The source of variation in ureolysis in artificial plaques cultured from human salivary bacteria

Artificial-mouth plaques were cultured for 7 days from the saliva of two individuals, one with high and one with low salivary ureolytic activity. There was a 7.5-fold range in the resulting plaque ureolysis rates (per mg of protein), and the composition of the flora varied widely. The average rate of ureolysis of artificial plaque was similar to that in natural plaques but higher than in salivary sediment (after correction of sediment rates for the presence of 50 per cent non-bacterial protein). The average rate of ureolysis per ureolytic bacterium was 2.5 times higher in the artificial plaques than in saliva. Although the saliva inocula were from subjects with a 3-fold difference in salivary ureolysis rate, this difference was not reflected in the ureolytic activity of the artificial plaques. Neither was this difference evident in the ureolytic activity of the corresponding natural plaques. The established hypothesis that plaque ureolysis is derived mainly from an unidentified active segment of the total ureolytic flora was tested in the artificial plaques by analysis of variance to determine the contribution of the known ureolytic bacteria. Plaque ureolysis rates were almost entirely explained (r2 = 86 per cent) by the percentage of total detectable ureolytic bacteria in the plaque flora. The plaque bacteria giving strong ureolytic reactions on agar plates were all Gram-positive cocci and in 6 of the 9 plaques were streptococci only. Therefore, in artificial plaques the physiologically significant bacteria comprise a high proportion of the total ureolytic flora and are Gram-positive cocci, mainly streptococci.