pathogenesis of the most important diseases associated with microorganisms (periodontitis and caries) differs. The following overview of biofilm models used in laboratories is structured according to the main focus of the study.
General Oral Biofilm Models
These models have been used to evaluate antimicrobial therapy (mainly oral healthcare products) on oral biofilms in general. Defined microorganisms or saliva as the microcosm source are used. Biofilms are generally formed on glass or hydroxyapatite (HA) disks, in part coated with saliva (Table 1).
Table 1. Examples of biofilm models with a more general focus on the therapy of oral diseases
Table 2. Biofilm models with focus on the therapy and the prevention of caries
Caries Biofilm Models
Models often use Streptococcus mutans as one of the main species associated with caries. Surfaces used for biofilm formation are glass, polystyrene HA disks, and enamel slices. When enamel specimens are applied surface hardness loss is often determined, but in part polysaccharides and metabolic activity are also measured (Table 2).
Endodontic Biofilm
To evaluate in vitro treatment options, most extracted teeth/roots are contaminated with a defined mixture of bacteria. Even if the samples were incubated for a longer time period, the term “biofilm” is only rarely used. In addition to the root canals, the surfaces for biofilm formation are often HA disks. Most single-species biofilms of Enterococcus faecalis are formed (Table 3).
Periodontal Biofilm Models
Different biofilm models have been introduced to evaluate periodontal therapy. Rarely, single species biofilms (Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis) are used. More often a mixture of defined bacteria (2–40 different species) is cultured on polystyrene or HA surfaces coated with saliva or serum. Microcosm biofilms originate from supra- and subgingival plaque or from saliva (Table 4).
Peri-Implant Biofilm Models
According to the importance of peri-implant diseases, biofilms formed on dental implant materials (titanium, zirconium oxide) have been introduced in recent years (Table 5).
Table 3. Biofilm models with focus on endodontic infections
Table 4. Biofilm models with focus on the therapy of periodontal diseases
Table 5. Biofilm models with focus on the therapy of peri-implant diseases
Table 6. Biofilm models with focus on the therapy of Candida spp. infections
Yeast Biofilm Models
Yeast infections are often associated with denture stomatitis. Accordingly, potential treatment options are evaluated in vitro using biofilms consisting of Candida sp. mostly formed on acrylic surfaces (Table 6).
Comparison of Own Biofilm Models
In our own laboratory, different periodontitis biofilm models have been used to evaluate therapy. In several studies, chlorhexidine digluconate formulations were applied mostly as a comparative. Four species were applied in a continuous flow system for 24 h before chlorhexidine in different formulations was applied for 1 min [19]. In static models, 6 or 12 species or a microcosm was used to form biofilms on polystyrene surfaces [13, 20]. Surprisingly, total counts of the untreated biofilms (CFU) did not differ much, with values between log10 CFU = 7.21 and log10 CFU = 7.68. After flushing the 4-species biofilm with 0.1% chlorhexidine solution, no colony was growing anymore, and application of a 1% gel reduced the CFU counts by 3.47 log10 CFU [19]. When applying 0.1% chlorhexidine digluconate for 1 min and 0.01% chlorhexidine digluconate for 18 h in the 6-species model, the reduction was 4.55 log10 CFU [20] (Fig. 1). Using the microcosm biofilm and forming the biofilm for 10 days with application of 0.1% chlorhexidine gel for 1 min and diluting to 10% for 1 h, the reduction was 0.64 log10 CFU [13]. Using dentine discs as the surface, the number was 5.95 log10 CFU. Applying a chlorhexidine-containing air-polishing powder for 10 s reduced the CFU counts by 4.06 log10 CFU [21]. The results suggest that a biofilm-reducing activity was shown in each of the different models; however, the extent of reduction seemed to be depended not only on the concentration and the antimicrobial formulation, but also on the biofilm model used.
Fig. 1. SEM images of a 5-day-old 6-species biofilm without exposure to antimicrobials (control, a) and exposed to 0.1% chlorhexidine digluconate for 1 min and 0.01% chlorhexidine digluconate for 18 h (b). (Photographs taken by Sandor Nietzsche, Center of Electron Microscopy, University Hospital of Jena, Jena, Germany.)
Conclusion
All the models are helpful in evaluating new antimicrobial treatment options. Conclusions regarding the tendency of the antimicrobial activity of the therapeutics can be drawn. However, there are limitations to the models and the final proof of a new therapy has to be determined in randomized controlled clinical trials.
Conflict of Interest Statement
The author has no conflicts of interest to declare.
Funding Sources
There is no funding to declare