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 a nd sorbate, were found to be sensitizers for acid killing of cells of Actin omyces naeslundii ATCC 19246 and Streptococcus sanguis NCTC 10904 in suspen sions or in mono-organism biofilms on glass slides. These bacteria are amon g 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 resist ant than cells in suspensions, especially in terms of the fraction of the i nitial population surviving acidification. The mechanism for sensitization to acid killing by fluoride and the other weak acids involved enhanced tran smembrane transport of protons, reflected by increases in measured proton p ermeabilities 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 indome thacin and higher molecular weight acids Which was interpreted in terms of diffusion limitation of sensitizer penetration. Overall, it seemed that wea k-acid sensitization to acid killing is a general phenomenon that occurs no t just for oral bacteria but also for organisms in food, soil, and other ac idified environments.