Sensitization of Actinomyces naeslundii and Streptococcus sanguis in biofilms and suspensions to acid damage by fluoride and other weak acids.

Fluoride and other weak acids, such as benzoate, indomethacin, salicylate and sorbate, were found to be sensitizers for acid killing of cells of Actinomyces naeslundii ATCC 19246 and Streptococcus sanguis NCTC 10904 in suspensions or in mono-organism biofilms on glass slides. These bacteria are among the more acid-sensitive organisms from dental plaque and were killed when acidified to pH values between 3.5 and 4.0. Biofilm cells were more resistant than cells in suspensions, especially in terms of the fraction of the initial population surviving acidification. The mechanism for sensitization to acid killing by fluoride and the other weak acids involved enhanced transmembrane transport of protons, reflected by increases in measured proton permeabilities of the cells. Thus, the weak acids thwarted the functions of F(H+)-ATPases in extruding protons and protecting cells against acid damage. Fluoride sensitization of biofilms or cells in suspensions to acid damage occurred rapidly. There was a delay in sensitization of biofilms by indomethacin and higher molecular weight acids which was interpreted in terms of diffusion limitation of sensitizer penetration. Overall, it seemed that weak-acid sensitization to acid killing is a general phenomenon that occurs not just for oral bacteria but also for organisms in food, soil, and other acidified environments.