increased in that biofilm. A. naeslundii seemed to be less sensitive to pH 5, but this was probably a short pH window that allowed multiplication of that species. At a lower pH (below pH 4) it was shown that A. naeslundii was killed, but demonstrated greater acid resistance when existing within a biofilm [16]. Interestingly, an initial pH of 5 or 5.5 also decreased the formation of the “cariogenic” biofilm. It can be assumed that biofilm formation begins at slightly higher pH values and the low pH leading to dissolution of enamel is a result of the bacterial metabolism in the deep layer of an advanced biofilm. This supports the different stages of the ecological caries theory [1]. In our study pH was only determined in the surrounding media, while differences within a biofilm are likely. pH values were measured both inside and outside of a single-species biofilm. With pH control (buffering of the media), the pH within the biofilm was about 6.1 and the pH in the surrounding media was 6.7 [17]. Thus, a “cariogenic” biofilm does not have a consistent pH. A recent study measuring pH values in different areas of an in vitro biofilm after sucrose-mediated biofilm formation found acidic regions (below pH 5.5) only in the interior of microcolonies, and in vivo analysis confirmed a spatial heterogeneity of pH, with acidic pH values only close to the enamel surface [18]. The metabolic activity of bacteria depends on the pH value. The expression of glucosyltransferases, which are major genes of S. mutans involved in biofilm formation, was demonstrated to be higher without pH control than in controlled conditions [17]. Also, the competence of S. mutans to internalize DNA from the environment depends on the environmental pH – it is only possible at a pH of around 7 [19]. The biofilm model focused only on enamel caries, and did not consider either root or dentine caries, which involve proteolytic bacteria that contribute to the proteolytic stage of the diseases [20].
A “periodontitis” biofilm is more metabolically active and has a higher quantity at slightly alkaline pH values. The pH of the gingival crevicular fluid increases in vivo from 6.9 to higher values with the severity of inflammation [21]. It was measured to be around pH 8.4 at inflamed sites and it was shown that the bicarbonate buffer system contributes most at that pH value [22]. Measuring the pH in periodontal pockets found levels below 7.0, with lower values in the case of acute inflammation, and alkaline values when the inflammation became chronic [23]. P. gingivalis growth is reduced at an acidic pH [24], which is in accordance with our results showing lower total counts and percentages in the in vitro biofilm assays. P. gingivalis is associated with periodontitis, whereas T. forsythia and T. denticola are present both in gingivitis and periodontitis [25]. In the present in vitro study, the “periodontitis” biofilm had a higher percentage of T. forsythia and T. denticola at lower pH values.
Fig. 4. Effect of different initial pH values on biofilm formation (quantity and metabolic activity), microbial composition, and the pH tendency in the environment of oral biofilms in general (grey), “caries” biofilms (blue), and “periodontitis” biofilms (purple), as well as possible associations with oral disease.
The dependency of biofilm formation on pH offers therapeutic options, and an increase of pH is of interest for preventing caries [26]. In recent years a major focus has been on arginine deiminase, an enzyme synthesized by several oral streptococci which utilizes arginine for alkali production [27]. Conversely, in therapy of periodontal disease a decrease of pH may be beneficial. In general, a relatively low pH seems to be favorable for wound healing by promoting an immune response [28]. However, this should be carefully balanced in periodontal therapy. In monkeys it was shown that short-term etching of root surfaces with a 37% orthophosphoric acid led to more connective tissue and a shorter epithelial junction, whereas a long-term etching for 3 min clearly impaired periodontal healing [29]. Also, the activity of antimicrobials can be affected by the environmental pH. For example, chlorhexidine is more active at an alkaline pH and hypochlorite at a more acidic pH [28]. Thus, investigating the influence of the pH value in periodontal therapy in more detail might be an interesting topic for further research.
Future in vitro research into the influence of pH values on oral biofilm formation should consider more complex models with more microorganisms and a different nutrient supply. More knowledge is needed about the influence of different pH values on the expression of important genes involved in biofilm formation and virulence.
In conclusion (Fig. 4), an initially low pH (pH 5 and 5.5) may suppress biofilm formation and could be associated with the development of erosive tooth wear. A slightly higher pH (around pH 6) might favor the development of caries if respective nutrients (sugar) are available. A pH slightly less than 7 appears to be associated with a higher percentage of Tannerella sp. and Treponema sp., an acute gingivitis, and an increase in the percentage of aciduric bacteria. An initial slightly basic pH contributes to a biofilm associated with periodontitis, and in particular P. gingivalis. Therapeutics leading to a high pH may be helpful in the prevention of caries. In periodontitis, modulation of the pH could be an alternative option in therapy but should also include caries-preventive measures.
Acknowledgements
The authors acknowledge the technical support by Anna Magdoń and Prashanthnj Sivapatham, Department of Periodontology, Laboratory of Oral Microbiology, School of Dental Medicine, University of Bern.
Conflict of Interest Statement
The authors have no conflicts of interest to declare.
Funding Sources
There was no funding in relation to this work.
Author Contributions
S.E. was responsible for the study design. L.B.S. and A.M. acquired and analyzed the data. S.E., A.S., and A.L. interpreted the data. S.E., L.B.S., and A.M. drafted the work. A.L. and A.S. revised it critically for important intellectual content. All authors approved to the version to be published and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
References
1Takahashi N, Nyvad B: Caries ecology revisited: microbial dynamics and the caries process. Caries Res 2008;42:409–418.
2Nascimento MM, Browngardt C, Xiaohui X, Klepac-Ceraj V, Paster BJ, and Burne RA: The effect of arginine on oral biofilm communities. Mol Oral Microbiol 2014;29:45–54.