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Colonization of oral cavity begins within hours after birth. From an ecologic view point, the oropharynx is considered an open growth system. A dynamic equilibrium exists between the adhesion forces of micro organisms and a variety of removal forces originating from-swallowing, mastication and blowing of nose; wash out effect of salivary, nasal and crevicular fluid out flow; tongue and oral hygiene implements; active motion of ciliae. Most organisms can survive only when they adhere to either soft tissues or hard surfaces in teeth, denture or implants. The high turn-over rate of intra-oral epithelial cells,especially gingiva, prevents permanent accumulation of large masses of microorganisms on these surfaces. This is a natural cleansing mechanism.
Structure and composition of dental plaque
dental plaque is defined clinically as a structured, resilient, yellow-grayish substance that adheres tenaciously to intra-oral hard surfaces, including removable and fixed restorations. Plaque is primarily composed of bacteria in a matrix of salivary glycoproteins and extra cellular polysaccharides. This matrix makes it impossible to remove the plaque by rinsing or use of sprays.
Materia alba refers to soft accumulations of bacteria and tissue cells that lack the organized structure of dental plaque, and is easily displaced with a water spray. Calculus is hard deposit that forms by mineralization of dental plaque, and it is generally covered by a layer of unmineralized plaque.
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Dental plaque is composed primarily of microorganism. One gram of plaque (net weight) contains approximately 1011 bacteria. Non bacterial micro-organisms include mycoplasma species, yeasts, protozoa and virus. The microorganisms exist within an inter-cellular matrix that also contains a few host cells, such as epithelial cells, macrophages and leukocytes. The material among bacteria in dental plaque is termed intercellular matrix. It contains organic acids and inorganic proteins . organic acid is composed of protein polysaccharide complex produced by micoorganisns. Carbohydrates like levans(fructans) and glucan (dextran) provide energy and act as organic skelton of plaque. Others include galactose and rhamnose.protein component is formed by glycoproteins. Small amount of lipids are also present.the inorganic component include Ca, P, Mg, K and Na.
Types of dental plaque
Dental plaque is classified broadly based on its position on the tooth surface towards the gingival margin as,
• Supragingival plaque is found at or above the gingival margin. When it is in direct contact with gingival margin, it is called marginal plaque. When it is in contact with only tooth surface it is called coronal plaque.
• Subgingival plaque, is found below the gingival margin, between the tooth and gingival pocket epithelium. It can be attached plaque and unattached subgingival plaque. Attached plaque can be tooth associated and tissue associated plaque.
Supragingival plaque typically demonstrates a stratified organization of multilayered accumulation of bacterial morphophytes. Gram positive cocci and short rods predominate at the tooth surface, where as gram-negative rods and filaments, as well as spirochetes, predominates in the outer surface of mature plaque mass.
Composition of subgingival plaque depends on the pocket depth. Subgingival microbiota differs in composition from supragingival plaque, primarily because of local availability of blood products and a low redox potential which characterizes the anaerobic environment. The tooth-associated cervical plaque, adhering to the root cementum contains Streptococcus mitis, S.sanguis, Actinomyces viscosus, A.naeslundii, and Eubacterium species. In the deeper parts of the pocket, filamentous organisms become fewer in number and in apical portion, they are virtually absent. The apical border of plaque mass is separated from junctional epithelium by a layer of host leukocytes, and the bacteria of this apical tooth associated region show an increased concentration of gram-negative rods.
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The layers of microorganisms facing the soft tissues predominated by Streptococcus oralis, S.intermedius, Peptostreptococcus micros, Porphyromonas gingivalis, Prevotella intermedia, Tannerella forsythia and Fusobacterium nucleatum.
The site specificity of plaque is significantly associated with diseases of periodontium. Marginal plaque, is of prime importance in the initiation and development of gingivitis. Supragingival plaque and tooth associated supragingival plaque are critical in calculus formation and root caries, where as tissue associated subgingival plaque is important in the tissue destruction that characterizes different forms of periodontitis.
