Acid-induced acid tolerance and acidogenicity of non-mutans streptococci

Acid tolerance and acidogenicity of non-mutans streptococci and their capac ity of acid adaptation were studied. The cells of non-mutans streptococci ( Streptococcus sanguis [Streptococcus sanguinis], Streptococcus gordonii, St reptococcus oralis and Streptococcus mitis) grown at pH 7.0 showed 0.0088% to 71% viability after acidification at pH 4.0 for 60 min, whereas the cell s of mutans streptococci (Streptococcus mutans) were not killed by the acid ification. Washed cells of non-mutans streptococci lowered pH to 4.04-4.33 in the presence of glucose, while mutans streptococci cells lowered pH to 3 .70. When the growth pH was shifted to 5.5 for 30-90 min, the viability of non-mutans streptococci after the acidification at pH 4.0 for 60 min increa sed (0.25% to 91%) and the minimum pH values of the cells in the presence o f glucose decreased (3.90-4.19). Along with the increase in acid tolerance and acidogenicity, non-mutans streptococci increased activities of H+-ATPas e and arginine deiminase and amounts of stress proteins cross-reacting with 60 kDa and 70 kDa heat shock proteins (Hsp60 and Hsp70). These results ind icate that non-mutans streptococci were capable of increasing their acid to lerance and acidogenicity in response to environmental acidification. Furth ermore, it is suggested that the acid adaptation observed in non-mutans str eptococci cells could involve the induction of H+-ATPase, arginine deiminas e and stress protein syntheses. The strains of non-mutans streptococci, whi ch are pioneer bacteria for dental plaque formation and predominant in plaq ue microbial flora, may play a significant role in shifting the dental plaq ue environment toward acidic and consequently promoting the colonization of more acid-tolerant and acidogenic bacteria such as mutans streptococci and lactobacilli.