Figure 1 shows a microscopic view of a smear of mature supragingival plaque. Note the presence of a variety of different bacterial forms (cocci, rods, filaments, as well as a eukaryotic cell.
I. Dental Plaque: Structural, Microbiological and Developmental Characteristics
II. Dental Plaque: Relationship to Dental Diseases
III. Dental Plaque: Recent Reclassification of Periodontal Microorganisms
Dental plaque is a soft deposit that accumulates on the teeth. Plaque can be defined as a complex microbial community, with greater than 1010 bacteria per milligram. It has been estimated that as many as 400 distinct bacterial species may be found in plaque. In addition to the bacterial cells, plaque contains a small number of epithelial cells, leukocytes, and macrophages. The cells are contained within an extracellular matrix, which is formed from bacterial products and saliva. The extracellular matrix contains protein, polysaccharide and lipids. Figure 1 shows a microscopic view of a smear of mature supragingival plaque. Note the presence of a variety of different bacterial forms (cocci, rods, filaments, as well as a eukaryotic cell.
Inorganic components are also found in dental plaque; largely calcium and phosphorus which are primarily derived from saliva. The inorganic content of plaque is greatly increased with the development of calculus. The process of calculus formation involves the calcification of dental plaque. The practical consequences of calculus formation are that the deposit is significantly more difficult to remove once calcified, and it leaves a rough surface on the root which is easily colonized by plaque. The calculus on the root surface of an extracted tooth is seen in Figure 2. 
Note the brown to black coloration of the subgingival calculus that extends to the apex of the distobuccal root, in contrast to the whitish color of the supragingival calculus.
Dental plaque can be classified in several different ways. Plaque is classified as supragingival or subgingival based on its relationship to the gingival margin. Supragingival plaque is evident on the tooth above the gingival margin 
(Figure 3). Plaque can also be classified by its relationship to the tooth surface, as either attached or unattached plaque. The unattached subgingival plaque is more closely associated with the wall of the subgingival tissues than is the attached plaque. Lastly, plaque has been classified by association with disease state as "health-associated" or "disease-associated". The latter classification is related to differences in the microbial composition of dental plaque in health versus disease.
The development of dental plaque has been studied in humans as well as non-human animal model systems. One of the most commonly used models of plaque development is referred to as the "experimental gingivitis" model (Loe, et al., 1965). This protocol involves the examination of subjects (usually dental students!) who abstain from any oral hygiene measures for a period of three weeks. These studies have provided much information on the structural and microbiological characteristics of dental plaque.
The pellicle is evident as lightly stained material on a tooth surface when patients use disclosing solution (Figure 4).
A newly cleaned tooth surface is rapidly covered with a glycoprotein deposit referred to as "pellicle". The pellicle is derived from salivary constituents which are selectively adsorbed onto the tooth surface. Components of the dental pellicle include albumin, lysozyme, amylase, immunoglobulin A, proline-rich proteins and mucins. The formation of pellicle is the first step in plaque formation.
The pellicle-coated tooth surface is colonized by Gram-positive bacteria such as Streptococcus sanguis, Streptococcus mutans, and Actinomyces viscosus. These organisms are examples of the "primary colonizers" of dental plaque. Bacterial surface molecules interact with components of the dental pellicle to enable the bacteria to attach or adhere to the pellicle-coated tooth surface. For example, specific protein molecules found as part of the bacterial fimbria (hair-like protein extensions from the bacterial cell surface) on both Streptococcus sanguis and Actinomyces viscosus interact with specific proteins of the pellicle (the proline-rich proteins) with a "lock and key" mechanism that results in the bacteria firmly sticking to the pellicle-coating on the tooth surface (Mergenhagen et al. 1987). Within a short time after cleaning a tooth, these Gram-positive species may be found on the tooth surface.
