in culture

Abstract

Fluoride (F) taken up by certain oral bacterial cells grown as pure or mixed cultures on F-containing media was present in two forms distinguished by acid-extractability. The F taken up inhibited the fall in pH when the cells were subsequently incubated in a buffer-glucose solution, especially if glucose was not added to the growth medium. Cells of Streptococcus sanguis H7PR3 suspended in F-containing buffer took up F which was readily extracted with dilute acid and was non-inhibitory. Cells of Streptococcus mutans FA1 did not take up F from buffers, but in separate tests were inhibited by external F to a similar extent to cells which took up F, suggesting that F-permeation of the cells is not essential for its inhibitory effect on this organism. Increased permeability to F at lower pH values has been suggested as an explanation of the greater sensitivity of bacteria to F at pH 5–5.5 compared with neutrality. This hypothesis was not supported by testing glucose utilization at pH 7 by cells pre-treated with F at pH 5–5.5. The increased sensitivity of bacteria to F at low pH was not explained by an effect of pH on F inhibition of enolase which is believed to be the inhibitory site of F in cells. The results suggest that both uptake of F into cells and their susceptibility to F inhibition vary with their metabolic state.