Mucous membrane: zones and colonization
The mucous membrane is the largest biotope of the oral cavity by surface area. Due to significant environmental variability across its various sites, high microfloral variability is observed here. Gram-negative anaerobic bacteria, facultative anaerobes, and microaerophiles requiring reduced oxygen levels predominantly inhabit the superficial epithelial layers.
In specific zones such as the sublingual region, inner cheek surfaces, deep folds, and crypts, the oxygen concentration is minimal. For this reason, obligate anaerobic species unconditionally dominate here: peptostreptococci, lactobacilli, and Veillonella. In addition, various streptococci, particularly Streptococcus mitis and Streptococcus salivarius, actively proliferate in these niches.
Special attention should be paid to the microflora of the hard and soft palate mucosa, palatine arches, and tonsils. These anatomical structures are characterized by an extremely high density of microbial colonization. The dominant groups of microorganisms here include:
- Streptococci (Streptococcus);
- Corynebacteria (Corynebacterium);
- Neisseria (Neisseria);
- Haemophilus (Haemophilus);
- Pseudomonads (Pseudomonas).
In addition to bacterial flora, nocardiae and yeast-like fungi may be detected as additional residents on the mucous membranes.
Gingival sulcus and gingival fluid
The gingival sulcus is a narrow space between the tooth and the gum, which fills with gingival fluid. This biotope is characterized by pronounced anaerobic conditions. By its physiological nature, gingival fluid is a transudate. As soon as it is secreted into the sulcus, it is instantly contaminated with microorganisms originating from the gingival mucosa and surrounding oral fluid.
The microbiological profile of this biotope is dominated by filamentous and convoluted obligate anaerobic bacteria. The gingival sulcus serves as the main habitat for such genera as Bacteroides, Prevotella, and Porphyromonas. Along with them, fusobacteria, spirochetes, campylobacters, actinomycetes, leptotrichiae, and spirilla vegetate here in massive numbers. Concomitant flora is represented by mycoplasmas, protozoa, and yeast-like fungi.
To assess the state of this biotope in the laboratory, quantitative methods for sample collection are used (micropipettes, special capillaries, or cannulas are employed).
Quantitative parameters of gingival fluid:
- Normal state: the concentration of microbial cells does not exceed $\le 10 \times 10^5$ (one million) per 1 ml.
- Pathological state (e.g., in gingivitis or periodontitis): a sharp, avalanche-like increase in bacterial load is observed, reaching tens or even hundreds of millions of cells per milliliter of transudate.
Oral fluid (mixed saliva)
Oral fluid acts as an integral biotope that physically and chemically connects absolutely all parts of the oral cavity. The basis of this liquid medium is the secretion of the salivary glands. Oral fluid serves not only as a transit zone ensuring interaction between local biotopes, but also as a crucial space for the implementation of regulatory mechanisms of the macroorganism itself.
The formation of the microbial composition of mixed saliva occurs due to the continuous influx of bacteria from the mucous membranes, the gingival sulcus, periodontal pockets, folds, and surface dental plaque.
Characteristic representatives of the oral fluid microflora include:
- Veillonella (Veillonella);
- Streptococci, among which facultative anaerobic species and microaerophiles stand out (especially Streptococcus salivarius);
- Aerococci and mycoplasmas;
- Motile forms of microorganisms, such as vibrios, selenomonas, spirilla, and spirochetes.
During quantitative cultural examination, an extremely high microbial contamination is always registered in the oral fluid: from several tens of millions to billions of cells per milliliter of saliva.
Briefly: answers to topical questions
Which streptococcal species are part of the resident microflora of the oral cavity?
The resident microflora of the oral cavity includes representatives of the viridans group streptococci. The main species include:
- Streptococcus salivarius (Streptococcus salivarius) — a permanent resident, predominates on the dorsum of the tongue and in saliva.
- Streptococcus sanguis (Streptococcus sanguis / S. sanguinis) — participates in the early colonization of the dental pellicle.
- Streptococcus mutans (Streptococcus mutans) — the main cariogenic species, dominates in dental plaque.