After the initial colonization of the tooth surface, plaque increases by two distinct mechanisms: 1 ) the multiplication of bacteria already attached to the tooth surface, and 2) the subsequent attachment and multiplication of new bacterial species to cells of bacteria already present in the plaque mass. The secondary colonizers include Gram-negative species such as Fusobacterium nucleatum, Prevotella intermedia, and Capnocytophaga species. A key property of these microorganisms appears to be the ability to adhere to Gram-positive species already present in the existing plaque mass. These organisms would typically be found in plaque after 1 to 3 days of accumulation.
Figure 5 shows the accumulation of plaque on a tooth surface one day after prophylaxis. When the structure of dental plaque from this time period is observed, the presence of a complex array of bacterial cocci, rods and filaments is apparent.
After one week of plaque accumulation, other Gram-negative species may also be present in plaque. These species represent what is considered to be the "tertiary colonizers", and include Porphyromonas gingivalis, Campylobacter rectus, Eikenella corrodens, Actinobacillus actinomycetemcomitans, and the oral spirochetes (Treponema species). The structural characteristics of dental plaque in this time period reveal complex patterns of bacterial cells of cocci, rods, fusiform, filaments, and spirochetes. In particular, specific associations of different bacterial forms have been observed. For example, the adherence of cocci to filaments results in a typical form referred to as "test-tube brushes" or "corn-cob" arrays and these structures can be seen in Figure 6. 
The structural interactions of the bacteria probably are a reflection of the complex metabolic interactions that are known to occur between different plaque microorganisms. One example of this is the production of succinic acid from Campylobacter species that is known to be used as a growth factor by Porphyromonas gingivalis. Streptococcus and Actinomyces species produce formate, which may then be used by Campylobacter species. Fusobacterium species produce both thiamine and isobutyrate that may be used by spirochetes to support their growth. The metabolic and structural interactions between different plaque microorganisms are a reflection of the incredible complexity of this ecological niche.
The overall pattern observed in dental plaque development is a very characteristic shift from the early predominance of Gram-positive facultative microorganisms to the later predominance of Gram-negative anaerobic microorganisms, as the plaque mass accumulates and matures. This developmental progression is also reflected in the shifts in predominant microorganisms that are observed in the transition from health to disease. Studies of plaque taken from sites of health or disease and examined either microscopically or by culturing have demonstrated distinct differences in health versus disease-associated microbial populations.
Microscopic studies of plaque have examined the presence of different morphological types ("morphotypes") of bacteria. 
Figure 7 is a Gram-stained smear of mature subgingival plaque illustrating different morphotypes (cocci, rods, filaments, spirochetes). These studies reveal an increase in the presence of motile rods and spirochetal organisms in gingivitis and periodontitis as compared to gingival health. A major limitation of studies of bacterial morphotypes is that many "health-associated" microorganisms are indistinguishable from "disease-associated" microorganisms (for example, Streptococcus species and Porphyromonas gingivalis, respectively). However, cultural studies also reveal characteristic distinctions between health- and disease-associated plaque. The percentage of Gram-positive rods and cocci decrease in gingivitis- and periodontitis-associated plaque as compared to health-associated plaque. Similarly, the percentage of microbiota compised of Gram-negative anaerobic species is greatly increased in gingivitis (approximately 25) and periodontitis (approximately 75) as compared to health (approximately 13, Slots, 1979). Specific microbial species that are important in plaque development and disease development are outlined below based on their categorization by cell wall morphology (Gram-positive, Gram-negative, or spirochetal) and their physiological status (facultative or anaerobic).
| SELECTED BACTERIAL SPECIES FOUND IN DENTAL PLAQUE | ||
| Facultative | Anaerobic | |
| Gram-Positive | Streptococcus mutans Streptococcus sanguis Actinomyces viscosus | |
| Gram-negative | Actinobacillus actinomycetemcomitans Capnocytophypa species Eikenella corrodens |
Porphyromonas gingivalis Fusobacterium nucleatum Prevotella intermedia Bacteroides forsythus Campylobacter rectus |
| Spirochetes | Treponema denticola (Other Treponema species) | |
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