Скачать книгу

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].

Img

      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.

      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