Biofilm is defined as a relatively undefinable microbial community associated with a tooth surface or any other hard, non-shedding material. (Widerer & Charasklis).
It is complex aggregation of microorganisms growing on a solid substrate. Its characterized by structural heterogenicity, genetic diversity, complex community interactions, and extracellular matrix of polymeric substances.
Basic properties of biofilm are:
1. co-operating community of various types of microorganisms
2. microorganisms are arranged in micro colonies.
3. micro colonies are surrounded by protective matrix.
4. within the micro colonies are differing environments.
5. micro oraganisms have aprimitive communication system
6. microorganisms in biofilm are resistant to antibiotics antimicrobials and host response.
PLAQUE AS A BIOFILM
Biofilms have an organized structure, composed of microcolonies of bacterial cells non randomly distributed in a shaped matrix of glycocalyx. It is bordered by a stationary sub layer and a fluid layer in motion. Nutrient components penetrate this fluid layer by molecular diffusuion. Dental plaque is host associated biofilm.
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The dental plaque biofilm has a similar heterogeneous structure, with open fluid filled channels running through the plaque mass. These water channels permit the passage of materials throughout the biofilm. Nutrients make contact with attached micro colonies by diffusion from water channels to the micro colony, rather than from the matrix. Biofilm matrix functions as a barrier. The intercellular matrix consists of organic and inorganic materials derived from saliva, gingival crevicular fluid and bacterial products. Organic constituent of the matrix include polysaccharides, proteins, glycoproteins and lipid material and albumin.
The lipid material consists of debris from the membranes of disrupted bacterial and host cells and possibly food debris. Glycoproteins from saliva initially coats a clean tooth surface. Polysaccharides produced by bacteria, of which dextran is predominant from, also contribute to the organic portion. They play a major role in maintaining the integrity of the film. The inorganic components are predominantly calcium and phosphorous, with trace amounts of other minerals, including sodium, potassium and fluoride. The sources of inorganic constituents of supragingival plaque is primarily saliva. As the mineral content increases, the plaque mass becomes calcified to become calculus. Calculus is found in areas of dentition adjacent to salivary ducts (ie. Lingual surfaces of mandibular anteriors, buccal surfaces of maxillary first molars), reflecting high concentration of minerals available from saliva. The inorganic component of subgingival plaque is derived from crevicular fluid; and fluoride largely from external sources.
• Biofilms have organized structure
• Structurally, dental plaque is now considered to be a biofilm of complex and dynamic microbial community.
• It is composed of micro colonies of bacterial cells non randomly distributed in a shaped matrix or glycocalyx
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• It contains areas of high and low bacterial biomass interlaced with aqueous channels of different size, which are nutrient channels.
• It adheres firmly to tooth surface and is resistant to mechanical removal
FORMATION OF DENTAL PLAQUE
The process can be derived into 3 major phases
1. Formation of Pellicle
Within nano seconds after vigorously polishing the teeth, a thin saliva-derived layer called acquired pellicle, covers all surfaces of the oral cavity, including teeth. This pellicle consists of numerous components including glycoproteins (mucins), proline-rich proteins, phosphor proteins (eg: α-amylase) and other molecules that can function as adhesion sites for bacteria (receptors).
The mechanism involved in enamel pellicle formation is electrostatic, Vander waals and hydrophobic forces. The physical and chemical nature of the solid substances effect several physico-chemical surface properties of pellicle including its composition, packing, density and its configuration.
2. Initial adhesion and attachment of bacteria.
Microbial adhesion to surfaces in aquatic environment is a four stage sequence.
Phase I: Transport to the surface:
Random contacts may occur, eg:- through Brownian motion, through sedimentation of micro organisms, through liquid flow or through chemotactic activity.
Phase II: Initial adhesion
Initial, reversible adhesion is initiated by interaction between the bacterium and the surfaces, from a certain distance (50 nm), through long range and short-range forces, including Vander waals attractive forces and electrostatic repulsive forces. For most bacteria, total Gibbs energy consists of a secondary minimum (where reversible binding takes place 5-20 nm from the surface), a positive minimum (located