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Oral biofilms are also more tolerant to the action of antimicrobials than planktonic bacteria. For example, a 50-fold MIC (minimal inhibitory concentration) of doxycycline and a 100-fold MIC of metronidazole against planktonic bacteria are required to be active against single species biofilms of P. gingivalis [52]. Still, mechanical removal of an oral biofilm associated with an oral disease is the gold standard in treatment, hence scaling and root planing as an anti-infective and anti-inflammatory therapy of periodontitis [53], or toothbrushing twice a day or more to prevent caries [54]. Still, there is a need for better methods to remove existing biofilms. In recent years new instrumentation treatment options, including certain lasers (e.g., Er:YAG), ultrasonication, or air polishing with special powders, have been introduced for the dentist. These options may combine more efficiency in biofilm removal with less damage of the natural tooth surface and better patient acceptance [55–57].

Novel approaches to overcome the antimicrobial inactivity against biofilms might be the targeted destruction of biofilm matrix molecules or the modification of the oral biofilm. Focusing on the dispersion of the biofilm matrix, enzymes might be of interest to destroy proteins or glucans, but their use is limited by a sensitivity to proteolysis or by limited penetrating ability [58]. A promising target seems to be eDNA; for example, sodium hypochlorite was shown in vitro to destroy eDNA as a component of the biofilm matrix (Fig. 4) [59]. Also, hydrogen peroxide simultaneously degraded the extracellular matrix and killed bacteria within an S. mutans biofilm [60].

Fig. 4. DNA staining of an in vitro-formed multi-species biofilm without (a) and with (b) exposure to 0.1% chlorhexidine digluconate solution, or 1% sodium hypochlorite solution (c) for 1 min.

Modification of the oral biofilm can be achieved by the application of probiotics – viable bacteria which are beneficial. Probiotics were shown to reduce early childhood caries when applied to milk [61] and in periodontal therapy they may help to reduce deep periodontal pockets; however, a long-term colonization within biofilms does not seem to occur [62]. Biofilms can also be modified by applying substances affecting the metabolism of bacteria; arginine, which is utilized by certain oral streptococci, increases the pH in a cariogenic biofilm, thus suppressing mutans streptococci [63].

In conclusion, biofilms represent a major form of microbial life in the oral cavity. Although knowledge has increased enormously over the last decade, more research is needed about the composition of the biofilm matrix in oral diseases and the functionality of the microbiome to develop efficient strategies to combat pathogenic biofilms.