Anatomy and Normal Microbiota of the Oral Cavity


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a) Structures of the head and neck: lips, jaw, nasal cavity (large space behind the nose), oral cavity (space in the mouth), tongue, uvula (structure in at the back of the mouth), pharyx (tube at the back of the mouth), esophagus (the pharyx is the top part of this tube which is now called the esophagus in the throat), and the larynx (this is also continuous with the pharynx but leads to the respiratory system). B) Components of the mouth region: teeth, sublingual gland (Below the tongue), submandibular gland (at the back and to the bottom of the mouth), and the parotid gland (a large gland at the very back of the mouth).
(a) When food enters the mouth, digestion begins. (b) Salivary glands are accessory digestive organs. (credit: modification of work by National Cancer Institute)

OpenStax Microbiology

Food enters the digestive tract through the mouth, where mechanical digestion (by chewing) and chemical digestion (by enzymes in saliva) begin. Within the mouth are the tongue, teeth, and salivary glands, including the parotid, sublingual, and submandibular glands. The salivary glands produce saliva, which lubricates food and contains digestive enzymes.

The structure of a tooth begins with the visible outer surface, called the crown, which has to be extremely hard to withstand the force of biting and chewing. The crown is covered with enamel, which is the hardest material in the body. Underneath the crown, a layer of relatively hard dentin extends into the root of the tooth around the innermost pulp cavity, which includes the pulp chamber at the top of the tooth and pulp canal, or root canal, located in the root. The pulp that fills the pulp cavity is rich in blood vessels, lymphatic vessels, connective tissue, and nerves. The root of the tooth and some of the crown are covered with cementum, which works with the periodontal ligament to anchor the tooth in place in the jaw bone. The soft tissues surrounding the teeth and bones are called gums, or gingiva. The gingival space or gingival crevice is located between the gums and teeth.

Structure of a tooth. The part above the gums (gingiva) is called the crown, the part below the gingiva is the root. The very top of the crown is a thick enamel, this is very thick at the region above the gingiva and much thinner in the root. The next layer in is the dentin and this is equally thick in both the crown and root. In the very center is the pulp which contains the pulp canal (root canal) and nerve and blood vessels. The root sits mainly in the bone region. There is a small space where the tooth extends past the gingiva, this is called the gingival crevice.
The tooth has a visible crown with an outer layer of enamel, a layer of dentin, and an inner pulp. The root, hidden by the gums, contains the pulp canal (root canal). (credit: modification of work by Bruce Blaus)

Microbes such as bacteria and archaea are abundant in the mouth and coat all of the surfaces of the oral cavity. However, different structures, such as the teeth or cheeks, host unique communities of both aerobic and anaerobic microbes. Some factors appear to work against making the mouth hospitable to certain microbes. For example, chewing allows microbes to mix better with saliva so they can be swallowed or spit out more easily. Saliva also contains enzymes, including lysozyme, which can damage microbial cells. Recall that lysozyme is part of the first line of defense in the innate immune system and cleaves the β-(1,4) glycosidic linkages between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in bacterial peptidoglycan. Additionally, fluids containing immunoglobulins and phagocytic cells are produced in the gingival spaces. Despite all of these chemical and mechanical activities, the mouth supports a large microbial community.


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at: