There are six stages of plaque biofilm development (Figure 3).
Stage 1: Formation of an acellular layer. Called the acquired pellicle, this layer of salivary glycoproteins, phosphoproteins, and lipids, but no bacteria, forms almost immediately on naked enamel surfaces.
Stage 2: Initial attachment. Free-floating early colonizers of the teeth, such as Streptococcus sanguinis, which are normal inhabitants on the mouth, form an initial attachment to the pellicle by weak and reversible van der Waals forces. If these bacteria are not removed, they eventually anchor themselves with adhesive structures, such as pili.
Stage 3: Irreversible attachment. Organisms that were unable to attach to the pellicle begin to adhere to the first layer of colonizers with irreversible attachments via specific adhesion-receptor interactions. The bacteria replicate and form microcolonies embedded in an extracellular matrix.
Stage 4: Early Maturation (also called Maturation I). As a result of the previous steps in which bacteria form attachments, early colonizers become established. This leads to increased dental plaque complexity due to allogenic factors, such as oxygen consumption within plaque creating anaerobic zones, food chains becoming established, and an increased range of receptor sites for bacterial attachments. Cell division and recruitment of new bacteria also allows the bacterial population to increase.
Stage 5: Late Maturation (also called Maturation II). In this stage, microbial diversity continues to increase, while rates of cell division decrease. The heterogeneous nature of plaque becomes apparent as a mosaic of microenvironments develop, particularly areas of different pH, oxygen concentrations, and secondary metabolite accumulations around and within microcolonies. The plaque microbial ecology reaches a pseudo-steady-state climax community, where there is a constant turnover of cells, but the overall composition remains roughly the same. At this point, a thick, three-dimensional layer of dental plaque biofilm has formed.
Stage 6: Dispersion. Enzymes that degrade the biofilm (such as dispersin B) allow some bacteria to detach themselves from the biofilm–sometimes in response to deleterious environmental conditions–in order to spread and colonize new surfaces in the oral cavity.
Figure 3. Development of Dental Plaque.
Confocal microscopy images of plaque growth showing the timescales involved (albeit in a laboratory model).
24h: monolayer of cells and microcolonies (stages i to iv).
48h: microcolonies grow and develop a 3-dimensional structure, seen in the cross-section at the bottom of the image (stages iii and iv).
96h: the entire surface is covered (stages iv and v).
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