Microflora of Saliva, Tongue, Dental Plaque and Gingival Crevice

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Saliva and gingival crevicular fluid are the main liquid substances washing oral cavity. The saliva is crucial in balancing oral microbial ecology. All the properties of saliva (secretion rate, viscosity, mineral contents, ionic potential, buffering capacity, multiple organic matter – amino acids, polysaccharides, vitamins, nucleotides, potent antimicrobial factors – mucins, secretory IgA antibodies, lysozyme) contribute to microbial composition of oral cavity.

Besides saliva, the bacteria are located preferentially in three zones of oral cavity: dental plaques upon tooth crown (or inside carious lesions in case of caries); within gingival crevices; and upon lingual body especially covering its back side.

Total amount of bacteria in saliva varies in the range from 40-50 mln to more than 5 billion per 1 ml, for about 750 mln an average. Microbial concentration in dental plaques and gingival crevices is almost 100-fold higher – nearly 200 bln microbial cells per 1 g of medium content. Besides microbial cells, the latter harbors about 80% of water.

As mentioned above, numerous microbial species and genera reside in oral cavity. More than one-half pertain to vast number of streptococcal species, e.g., S. mutans, S. mitis, S. sanguis, S. sobrinus and others – except beta-hemolytic streptococci that can be found solely as transient microflora. Various coccal species occupy certain compartments within the mouth. For instance, most of enterococci are located inside gingival crevice and upon the body of tongue; S. mutans is typically found in dental plaque upon crown.

Viridans streptococci and veillonellas produce the great mass of salivary microflora. Mostly they shed there from tongue body. The number of gram-negative anaerobic rods (bacteroidal species and fusobacteria) together with diphtheroids increases in gingival crevices.

Total quantity of microbial cells undergoes daily alterations. It depends mainly on amount of saliva secretion that greatly declines at night. Dental loss leads to marked reduction of dental microflora.

Multiple factors can impact certain members of oral microbiota. For example, any antibiotic treatment inhibits the target group of defined microbial species thus impairing normal microbial balance. Protein-enriched diet doubles the number of facultatively anaerobic gram-positive rods. Large part of bacteria needs vitamins or other supplements for their successful propagation; lack of growth factors results in suppression of activity of selected bacterial groups.

Qualitative and quantitative composition of dental microflora is greatly influenced by various diseases. For instance, С. albicans recovery from oral samples is made with highest rate in diabetes patient (up to 80% against 50% in healthy individuals). Lactobacilli grow high in caries patients and fall down after lesions treatment.

It can be indicated also that S. mutans, S. sanguis, lactobacilli, yeasts and spirochetes seriously disappear after massive dental loss, whereas the amount of S. salivarius elevates in the course of time. In first 2 weeks after mounting of removable dentures the levels of streptococci look high, whereas the quantity of lactobacilli and yeasts rapidly goes down. In 3-5 weeks the count of lactobacilli and yeasts tends to restore but the number of streptococci declines. Overall, the total number of streptococci doesn’t alter significantly in all periods of life.

Polymicrobial adherence to dental surface leads to dental plaque formation.

Dental plaque is a complex matrix (or microbial biofilm) made of immensity of microbial bodies, their extracellular products and wastes, and salivary compounds.

Dental plaques are divided into supragingival and subgingival. Supragingival plaques play substantial role in caries. Likewise, subgingival palques participate in periodontal disease progression.

Composition of dental plaque differs depending on site of adherence and plaque’s maturation stage. It grows predominantly on dental surfaces that avoid mechanical cleaning – approximal surface between two teeth, fissures and pits of the tooth crowns, gingival crevices.

The process of plaque formation commences from adhesion of poorly soluble polymeric carbohydrates such as dextrans together with mucins and salivary proteins to dental enamel. Acid glycoproteins react with calcium ions of enamel whereas basic proteins bind to phosphates of hydroxyapatites. Primary biofilm is known as pellicle.

Attachment of bacteria demonstrates rapid progression. By 5 minutes the number of microbes arises up to 105-106 of bacterial cells per 1 cm2. Initial microbial bodies land within tooth pits and fissures; later they spread to smooth dental areas. Further microbial propagation and their exopolysaccharide excretion facilitate the growth of soft dental plaque.

