Relevance of biofilms in the oral cavity in the formation of dental plaque, caries and gum disease
Introduction to Biofilms
Biofilms are communities of microorganisms that grow on many solid environmental surfaces, including the human body  . Biofilms are highly organized and are formed by the initial adherence of free floating single cells, termed planktonic cells, to a surface, followed by recruitment of additional cells, multiplication and growth . Extracellular matrix secreted by microorganisms in biofilms provide structural integrity, holding microbial communities together as masses . Microorganisms within biofilms communicate through the mechanism of quorum sensing, by which secreted signaling molecules called autoinducers regulate group activity, at the level of gene expression . Microorganisms contained in biofilms are subject to various benefits including increased resistance to antimicrobial agents, phagocytosis by immune cells, and physical removal  . In contrast, planktonic cells are highly susceptible to many of these factors .
The Oral Cavity Environment
One extensively studied biofilm in the human body is dental plaque in the oral cavity. The oral cavity normal flora is highly diverse, containing over 700 different species . Conditions in the mouth such as warmth, moisture and neutral pH promote the growth of many microorganisms . The oral cavity consists of tissues that can be shed and replaced, but stable structures like teeth allow microorganisms to adhere and form dental plaque biofilms .
The tooth consists of smaller sub-environments where certain species will colonize and predominate based on their metabolic requirements . On the tooth surface, supragingival plaque consists of aerobic species that cause caries, while below the tooth surface, subgingival plaque consists of anaerobic species that cause periodontal disease  .
Formation of Dental Plaque, Caries and Gum disease
Formation of Dental Plaque
The formation of dental plaque occurs in the general stages: lag phase, growth phase, and steady state .
Glycoproteins present in saliva bind to the surface of teeth forming a structure called the pellicle . The pellicle acts as a binding site for further attachment of organisms . Species including Streptococcus, Actinomyces, Haemophilus, Neisseria and Veillonella, most of which are Gram positive, are often primary colonizers of the pellicle .
Species secrete extracellular matrix to hold the community together . Secondary colonizers join the biofilm by attaching to primary colonizers in a process called co-aggregation (1, 4). Many secondary colonizers depend on metabolic products formed by primary colonizers, for instance, Tanerella depends on previously made N-acetylmuramic acid . Biofilm composition shifts from abundant Gram positive species to predominantly Gram negative species . Examples of secondary colonizers include Gram negative Porphyromonas gingavalis and Fusobacterium nucleatum . Secondary colonizers increase the diversity of the plaque, and maturation and growth of the biofilm occurs (1,4).
Detachment /Steady State
Biofilm growth slows or stops and bacterial cells can detach and freely colonize new sites .
Streptococcus mutans is the main species in dental plaque responsible for tooth decay, or dental caries . When sugars are consumed in diet, carbohydrate fermenting bacteria produce lactic acid and related products, lowering the local pH in the mouth . S. mutans, S. sanguinis and Lactobacilli grow well under acidic conditions, and also ferment carbohydrates producing acid products, further dropping the pH, inhibiting the growth of other bacterial species (2,4,8). Acid production results in demineralization, the loss of minerals in tooth enamel, resulting in dental caries (4,7).
Gingivitis occurs when microorganisms in mature dental plaque or their produced products are either in contact with or enter gingival tissue (1,4). Recognition of these compounds by resident immune cells in gingival tissue results in the recruitment of phagocytes and cytokines to fight infection, resulting in inflamed tissue . Dental plaque that is not removed regularly hardens to form calculus (tartar) that physically irritate gingiva, promoting inflammation . In gingivitis, Gram negative Actinomyces species predominate over Gram positive Streptococcus species .
Progression of gingivitis leads to chronic infection known as periodontitis (4, 7). In contrast to gingivitis, damage and loss of the tissue supporting the teeth occur in periodontitis . Porphyromonas, Actinobacillus, Treponema, Selenomonas, Fusobacterium and Tannerella are common secondary colonizers, most of which are Gram negative, associated with periodontitis (4,7). P. gingivalis can enter gingival tissue and secrete proteases and other destructive enzymes damaging the tissues .
Removal of dental plaque requires disruption of the biofilm . The extracellular matrix of dental plaque keeps cells strongly associated even in the presence of fluid movement from saliva . Disruption of biofilms can be achieved mechanically through brushing one's teeth, or chemically, where antimicrobial agents penetrate the plaque to inhibit bacterial enzyme function . Chemical disruption is the basis for mouthwashes and requires the concentration of the chemical to be above a certain minimum bactericidal concentration (1,3). Disruption of the biofilm results in planktonic cells, which are far more susceptible to cell death . Mouthwashes containing essential oils like methyl salicylate, such as Listerine, or mouthwashes with 0.12% chlorhexidine are particularly effective (10,11).
Impact on Human Health
Dental caries and periodontal disease caused by biofilms are the most common chronic infections humans encounter . Biofilms in the oral cavity are however, important in human health because the normal flora biofilms that colonize the mouth occupy adherence sites, preventing the colonization of pathogens that may harm the host . Although the biofilm benefits the host in some respect, over-accumulation of dental plaque can lead to caries and gum disease, therefore it is important to achieve a balance by proper oral care .
Currently, it is shown that there is a link between oral health and systemic conditions such as diabetes, respiratory disease and cardiovascular disease (1,7). It is believed that pathogens, particularly Gram negative anaerobes such as P. gingavalis, can enter gingival tissue during periodontal inflammation, travel through the circulatory system to other sites of body, and establish infection at new sites . For further information on the importance of oral health please see Health Canada's webpage.
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