According to Dawes et al (1963), dental plaque is "the soft tenacious material found on tooth surfaces which is not readily removed by rinsing with water." It is estimated that 1 mm3 of dental plaque, weighing about 1 mg, will contain more than 200 million bacteria. Other microorganisms, such as mycoplasma, "yeasts," and protozoa, also occur in mature plaque; sticky polysaccharides and other products form the so-called plaque matrix, which constitutes 10% to 40% by volume of the supragingival plaque.
The most readily discernible plaque on the smooth surfaces of the teeth, along the gingival margin, is termed dentogingival plaque. Dentogingival plaque on the approximal surfaces, apical to the contact points, is called approximal dental plaque.
Plaque occurring below the gingival margin, in the gingival sulcus or in the periodontal pocket, is known as subgingival plaque (Theilade and Theilade, 1976).
Occlusal or fissure plaque may also be formed, particularly in erupting molars.
Although there are more than 350 species of bacteria in the oral cavity, only a few have the ability to colonize a newly cleaned tooth surface. This initial association depends on the presentation and interaction of surface molecules on the bacteria and the pellicle-coated tooth surface. These molecules are vulnerable to alteration by chemical agents. Plaque adhesion is especially favored by high free energy of the tooth surfaces and the microorganisms.
The initial bacteria are called pioneer colonizers, because they are hardy and successfully compete with the other members of the oral flora for a place on the tooth surface (Gibbons and Van Houte, 1980). These pioneer colonizers are mainly the streptococcal strains S oralis, S mitior, and S sanguis. The deposition of these pioneer species is not a chance occurrence, but the outcome of an exquisitely sensitive interaction between protein adhesions on the surface of the colonizing bacteria and carbohydrate receptors on the salivary components adsorbed to the tooth surface.
After initial deposition, clones of pioneer colonizing bacteria, in particular Streptococcus sanguis, begin to expand away from the tooth surface to form columns that move outwardly in long chains of pallisading bacteria. These parallel columns of bacteria are separated by uniformly narrow spaces. Plaque growth proceeds by deposition of new species into these open spaces (Listgarten et al, 1975).
illustrates a cross section of such columns and open spaces.
These newly deposited species attach to pioneer species in a specific, molecular locking manner. Expansion of existing species in a lateral direction causes the interbacterial spaces to merge. It is hypothesized that, when the spaces are close enough, a starter substance is secreted by bacteria within the plaque matrix, signaling the surrounding bacteria to undergo a growth spurt. Within a short time, the tooth surface adjacent to the gingiva is covered by intermeshed bacteria. New bacteria derived from saliva or surrounding mucous membranes now sense only the bacterialaden landscape of the tooth surface and attach by a bonding interaction to bacteria already attached to the plaque. These associations, called intergeneric coaggregations, are mediated by specific attachment proteins that occur between two partner cells (Di Renzo et al, 1985; Kolenbrander, 1988).
All this activity occurs within the first 2 days of plaque development and, for descriptive purposes, is called phase I of plaque formation (Theilade et al, 1976).
After 24 to 48 hours, continuous plaque has formed along the gingival margin.
The plaque is dominated by cocci and a few rods.
In phase II of plaque development, the remaining interstices are occupied by increasing levels of gram-positive rods, such as Actinomyces viscosus, and gramnegative cocci, including Neisseria and Veillonella species. The outer surface of the gingival plaque is covered by tall rods.
illustrates the thickness of freely accumulated gingival plaque after 2, 3, and 4 days. There is a dramatic increase in plaque thickness after 3 and 4 days compared to the first 2 days. Now the gingival plaque is mature, and so-called homeostasis is established among the different
microorganisms.
In phase III, 5 to 7 days after initiation, plaque begins to migrate subgingivally, and bacteria and their products permeate and circulate in the pocket. In phase IV, 7 to 11 days after initiation, the diversity of the flora increases to comprise motile bacteria, including spirochetes and vibrios as well as fusiforms. Attached gingival plaque fills the gingival sulcus, while spirochetes and vibrios move around in the outer and more apical regions of the sulcus.