Many bacterial cells can’t attach firmly to clean dental surface but easily bind to primarily absorbed microbial layer. For instance, when coccoid flora surrounds embedded rod-like and filamentous bacteria, it produces mixed cellular clusters known as corncob formations.

The bacteria composing dental plaque can be divided into two large groups. The first comprises acidophilic agents able to propagate in acidic environment – lactobacilli, actinomycetes, peptococci, leptotrichia, corynebacteria and some others. The second one embraces bacteria with prominent proteolytic activity – veillonellas, fusiform bacteria, neisserias, vibrios, or spirochetes.

At initial steps of maturation the dental plaque has larger amounts of aerobic and facultatively anaerobic bacteria with dominating role of oral streptococci.

Oral viridance streptococci together with lactobacilli ferment sucrose resulting in overproduction of lactic acid and next sharp decline of local oral pH. Lactate can be further utilized by veillonellas, neisserias and other microbials accumulating more organic acids (eg. acetic, propionic or formic). All these changes influence microbial composition of dental plaque.

Exuberant consumption of sucrose and other simple carbohydrates from nutrients worsens the situation and intensifies enamel destruction, microbial retention and plaque maturation. In addition, elevated levels of carbohydrates in oral cavity lead to their polymerization by local microbiota. Synthesis of extracellular polysaccharides such as soluble or insoluble dextran and levan is typical for oral streptococci especially S. mutans. They facilitate microbial tooth adhesion and consolidate the matrix of microbial biofilm within dental plaque.

The synthesis of bacterial exopolysaccharides ceases at pH below 5,0.

Supragingival dental plaque predominantly harbors facultatively anaerobic gram-positive bacteria, mainly the broad spectra of streptococci and actinomycetes. Gram-negative representatives that pertain to Veillonella, Bacteroides and Haemophilus species are present constantly but in lower concentrations.

Similarly, subgingival dental plaque also confines the most common gram-positive microorganisms – streptococci and Actinomyces spp. Non-affected subgingival crevice carries moderate number of microbes; their total number varies from 103 to 106 cells per site.

Composition of bacterial plaques is also different on teeth of upper and lower jaws. A large proportion of streptococci and lactobacilli is present within dental plaques of upper jaw. Veillonellas and filamentous bacteria can be often found on teeth of mandibular bone.

Gram-positive cocci and rods prevail on approximal dental surfaces (between teeth) and within fissures. First day of plaque emergence is characterized by swift primary microbial colonization. After plaque maturation their microbial composition remains stable for a long time.

Next plaque progression is followed by lowering of its redox potential under the action of aerobic and facultatively anaerobic bacteria, thus engendering the growth of obligate anaerobic organisms. The dental plaque progressively accumulates bacteroids, porhyromonads, prevotellas, fusobacteria, leptotrichias and many others. Their metabolism results in alkaline byproducts (e.g., ammonia, urea, etc.) thereby elevating dental pH and dampening further plaque growth.

Sequential change of microbial communities, basic character of elderly plaque biofilm, accumulations of calcium and phosphates predispose to the formation of dental stone (calculus, or tartar). It begins to grow on dental surface especially in the area of gingival margin that impedes circulation of crevicular fluid.

Dental stone (calculus) is the solid formation tightly attached to dental crown and/or radix that is resulted from consolidation and calcification of contents of long-term dental plaque (degraded microbial bodies and polymeric matrix, inorganic matter, etc.)

Dental stones are also divided into supragingival and subgingival. Supragingival stones can be ordinarily found nearby the openings of ducts of salivary glands or upon the lingual surface of lower molars. Subgingival attach to dental radices. This stimulates progression of dental pockets impacting gum detachment.

Overall, dental plaques and dental stones impair the normal self-cleaning of dental areas and promote the development of most common aggressive disorders – i.e., caries and periodontitis.

Efficient prophylaxis of these widespread dental diseases depends on the number of medical and hygienic measures for prevention and removal of dental plaques such as brash cleaning of teeth and dental flossing; the use of proper tooth pastes and powders that ensure plaque withdrawal.