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Total counts of bacteria within each biofilm (log₁₀ CFU) differed significantly between the initial pH values at 2 and 6 h (each p < 0.001; Fig. 1a). At 2 h, the lowest values were counted at pH 5 and 5.5 with approximately 5.8 log₁₀ CFU, while the highest counts were detected at pH 7 with a mean of 6.6 log₁₀ CFU. Post hoc analysis showed differences between pH 5, 5.5, 7.5, and 8 on the one hand, and pH 7 on the other (each p < 0.01). At 6 h, the lowest CFU counts remained at 5.7 log₁₀ when the initial pH was 5, the highest value increased to 7.3 log₁₀ (pH 7.5). Also at 6 h, the CFU count differences were each statistically significant between pH 5, 5.5, and 7 (each p < 0.01).

Fig. 1. Total bacterial counts (mean and SD; a), bacterial composition (b), metabolic activity (mean and SD; c), and biofilm mass (mean and SD; d) of the two-species biofilm representing oral health in relation to the initial pH. Differences versus pH 7 are presented: * p < 0.05, ** p < 0.01 (the asterisk colors correspond with the composition of biofilms in b).

At 2 h, the quantity of the two species did not differ significantly between the different pH levels. At 6 h, there was a significantly higher percentage of A. naeslundii and a lower percentage of S. gordonii at pH 5 in comparison with pH 7 (each p < 0.001; Fig. 1b).

The metabolic activity differed between the different pH values at 2 and 6 h (each p < 0.001; Fig. 1c). At both time points, the lowest activities were at pH 5 and 5.5, each with a statistically significant difference to pH 7 at 2 h (each p < 0.05) and at 6 h (each p < 0.01).

Regarding biofilm mass, no statistically significant difference from the initial pH values was found, either at 2 or 6 h (Fig. 1d). The pH measurements performed at 6 h still showed a gradient from pH 5.15 (initially pH 5) to pH 7.05 (initially pH 8; Table 2).

Table 2. Initial pH values and values (mean ± SD) in the surrounding media after 6 h of formation of the two-species biofilm representing “oral health”

Fig. 2. Total bacterial counts (mean and SD; a), bacterial composition (b), metabolic activity (mean and SD; c), and biofilm mass (mean and SD; d) of the five-species biofilm representing caries in relation to the initial pH. Differences versus pH 7 are presented: * p < 0.05, ** p < 0.01 (the asterisk colors correspond with the composition of biofilms in b).