Environmental pH as a factor in the competition between strains of the oral streptococci Streptococcus mutans, S. sanguis, and "S. mitior" growing in continuous culture

Abstract

Strains of Streptococcus mutans (biotype 1), Streptococcus sanguis, and Streptococcus mitior have been grown in mixed continuous culture in a semidefined medium under glucose limitation at a growth rate of D = 0.1 h−1. The effect of varying the environmental pH on the proportions of the different populations within the community has been determined. Initially the populations were allowed to reach steady state at pH 7.0 when S. sanguis was dominant with S. mutans and "S. mitior" maintaining similar populations. The medium pH was then lowered in steps of 0.5 pH units from pH 7.0 to 4.5, and the community was grown at each step for at least 15 generations. Viable counts of each species were made at 24-h intervals. The population ratios established at pH 7.0 remained relatively stable when the environmental pH was set at pH 6.5. However, after the medium pH was lowered to 6.0 (days 18–27), the population of S. mutans began to increase and the S. mitior population began to decline. A further change was seen at pH 5.5 (days 27–34) when S. mutans became dominant, S. sanguis declined, and S. mitior was not detectable. At pH 4.5, both S. mutans and S. sanguis were reduced in numbers, but survived until the experimental run was terminated (44 days). Samples of culture fluid were taken throughout the experiment and analyzed for the presence of the acid products of glucose metabolism. The amounts of lactic acid produced by the community increased as the environmental pH was lowered. The results show that there was variation in the ability of Streptococcus species to compete in acidic environments. Streptococcus mutans is most favoured by an environment below pH 6.0, while "S. mitior" is relatively sensitive to low pH, therefore a low environmental pH can be selective for S. mutans. Comparison of these results to two similar studies carried out recently suggests that there is variation in the ability of given strains of S. sanguis and "S. mitior" to compete with S. mutans in acid environments. Therefore, it is not valid to take data from studies of a single strain and interpret it as applicable to the species as they are presently defined.