Urea concentration in minor mucous gland secretions and the effect of salivary film velocity on urea metabolism by Streptococcus vestibularis in an artificial plaque

Our purpose was to determine the urea concentration in minor mucous gland (MMG) secretions and the pH at proximal and distal aspects of the lower surface of artificial plaque in vitro during infusion of urea solutions over the surface, at different film velocities. Saliva is present in the mouth as a slowly moving film (ca. 0.1 mm thick) with an estimated velocity in the range of 0.8–8.0 mm/min. At low velocities, due to the accumulation of bacterial products, a progressive increase in their concentration may occur in both the plaque and the overlying salivary film at the distal edge (where the film leaves the plaque). S. vestibularis, an oral micro‐organism possessing ureolytic activity, was combined with 1% agarose, to give a urease Vmax similar to that of natural plaque. The artificial plaque was in the chamber (6.0 × 6.0 square and 0.5 or 1.5 mm deep) of a diffusion apparatus, and a urea‐containing artificial saliva (3.3 or 13.2 mmol/ 1) was infused over the surface, as a film 0.1 mm deep, at velocities of 0.8, 8.2 and 86.2 mm/min. At the lower (physiologically normal) urea concentration and the two lower film velocities, most urea appeared to be metabolized at the proximal end of the plaque, which developed a higher pH. At the higher urea concentration, and a film velocity of 8 mm/min, a higher pH was found at the distal end. This was probably due to the combination of greater urea availability and a reduced rate of ammonia loss distally. At a film velocity of 86.2 mm/min, proximal/distal pH gradients did not develop. Thus the salivary film velocity may influence the pH and ammonia level in plaque and perhaps the development of gingivitis. In unstimulated and stimulated MMG secretions, which are the secretions in contact with the buccal surfaces of most teeth, the urea concentrations were 5.27 ± 1.07 and 4.38 ± 1.51 mmol/1, respectively