- Streptococcus mitis (Streptococcus mitis) — predominates on the buccal mucosal epithelium.
- Streptococcus milleri (Streptococcus milleri) — possesses high adhesive properties to epithelial cells.
Also, S. gordonii and S. oralis participate in early colonization.
Which bacteria perform primary colonization of the dental pellicle?
Primary colonization of the dental pellicle is performed by aerobes and facultative anaerobes. Primary colonizers include:
- streptococci (Streptococcus) — specifically S. gordonii, S. mitis, S. oralis, S. sanguinis;
- corynebacteria (Corynebacterium);
- actinomycetes (Actinomyces).
Early colonizing streptococci bind to salivary receptors on the dental pellicle. The role of primary colonizers is to create oxygen-free conditions, after which the plaque is colonized by strict anaerobes.
What microorganisms constitute the basis of mature dental plaque microflora?
The microflora of dental plaque changes as it matures.
The following microorganisms are indicated for dental plaque:
- streptococci;
- bacilli;
- vibrios;
- actinomycetes.
In the dynamics of plaque maturation, the following is noted:
- on days 2–4 — a decrease in the number of streptococci, an increase in the number of facultative and anaerobic actinomycetes, as well as Gram-negative cocci and rods;
- after a week — the appearance of spirochetes and motile forms.
During late colonization, Porphyromonas gingivalis, Prevotella denticola, Tannerella forsythia, Treponema denticola, Capnocytophaga spp., Actinomyces spp., and Aggregatibacter actinomycetemcomitans join the formed base.
Which bacterial species are specific to the dorsum of the tongue biotope?
The biotope of the dorsum of the tongue is characterized by:
- Streptococcus salivarius — predominates on the dorsum of the tongue; easily washed off by saliva and forms the basis of oral fluid microflora;
- Actinomycetes (Actinomyces) are found in large quantities on the tongue.
The dorsum of the tongue is one of the oral cavity regions with a high concentration of microorganisms.
What antimicrobial factors are present in mixed saliva?
Saliva contains antimicrobial and protective factors:
- lysozyme;
- secretory immunoglobulin A (sIgA);
- lactoferrin;
- peroxidase;
- antimicrobial peptides.
Secretory IgA and antimicrobial peptides are involved in blocking the attachment of microorganisms to the epithelium; the constant flow of saliva also prevents pathogens from adhering to the epithelium and teeth.
To remember typical inhabitants of mixed saliva (oral fluid), use the phrase "Cheerful Streptococcus Seeks Mobile Mycoplasmas": Veillonella, Streptococci, Motile forms (vibrios, spirochetes), Mycoplasmas (as well as aerococci).
Class and exam questions
Which oral biotope is called integral and why?
Oral fluid (mixed saliva) serves as an integral biotope. It receives microbial flora from all other sites (mucosa, sulci, plaque) and interconnects them, acting as a medium for interaction.
Which bacteria predominate in the microflora of the gingival sulcus?
Filamentous and convoluted obligate anaerobic bacteria dominate in the gingival sulcus. This zone serves as the primary habitat for periodontal pathogens of the genera Bacteroides, Prevotella, and Porphyromonas.
How does the quantitative composition of gingival fluid change in gingivitis?
Normally, the number of microorganisms in the gingival fluid does not exceed one million cells per 1 ml. With the development of pathology such as gingivitis or periodontitis, this figure rises sharply to hundreds of millions of cells.
What microorganisms are characteristic of specific zones of the mucous membrane (folds and crypts)?
In the crypts and folds of the mucosa, where oxygen access is limited, obligate anaerobic species dominate: peptostreptococci, veillonellae, lactobacilli, as well as streptococci (Streptococcus mitis, Streptococcus salivarius).
What else to study on this topic
- Mechanisms of oral fluid microflora formation
- Characteristics of anaerobic bacteria in the gingival sulcus
- Quantitative research methods for oral biotopes
- The role of veillonellae and streptococci in mucosal microbiocenosis
- Changes in the microbial landscape in periodontitis and gingivitis
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete guide on "Oral Biotopes" and 4,383 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the «Clinical Microbiology» section
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.