In-vitro urea-dependent pH-changes by human salivary bacteria and dispersed, artificial-mouth, bacterial plaques

The pH effects of urea metabolism were studied in washed salivary-sediment bacteria from subjects that had up to 10-fold variation in oral ureolytic activity, and in dispersed artificial-mouth plaques. Adequate evaluation required analysis of the [OH-] as well as the pH curve. An initial constant rate of pH-change, lasting until pH 7.8, was derived from the pH curve; this gave the best correlation (r = 0.95) with the ureolysis rate. From the [OH-]-curve, between pH 7.8 and 8.3 (approx.), a constant and maximal rate of change in [OH-] was determined. Although theoretically this was directly related to the rate of ammonia release, it was 10(-2) to 10(-3) times its value and correlated less well (r = 0.83) with ureolysis. Together with the initial and final pH, these two rates largely described urea-induced pH changes. After 12.5-fold dilution of the cells, changes in the pH curve were minor. Although the rate of ureolytic ammonia release was proportional to cell-protein concentration, the reduction in ureolytic activity was compensated by a corresponding reduction in cell pH-buffering. Consequently, in order to relate pH and [OH-] changes to ureolysis, it was necessary to control, or correct for, variations in the cell mass present. Buffering capacity in plaques was greater than in sediments. The 10-fold range in oral ureolytic activity by salivary bacteria gave a 10-20-fold range in base changes.