ACID TOLERANCE RESPONSE AND SURVIVAL BY ORAL BACTERIA

Using 21 species of oral bacteria, representing six acidogenic genera, we undertook to determine whether the pH-limiting exponential growth is related to the ability of the organisms to generate an acid-toleran ce response that results in enhanced survival at low pH. The lower pH limit of exponential growth varied by more than two units with that of Neisseria A182 at pH 6.34; growth of Lactobacillus casei RB1014 stopp ed at pH 3.81, with species of Actinomyces, Enterococcus, Prevotella a nd Streptococcus falling between these limits. The working hypothesis was that the organisms with the higher pH limits for growth are unable to respond to acidic environments in order to survive, whereas the mo re aciduric organisms would possess or acquire acid tolerance. Adaptat ion to acid tolerance was tested by determining whether the prior expo sure of exponential-phase cells to a low, sub-lethal pH would trigger the induction of a mechanism that would enhance survival at a pH killi ng pH 7.5 control cells. The killing pH varied from pH 4.5 for Prevote lla intermedia ATCC 25611 to pH 2.3 for the three Lactobacillus casei strains in the study, with the three Streptococcus mutans strains kill ed at pH 3.0 for 3 h. The adaptation experiments revealed three groups of organisms: non-acid-responders, generally representing strains wit h the highest terminal pH values; weak acid-responders in the middle o f the pH list, generating low numbers of:survivors at one or two pH va lues, and the aciduric, strong responders generating a high number of survivors at pH values in the range 6.0 to 3.5, but not at pH 7.5. Pre dominant among the latter group were the S. mutans and Lactobacilli ca sei strains, with the most significant adaptive response exhibited by S. mutans LT11 and S. mutans Ingbritt, involving a process that requir ed protein synthesis. Time course experiments with the latter organism s indicated that 90-120 min was required after exposure to the trigger ing pH before the acid response was fully functional. These results in dicate that the sudden exposure of strains of oral streptococci and la ctobacilli, as well as Enterococcus faecalis, to pH values between 6.0 and 3.5 results in the induction of an acid tolerance response that e nhances the survival of these strains at or below pH 3.5.