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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.

© 2021 S. Karger AG, Basel

Two major oral diseases, caries and periodontal disease, are associated with biofilm formation on teeth. Caries is a disease affecting dental hard tissues. A collection of different microorganisms in a biofilm produces acids, decreases the pH, and moves the balance between demineralization and remineralization towards demineralization. The acidogenic stage is followed by an aciduric stage where microorganisms (including mutans streptococci) are still able to survive at low pH values [1]. Treatment options that increase the pH within supragingival biofilms are currently under discussion. For example, arginine as a supplement of toothpaste aims to modify the microbial biofilm composition to be less acidogenic [2].

Table 1. Bacterial strains used in the experimental biofilms

Periodontitis is a disease characterized by the destruction of the tissue surrounding teeth and, if untreated, ultimately leads to tooth loss. Among the many different bacteria found in subgingival biofilm, Porphyromonas gingivalis was postulated as being a keystone pathogen in the development of periodontitis by modifying the host response and finally the composition of the biofilm [3]. A recent analysis in adolescents showed that the severity of periodontitis was inversely associated with enamel caries, whereas the extent of periodontitis (numbers of affected sites) showed a positive association with dentin carious lesions [4]. After initial periodontal therapy, periodontopathogenic bacteria decrease while Streptococcus mutans increases [5]. This suggests a dependency of the composition of the oral biofilm on the surrounding micromilieu. One important factor might be the pH value.

The aim of this study was to investigate the influence of initially different pH values on biofilm formation. For this, nutrient media containing buffers were adjusted to pH values in a range from 5 to 8 in increments of 0.5. Defined microbial strains representing oral health, caries, and periodontal disease were studied for their ability to form biofilms regarding bacterial counts, microbial composition, biofilm mass, and metabolic activity.