Bacteria are present throughout life in a myriad of sites on and in the human body. The bacteria may be beneficial for the host, or of no consequence (commensal), or injurious. In the oral cavity, over 530 different species of microorganisms have so far been identified; fortunately, for the most part these organisms remain in ecological balance and do not cause disease. Certain facultatively pathogenic (“opportunistic”) bacteria are occasionally observed in high numbers, e.g., in cases of disease (periodontitis, mucosal infection). It remains unclear whether these bacteria alone represent the cause of the diseases, or whether they simply find favorable living conditions in the disease milieu. Non-specific supragingival plaque (mixed flora) will elicit gingivitis within ca. seven days. If the plaque is removed, gingivitis regresses in a short period of time (“reversibility”). On the other hand, for the various forms of periodontitis, especially the aggressive, rapidly-progressing forms, specific bacteria are associated.
42Experimental GingivitisIn 1965, Löe et al. published the classic experimental proof of the bacterial etiology of gingivitis. In plaque- and inflammation-free subjects, plaque begins to accumulate if all oral hygiene is ceased. For the first few days, this plaque is composed of gram-positive cocci and rods, then later of filamentous organisms and finally spirochetes (gram-negative). Within a few days, a mild gingivitis ensues. If the plaque is removed, the gingiva quickly returns to a state of health.43Healthy Gingiva and Initial GingivitisLeft: “Almost” clean central incisors. An extremely thin biofilm is compatible with clinically healthy gingiva. A few bacteria may even serve to maintain the “memory” of the immune system (p. 55).Right: Plaque accumulation following seven days without oral hygiene. Gingivitis has developed. The proliferation and relative increase in numbers of gram-negative microorganisms are responsible for the gingivitis.
Biofilm—Plaque Formation on Tooth and Root Surfaces
The oropharynx is an open ecosystem wherein bacteria are always present; bacteria attempt to colonize in all favorable locations. Most bacteria, however, can only persist after the formation of a biofilm upon desquamation-free surfaces, i.e., hard substances (tooth and root surfaces, restorative materials, implants, prostheses etc.). In the presence of healthy dental and gingival relationships, there is a balance between the additive and retentive mechanisms of biofilms vis-à-vis the abrasive forces that tend to reduce biofilm formation, e.g., self-cleansing by the cheeks and tongue, diet and mechanical oral hygiene measures.
The existence of a biofilm results within a matter of hours or days, in the phases described below (Darveau et al. 1997, Descouts & Aronsson 1999, Costerton et al. 1999).
The establishment and stabilization of bacteria within a biofilm are important not only for the etiology of periodontitis, but also for adjunctive systemic and topical medicinal treatment for periodontitis (p. 287): Biofilm bacteria imbedded within a matrix of extracellular polysaccharides are more than 1,000 times less sensitive to antimicrobials (e.g., antibiotics) than free-floating (“planktonic”) bacteria.
44Dental Plaque—DevelopmentWithin minutes after completely cleansing the tooth surface, a pellicle forms from proteins and glycoproteins in saliva.AAssociation: Through purely physical forces, bacteria associate loosely with the pellicle.BAdhesion: Because they possess special surface molecules (adhesins) that bind to pellicle receptors, some bacteria become the “primary colonizers,” particularly streptococci and actinomyces. Subsequently, other microorganisms adhere to the primary colonizers.C Bacterial proliferation ensues.DMicrocolonies are formed. Many streptococci secrete protective extracellular polysaccharides (e.g., dextrans, levans).EBiofilm (“attached plaque”): Microcolonies form complex groups with metabolic advantages for the constituents.FPlaque growth—maturation: The biofilm is characterized by a primitive “circulatory system.” The plaque begins to “behave” as a complex organism! Anaerobic organisms increase. Metabolic products and evulsed cell wall constituents (e.g., lipopolysaccharides, vesicles) serve to activate the host immune response (p. 38). Bacteria within the biofilm are protected from phagocytic cells (PMN) and against exogenous bacteriocidal agents.
