Normal Oral Microbiota

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Oral cavity provides favorable conditions for growth and propagation of multiple microbial inhabitants. They can be found in great amounts on mucous membranes of tongue, cheeks, teeth, gingival crevices and pockets. Species composition of oral microflora is extremely variable.

Oral cavity harbors above 1000 of diverse bacterial species. Their absolute quantity is also enormously high. For instance, total salivary microbial count exceeds 1 billion cells per 1 ml.

These bacteria encompass mixed microflora from various compartments of oral cavity. Most of them participate in dental plaque formation.

There are two main groups of bacteria that make oral microbiota – autochthonous and allochthonous microflora.

Autochthonous or indigenous bacteria are the resident inhabitants of oral cavity (obligate microflora), whereas allochthonous microorganisms are temporary for this site (or transient) arising largely from external source. Nevertheless, transient oral microflora comprises more likely pathogenic and opportunistic bacterial species in comparison with resident ones.

Allochthonous microorganisms enter oral cavity from other biotopes of human body (e.g., large intestine) or from external environment.

The group of resident aerobic and facultatively anaerobic gram-positive cocci encompasses mainly viridans streptococci. They produce green zone of hemolysis when grown onto blood agar medium. Most common here are S. mutans, S. mitis, S. sanguis, S. salivarius. Their quantitative distribution depends on many variable external and internal factors: person’s diet, oral cavity personal hygiene, state of local immune response, genetic factors, etc.

Streptococci can produce hydrogen peroxide and ferment carbohydrates yielding organic acids. This lowers local pH below 5.0 resulting in dental enamel demineralization and teeth decay. Furthermore, streptococci are capable of making polysaccharides from sucrose taken from sucrose-containing foodstuffs. Soluble oligosaccharides are metabolized by other bacteria thereby intensifying acid formation. Non-soluble polysaccharides actively promote adhesion of oral streptococci to dental surface thus fostering dental plaque growth.

Gram-positive anaerobic cocci are represented by peptococci that intensively utilize peptides and amino acids. Unlike streptococci they demonstrate slow carbohydrate fermentation.

Resident oral gram-negative anaerobic cocci, e.g., Veillonella genus members, play an important role in metabolic balance within oral cavity. They don’t ferment mono- and disaccharides but utilize numerous organic acid (lactate, pyruvate, acetate and others) yielding СО2 and Н2О end products. This leads to acid content neutralization and pH rise that ameliorates local environment. Taking into account virtually similar amounts of viridans streptococci and veillonellas in saliva, the latter degrade lactic acid produced after streptococcal fermentation thus protecting against caries.

Gram-negative diplococci from Neisseria genus are facultatively anaerobic. They can be found at an early stage of dental plaque initiation and growth. Unlike streptococci they demonstrate a slow rate of propagation. Their most common species are N. sicca that produce various polysaccharides and N. subflava.

Oral gram-positive aerobic and facultatively anaerobic rods comprise lactobacilli, corynebacteria and some other representatives.

The members of Lactobacillus genus generate ample quantities of lactic acid upon carbohydrate fermentation that actively stimulates caries progression. Corynebacteria lower redox potential in local dental surroundings ensuring beneficial conditions for anaerobic bacteria overgrowth (e.g., bacteroids, prevotellas, porphyromonads, fusobacteria, spirochetes, and many others). Moreover, corynebacteria produce vitamin K that is used as a potent growth factor by many oral bacteria.

Two genera from Actinomycetaceae family, namely Actinomyces and Bifidobacterium, can be found in oral microflora as well.

Actinomycetes easily settle upon mucous layer of oral cavity; they are typical microbial constituents of dental plaque and dental stone. Actinomycetes are commonly isolated from ducts of salivary glands, gingival pockets, and carious cavities. These bacteria possess weak proteolytic activity but intensively ferment carbohydrates accumulating broad spectra of organic acids (lactate, acetate, succinate, formate and others).

The species A. israelii, A. naeslundii genospecies 2 (former A. viscosus) contribute to caries and periodontal disease progression.

Bifidobacteria ferment numerous carbohydrates with lactic and acetic acid end products predisposing to decay of dental enamel and caries.

Gram-negative rods predominantly comprise obligate anaerobic bacteria from genera Bacteroides, Porhyromonas, Prevotella, Fusobacteium, Leptotrichia. These agents are autochthonous representatives of oral microbiota. They lack catalase, ferment carbohydrates with gas and hydrolyze proteins to amino acids.

Multiple bacteroidal members of microbial community belong to B. forsythus, B. gracilis, B. urealyticus, and many other species. In association with streptococci and fusobacteria they may exert periodontal disorders.

Pigment bacteria Porphyromonas gingivalis and P. endodontalis are isolated from periodontal tissues. They are typically indole-producing. P. gingivalis expresses collagenase that acts detrimentally on dentin layer and destroys fibrinogen. These pathogens are found in gingivitis and periodontal pathologies.

Very common oral pathogens are Prevotella melaninogenica, P. oralis, P. denticola, P. buccalis. Their carbohydrate-fermenting capacity is low. P. melaninogenica is a constant habitant of dental pockets in adults. By secretion of phospholipase A it breaks cell membrane integrity thereby stimulating periodontal diseases.

Fusobacteria are spindle-like polymorphic rods that grow in dental pockets in association with other bacteria (e.g., spirochetes). They weakly ferment carbohydrates and peptone, releasing butyric, and lesser amounts of lactic and acetic acids; prodice indole. Typical representative is F. nucleatum.

Leptotrychia are granular polymorphic rods, some of them are filamentous. Most frequent agent here is L. buccalis, capable of glucose fermenting with large amounts of lactic acid. The amount of these bacteria rises in case of periodontal disease progression..

Conventional members of dental microflora embrace numerous spirochetal species of Treponema, Borrelia and Leptospira genera. Typical oral treponemas are T. oralis, T. macrodentium, T. denticola and others.

Treponema vincentii is ordinarily found in oral folds and dental pockets. It produces modest amounts of acetic and butyric acids. In persons with weakened immunity Treponema vincentii together with prevotellas and fusobacteria exerts acute necrotizing ulcerative gingivitis (or ANUG), demonstrating sudden onset.

Gingival pockets often harbor Borrelia buccalis – large spirochetes that live in symbiotic associations with fusiform bacteria.

Most of oral mycoplasmas pertain to saprophytic M. orale and M. salivarium species.

Candida fungi participate in colonization of oral mucosa in closest interrelationships with neighboring bacteria. In most situations they don’t evoke pathological changes. However, in cases of indiscriminate use of antibiotics or secondary immune deficiencies oral candidiasis can arise thus indicating deep shift in local microbiota composition that results in dysbiosis.