The Impact of the pH Value on Biofilm Formation

The pH value of a biofilm influences the pathogenesis and therapy of oral diseases such as caries and periodontitis. This study aimed to investigate the influence of different initial pH values on the microbial composition, bacterial counts, metabolic activity, and quantity of three defined biofilms representing oral health, caries, and periodontal disease. Respective bacterial suspensions in the nutrient broth were initially adjusted to pH values between 5 and 8. Then biofilms were cultured on polystyrene surfaces coated with a proteinaceous solution for 2 h ("healthy" biofilm), 6 h ("healthy," and "cariogenic" biofilms), 24 h ("cariogenic," and "periodontitis" biofilms), and 48 h ("periodontitis" biofilm). In all biofilms, total bacterial counts were lower at an initial pH of 5 or 5.5 than at higher pH values. In the biofilm representing caries, the percentage of cariogenic bacteria (Streptococcus mutans, S. sobrinus, Lactobacillus acidophilus) was higher at a low pH, the metabolic activity was highest at pH 6-6.5, and biofilm mass was greatest at pH 7-7.5. In the biofilm representing periodontitis, the percentage of Porphyromonas gingivalis increased with the pH. Also, the metabolic activity was highest at pH 8, whereas mass had the highest value at pH 7. In conclusion, the initial pH value influences biofilm formation. In particular, metabolic activity and the amount of bacteria associated with disease correlated with the respective pH known to be of importance in the development of caries (relatively low pH) and periodontitis (higher pH). Modifying the pH level in oral biofilms might be an alternative concept in (primary) prevention and treatment, not only of caries but also of periodontitis.