Supragingival Plague
… and its Initial Subgingival Expansion
The first bacteria that accumulate supragingivally on the tooth surface are mostly gram-positive (Streptococcus sp, Actinomyces sp.). In the course of the following days, gramnegative cocci as well as gram-positive and gram-negative rods and the first filamentous forms begin to colonize (Listgarten et al. 1975, Listgarten 1976). By means of a variety of metabolic products, the bacterial flora provoke the tissue to increased exudation and migration of PMN leukocytes into the sulcus (“leukocyte walls” against the bacteria).
The increase in PMN diapedesis and the flow of sulcus fluid lead to initial disintegration of the junctional epithelium. This makes it possible for bacteria to more easily invade between the tooth and the junctional epithelium, and invade the subgingival area (gingivitis, gingival pocket formation). In the total absence of oral hygiene, plaque formation and an initial host defensive response within gingival tissue occur. With optimum—including interdental—oral hygiene, the formation of biofilm is repeatedly disrupted and gingival health is maintained.
45One-week-old Plaque—InteractionsThick zone of early colonizers on the enamel surface and the column-like structures that result from rapid proliferation of streptococci. On the plaque surface, one observes rods and filaments.Left: Interaction between host and plaque. Chemotactically regulated immigration of polymorphonuclear granulocytes (PMN, arrow). The black horizontal line indicates the level from which this sample of plaque was taken.46Three-week-old PlaqueThe composition of the supragingival plaque has changed markedly. Filamentous organisms now predominate. Conspicuous forms resembling “corn cobs” are observed at the plaque surface.Left: In this transmission electron photomicrograph, the structure of such a “corn cob” is revealed. At the center is a gram-negative filamentous organism (F), surrounded by gram-positive cocci (C).Histology and TEM courtesy M. Listgarten47Expansion of Supragingival Plaque—Gingival PocketMiddle and Right: Weakening of the epithelial attachment to the tooth permits apical migration of gram-positive plaque bacteria in a thin layer between the tooth and the junctional epithelium (thin arrow). Gram-negative bacteria colonize subsequently, and a gingival pocket forms (Fig. 150).Histology courtesy G. CimasoniLeft: Schematic representation of the interaction between plaque and host tissue.
Natural Factors Favoring Plaque Retention
The formation of a plaque biofilm can be enhanced by natural retention factors, which can also render biofilm removal by means of oral hygiene more difficult. These retention factors include:
Supra- and subgingival calculus
Cementoenamel junctions and enamel projections
Furcation entrances and irregularities
Tooth fissures and grooves
Cervical and root surface caries
Crowding of teeth in the arch.
By itself, calculus is not pathogenic. However, its rough surface presents a retention area for vital, pathogenic bacteria. At the microscopic level, the cementoenamel junction is very irregular, and offers retentive roughness. Enamel projections and “pearls” also inhibit soft tissue attachment.
Furcation entrances, fissures, etc. are retentive niches for plaque. Carious lesions represent a huge bacterial reservoir. Crowding of teeth reduces self-cleansing and renders oral hygiene more difficult.
48Supragingival CalculusLingual surfaces of mandibular incisors and buccal surfaces of maxillary molars near the orifices of salivary ducts often exhibit massive accumulations of supragingival calculus.Right: TEM of old supragingival calculus. Calcified plaque (A) close to the tooth surface. Note the accumulation of cell-free hexagonal monocrystals (B) upon the calcified plaque.Courtesy H. Schroeder49Subgingival CalculusIn this patient with long-standing periodontitis, the gingiva has receded. Calculus that was formerly subgingival is now supragingival.Right: Subgingival calculus is observed clinically after reflecting the gingival margin. Subgingival calculus is usually dark in color (Fe minerals) and harder than the more loosely structured supragingival calculus (calcium phosphates). The cementoenamel junction is indicated by the dashed line.50Crowding, Enamel Projection (Enamel “Pearl”)The lingually displaced mandibular incisors do not benefit from the natural self-cleansing action of the lower lip. Oral hygiene is also rendered more difficult.Right: The furcation on this molar is filled by a projection of enamel that ends in a bulbous pearl, extending into the interradicular area. When a pocket forms in such an area, plaque control is particularly difficult.
