Microbiology of Gingivitis
Master the INBDE with Dental Panda: Your go-to resource for expert practice exams and tailored study resources!
In a clinically healthy periodontium the bacterial profile favors gram-positive facultative cocci (Streptococci) and filamentous bacteria (Actinomyces). Normal inhabitants of a healthy oral environment include:
- Gram-positive bacteria:
-
- Streptococcus.
- Peptostreptococcus.
- Actinomyces.
- Lactobacillus.
- Gram-negative bacteria:
-
- Veillonella.
- Campylobacter.
- Fusobacterium.
- Eikenella.
- Corynebacterium.
Gram-negative species and spirochetes are present, but in lower proportional numbers. Viridans streptococci are a large group of gram-positive streptococcal bacteria that are α-hemolytic (produce green coloration on blood agar plates), and include:
- Streptococcus salivarius.
- Streptococcus mutans (only present when teeth are present, ~6+ months old).
- Streptococcus sanguis (only present when teeth are present, ~6+ months old).
- Streptococcus mitis.
A distinct shift towards gram-negative anaerobes is a key sign in developing periodontal disease. The major factor in determining the bacterial makeup is the relative presence of oxygen (redox potential). A periodontal lesion will likely contain a wide range of bacterial species, however, as periodontal disease progresses, there is a generalized shift in the bacterial profile:
- From gram positive to gram negative.
- From Cocci to rods.
- From non-motile to motile bacteria.
- From facultative anaerobes to obligate anaerobes.
- From fermenting species to proteolytic species.
Chronic periodontitis is composed predominantly of:
- Porphyromonas gingivalis.
- Actinobacillus actinomycetemcomitans.
- Prevotella intermedia.
- Tannerella forsythia.
- Campylobacter rectus.
- Eikenella corrodens.
- Fusobacterium nucleatum.
- Treponema denticola.
- Eubacterium nodatum.
- Peptostreptococcus micros.
You will need to be able to associate certain bacterial species with the conditions they are most strongly linked to:
- Aggressive periodontitis (molar-incisor distribution) – A. actinomycetemcomitans.
- Necrotizing gingivitis and periodontitis – Prevotella intermedia, Fusobacterium.
Periodontal abscesses – F. nucleatum, P. intermedia, P. gingivalis, P. micros, and T. forsythia.
DENTAL PLAQUE COMPLEXES
Dental plaque complexes were constructed from DNA-DNA hybridization studies (“checkerboard” analyses) and complexes of periodontal microorganisms were given color designations. Early (primary) colonizers are members of the yellow (Streptococcus spp.) or pulpe (Actinomyces odontolyticus) complexes. Complexes associated with late (secondary) colonizers include:
- Red complex – Porphyromonas gingivalis, Tannerella forsythia, and Treponema denticola. Microbes within this complex are recognized as the most important in adult periodontal disease, associated with severe periodontitis, and linked to bleeding on probing.
- Orange complex – Fusobacterium species, Prevotella species, and Campylobacter species. These bacteria act as “bridge species”, forming a link between the early colonizers and the highly pathogenic bacteria of the red complex.
- Orange-associated Complex – Campylobacter rectus, Eubacterium nodatum.
- Green complex – E corrodens, Capnocytophaga species (gingivalis, ochracea, sputigena).
Aggregatibacter actinomycetemcomitans is a nonmotile, facultative anaerobic, capnophilic (CO2), gram-negative rod that grows as smooth, white, non-hemolytic colonies on blood agar plates. It is most closely associated with aggressive periodontal disease with a molar-incisor distribution (formerly termed localized aggressive periodontitis). Formerly called Actinobacillus actinomycetemcomitans, specific virulence factors include:
- Leukotoxins – cytotoxic to neutrophils, monocytes, and lymphocytes.
- Lipopolysaccharide (LPS) – endotoxin found in the outer membrane of gram-negative bacteria.
- Collagenase – enzymes that break the peptide bonds in collagen.
- Proteases – cleave immunoglobulins (IgG).
Porphyromonas gingivalis is a nonmotile, anaerobic, gram negative pleomorphic rod that grows darkly pigmented hemolytic colonies on blood agar plates. It is most closely associated with chronic periodontitis and is a member of the red complex of bacteria. Specific virulence factors include:
- Fimbriae – long filamentous polymeric protein structures that enable the bacteria to bind to specific receptor structures to colonize specific surfaces.
- Capsule – polysaccharide layer that protects cells from phagocytosis, desiccation, and hydrophobic toxic materials such as detergents.
- Proteases – cleave immunoglobulins and complement.
- Collagenase – enzymes that break the peptide bonds in collagen.
- Hemolysin – destroys red blood cells and liberates hemoglobin.
Prevotella intermedia and Prevotella nigrescens are both nonmotile, anaerobic, gram-negative rods that grow darkly pigmented hemolytic colonies on blood agar plates. P. intermedia is most closely associated with pregnancy gingivitis and necrotizing gingivitis/periodontitis.
