Ultrastructure, growth, and adherence of Streptococcus mutans after treatment with chlorhexidine and fluoride

Ultrastructure, growth, and adherence to hydroxyapatite of Streptococcus mutans ATCC 27351 was studied after treating bacterial suspensions for 1 h with 0.1% chlorhexidine gluconate (CHX), 0.1% sodium fluoride (F), or a combination of these two (CHX + F). Cells treated with 0.1 M phosphate-buffered saline were used as controls. Electron microscopy revealed that the treatment with CHX or CHX + F caused disruption of streptococcal cells with vacuolization and sloughing and leakage of cytoplasmic constituents. The F-treated specimens appeared the same as the controls, where the ultrastructure was mostly normal. In the adherence assay, radiolabelled bacteria attached themselves in 77.9% to hydroxyapatite in the control series. Treatment with F reduced the adsorption to 63.7%, and treatment with CHX and CHX + F to 57.4 and 43.4%, respectively. The reduction in CHX and CHX + F series when compared with the controls was statistically highly significant. The synergistic effect of CHX and F was further verified in minimum inhibitory concentration titrations where total inhibition was observed at 0.5% F and 0.1% CHX, respectively, while CHX + F inhibited growth at 0.05%. Thus, F does not interfere with the toxic effect of CHX on S. mutans cells, but, on the contrary, the two chemicals seem to exert a synergistic mode of action.