Iatrogenic Factors Favoring Plaque Retention
Restorative dentistry—from a simple restoration to a fullmouth reconstruction—can do more harm than good to the patient’s oral health if performed improperly! Placing only optimum restorations is synonymous with preventive periodontics (tertiary prevention, p. 198).
Fillings and crowns that appear to be perfect clinically and macroscopically almost always exhibit deficiencies at the margins when viewed microscopically. When margins are located subgingivally, they always present an irritation for the marginal periodontal tissues.
Overhanging margins of restorations and crowns accumulate additional plaque. Gingivitis ensues. The composition of the plaque changes. The number of gram-negative anaerobes (e.g., Porphyromonas gingivalis), the organisms responsible for initiation and progression of periodontitis, increases rapidly (Lang et al. 1983).
Gross iatrogenic irritants such as poorly designed clasps and prosthesis saddles may exert a direct traumatic influence upon periodontal tissues.
51Amalgam Restoration—Clinical View and SEMLeft: Viewed in the scanning electron microscope, a clearly visible margin defect is observed. Such a defect is a perfect niche for the accumulation of plaque. The amalgam restoration (A) is at the top of this figure, the adjacent enamel below. The white dot under the 25 μm legend are representative of the size of coccoid microorganisms (ca. 1 μm).Courtesy F. Lutz52Amalgam—Proximal OverhangGross overhangs such as this, located subgingivally, invariably lead to plaque accumulation and to gingivitis (note hemorrhage). Pathogenic, gram-negative anaerobes are frequently observed. In contrast to amalgam and especially gold restorations, composite resin restorations are particularly retentive of bacteria.Left: Radiograph of the same case.53Crown Margin Overhang and Open MarginsThe cement that was used to cement this crown has begun to extrude from the open margin. The massive retention of plaque between the crown and the prepared tooth leads to severe gingivitis and establishment of a pathogenic bacterial flora.Left: Section through a porcelain-fused-to-metal crown with a margin that is both overhanging (arrows) and open. Calculus (C) and plaque have accumulated apically.
Subgingival Plaque
Extending apically from the supragingival region, a subgingival plaque biofilm will often form within the existing gingival sulcus/pocket; this was previously called the “adherent” plaque. In addition to gram-positive bacteria such as streptococci, actinomyces, etc., as the probing depth increases so does the number of anaerobic gram-negative bacteria (p. 36).
This subgingival biofilm can also calcify. A dark, hard and difficult to remove calculus (“serum calculus”) accumulates. In addition, the gingival pocket also contains loose agglomerates of non-adherent, often mobile bacteria (with a high concentration of gram-negative anaerobes and spirochetes). In acute phases, periodontopathic bacteria often increase dramatically. These include Actinobacillus actinomycetemcomitans, P. gingivalis, T. forsythia, spirochetes etc. (pp. 30, 33, 38). Despite these alterations in the subgingival plaque, periodontitis, even in the acute stage, cannot be characterized as a “highly specific” infection because large differences have been reported in the bacterial composition between patients and even within different pocket locations in the same patient (Dzink et al. 1988, Slots & Taubmann 1992, Lindhe 1997).
54Subgingival Pocket FloraThis is a relatively thin adherent biofilm (blue-violet). One observes loose accumulations of gram-negative, anaerobic, and also motile bacteria. Formations resembling test tube brushes, consisting of filamentous bacteria are also observed (inset).Histology courtesy M. ListgartenRight: As pocket depth increases (arrow), the resident flora becomes increasingly gram-negative and anaerobic.55Surface of the Biofilm on the RootWithin a pocket, the root surface of a tooth manifesting periodontitis is covered with a densely intertwined bacterial colonization composed of many different bacterial morphotypes (scanning electron photomicrograph).The morphology of the bacteria permits neither a determination of the species nor any clues concerning pathogenicity.56Microorganisms of the Non-adherent Plaque—“Planktonic” PhaseIn a dark-field preparation, motile rods and small to larger spirochetes predominate, while cocci and filaments are rare: Typical signs of an active pocket (exacerbation; cf. p. 63).Right: Intact phagocytes (PMN) in the pocket exudate do not lose their capacity for phagocytosis. The arrow depicts a spirochete being engulfed.Courtesy B. Guggenheim
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