Treponema denticola (also pectinovorum, socranskii, vincentii) is a motile, obligate anaerobic, gram-negative spirochete. It is highly proteolytic, producing enzymes that can degrade collagen and destroy immunoglobulins and complement factors. T. denticola is a red complex bacteria that is closely associated with chronic periodontitis and necrotizing periodontal disease.
Tannerella forsythia is a nonmotile, anaerobic, gram-negative pleomorphic rod. T. forsythia is associated with various forms of the periodontal disease including chronic and aggressive periodontitis. It is a member of the red complex of bacteria. Specific virulence factors include:
- proteases – liberates peptides, free amino acids and heme for growth, cleaves components involved in innate (cytokines/chemokines, complement factors) and adaptive immunity (immunoglobulins).
- surface lipoproteins – activate host cells to release proinflammatory cytokines and induce cellular apoptosis of human cells including gingival fibroblasts.
GINGIVITIS
Though there is an acceptable range, healthy gingival tissues:
- appear coral-pink, though physiological pigmentation may be present.
- are firmly attached to the bone and teeth, following the contour of the underlying anatomy.
- exhibit visible stippling and scalloped gingival margins.
- fill the interproximal spaces (gingival papilla).
Gingivitis describes a form of periodontal disease that results from an inflammatory response to plaque that builds up on the external surface of teeth. Dental plaque is a soft deposit that consists of a complex microbial biofilm housed within an extracellular matrix. The major organic constituents of the plaque biofilm are polysaccharides, proteins, glycoproteins, and lipids. The major inorganic constituents are calcium and phosphorus with trace amounts of fluoride, sodium, potassium. Saliva is the main source of inorganic components in supragingival plaque. Gingival crevicular fluid is the main source of inorganic components in subgingival plaque.
Dental plaque formation begins with the attachment of salivary proteins and glycoproteins to the tooth surface. This initial film is called the acquired pellicle. The first bacteria (primary colonizers) weakly bond to the acquired pellicle via van der Waals forces. Early colonizers are primarily gram-positive cocci and short rods, mainly Actinomyces, Streptococci, and Neisseria. Gram-positive colonizers use saccharides as an energy source (saccharolytic) and saliva as a carbon source. The growth and metabolism of these initial species prime the surrounding environment for late colonizers by:
- changing the surrounding pH.
- altering substrate availability.
- the development of food chains e.g. lactate metabolism by Veillonella spp.
- changing the concentration of oxygen, facilitating the proliferation of obligate anaerobes.
As the plaque matures, secondary/late colonizers attach to the primary colonizers (coaggregation) via specific intermolecular interactions. Fusobacterium nucleatum serves as an important “bridging” microorganism because of its ability to coaggregate. Dental plaque exhibits vertical and horizontal stratifications within the biofilm when organized by microbial interactions. Supragingival and subgingival plaque differ in their bacterial profile because of the difference in oxygen. Deeper in the gingival sulcus the environment favors gram-negative rods, cocci, filaments, flagellated rods and spirochetes. In periodontal diseases, there is a shift in the composition of the biofilm to a more proteolytic gram negative anaerobic rods, filaments and spirochetes. These anaerobic bacteria in mature plaque are not adapted to use saccharides as an energy source (asaccharolytic) and use amino acids/small peptides instead. Late colonizers include:
- Prevotella (intermedia, loescheii).
- Capnocytophaga species.
- Porphyromonas gingivalis.
- Actinobacillus actinomycetemcomitans (AA).
CALCULUS
An inorganic shift in dental plaque can lead to the development of calculus (tartar), a mineralized form of plaque caused by the precipitation of minerals from saliva and gingival crevicular fluid (GCF). Supragingival calculus is often white/pale yellow in appearance and is more easily removed. Supragingival calculus accumulates above the free gingival margin, most often adjacent to the parotid/Stensen duct (buccal side of maxillary molars) and the submandibular/Wharton’s and Bartholin’s ducts (lingual side of lower incisors). Supragingival calculus is primarily composed of inorganic material (70-90%) from saliva. It is known to contain calcium phosphate (75%), calcium carbonate (3%), and traces of magnesium phosphate and other metals. The inorganic material is organized into different crystalline structures, the most important of which are hydroxyapatite (58%), magnesium whitlockite (21%), octacalcium phosphate (12%), and brushite (9%).
Subgingival calculus is often darker and more dense than supragingival calculus due to blood pigments, but the composition is similar to supragingival calculus. Subgingival calculus accumulates below the free gingival margin. The organic components of calculus are similar to that of dental plaque, a mixture of protein-polysaccharide complexes, desquamated epithelial cells, and microorganisms. Calculus does not serve as a mechanical irritant to the gingival tissues, but it is always covered with a layer of bacterial plaque which serves as the primary irritant. The serum transudate gingival crevicular fluid is the source of minerals needed for subgingival calculus formation.
