Dental plaque is a biofilm, usually a pale yellow, that develops naturally on the teeth. Like anybiofilm(生物膜), dental(牙科的) plaque is formed by colonizing(移于殖民地) bacteria trying toattach(依附) themselves to the tooth‘s smooth surface.^[1] It has been speculated(推测) that plaque forms part of the defense systems of the host by helping to prevent colonization of microorganismsthat may be pathogenic.^[2][3]

The oral cavity contains the only known anatomical(解剖的) aspect(方面) of the human body that does not have a regulated(调节) system of shedding(脱落) surfaces: the teeth. This allowsmicroorganisms(微生物) to adhere(坚持) to the surface of teeth for long periods of time.^[4] These multiple species(物种) of bacteria become dental(牙科的) biofilm(生物膜). Dental biofilm, more commonly referred to as dental plaque(匾), is composed(构成) of about a thousand species of bacteria that take part in the complex(复杂的) ecosystems(生态系统) of the mouth. The natural, non-frequent regulation of tooth shedding plays a large role in making dental biofilm the mostdiverse(不同的) biofilm in the human body despite(尽管) the relatively small size of the teeth.

The human oral cavity is also called the human oral microbiome. This is because the human oralcavity(腔) can contain several environments at a given moment that could vary(变化) from tooth to tooth.^[5] Additionally(附加的) it has been estimated(估计的) that 25,000 species of bacteriareside(住) in the mouth.^[6] This is in contrast(对比) to the previously estimated 700+ species.^[5]Studies have found that out of the 25,000 species(物种) that exist in the oral cavity(腔), about 1,000 species can exist as part of the dental(牙科的) biofilm(生物膜) ecosystem(生态系统).^[7] This is also in contrast(对比) to the previous estimate(估计) of more than 500 species as part of the dental biofilm.^[8] These 1,000 species have the ability to change their environment through a series of biotic(生物的) relationships.

At first, the biofilm is soft enough to come off by using the fingernail. However, it starts toharden(变硬) within 48 hours, and in about 10 days the plaque(匾) becomes dental calculus(tartar), which is hard and difficult to remove.^{[citation needed]}

Dental plaque can give rise to dental caries (tooth decay(腐烂))—the localised(限局性的)destruction(破坏) of the tissues(纸巾) of the tooth by acid(酸) produced from the bacterial(细菌的) degradation(退化) of fermentable sugars^[2][3]—and periodontal problems such as gingivitis andchronic periodontitis.

Basic plaque formation

Microscopic(微观的) view of some of the bacteria of whichplaque(匾) is composed(构成). Numbered ticks are 10 µmapart(相距).

The mechanisms(机制) of plaque formation(形成) include:^[2][3]

• Adsorption of proteins(蛋白质) and bacteria to form a film on the tooth surface.
• The effect of van der Waals and electrostatic forces between microbial(微生物的) surfaces and the film to create reversible adhesion to the teeth.
• Irreversible adhesion(粘附) due to intermolecular(分子间的) interactions(相互作用) between cell surfaces and the pellicle(薄膜).
• Secondary(第二的) colonisers attach(依附) to primary colonisers by intermolecular interaction.
• The cells divide and generate(形成) a biofilm(生物膜).

Microorganisms

As previously mentioned there are about 1,000 out of the 25,000 species of bacteria that areinvolved(包含) with the formation(形成) of dental(牙科的) biofilm. Due to this fairly large number there is fierce competition among the bacteria present on dental biofilm for nutrients(营养物)present in the mouth. Only about fifty percent of the 1,000 species have been cultured for study.^[7] Scientists at the Forsyth Institute in Cambridge, Massachusetts have begun the Human Oral Microbiome Project to identify bacteria and study the complex(复杂的) ecosystems(生态系统) of the mouth. On their website, one can find information about the identified(被识别的) organisms(有机体)as well as the method being utilized(利用) to identify the organisms.

Environment

The ecological(生态的) factors(因素) provided by the environment of the oral cavity(腔) are directly proportional(比例的) to the species(物种) richness(丰富) and species biodiversity(生物多样性) of the microorganisms(微生物) that reside(住) on the teeth.^[4] The main ecological factors are pH, saliva, temperature and redox reactions.^[4][9] The majority of microbial(微生物的)organisms(有机体) prefer neutral(中立国) pH levels (pH 7). Saliva(唾液) acts as a buffer,maintaining(维持) the pH in the mouth between 6.75 and 7.25.^[4] In addition to acting as abuffer(缓冲区), saliva is also a main source of nutrients(营养物) for the thousands of bacteria(note: gingival crevicular fluid is also a nutrient(营养的) source but a smaller one ). A two degree (°C) change has been shown to drastically(彻底地) shift(移动) the dominant(显性的)species(物种) in the plaque(匾).^[9] The normal temperature of the mouth ranges between 35–36°C (Marsh). Redox reactions are carried out by aerobic bacteria. This keeps the oxygen levels in the mouth at a semi-stable homeostatic(自我平衡的) condition. This allows other bacteria to survive(幸存), which will be discussed in the next section.^[9]

Biotic relationships

The microorganisms(微生物) in the oral cavity live with one another in commensal or mutualisticsymbiotic relationships.^[4] Typically, anaerobic bacteria would succumb(屈服) to high levels of oxygen, but with the redox(氧化还原剂) reactions(反应) discussed in the previous section they are able to survive(幸存). This commensal(共餐者) relationship allows a mixture of aerobic(需氧的) andanaerobic(厌氧的) bacteria to live in the same area. The formation(形成) begins by the adsorptionof early colonizers(移于殖民地) onto an acquired pellicle through chemical processes.^[10] Anacquired pellicle is a layer of saliva(唾液) that is composed(构成) of mainly glycoproteins and forms shortly after cleaning of the teeth or exposure(暴露) of new teeth.^[10] These bound earlycolonizers(移于殖民地) manipulate(操纵) the environment for the immediate benefit(利益) of other bacteria. Once the environment has been manipulated other bacterial(细菌的) colonizers are able to co-adhere to the early colonizers. This is done repeatedly resulting in layers of bacteria. Once new bacterial cells co-adhere to one another they gain the ability to communicate to one another. They are able to communicate to one another through a biochemical(生物化学的) process calledquorum(法定人数) –sensing.^[7] Quorum –sensing virtually(事实上) allows all the bacteria to benefit from one another. This ability can allow a bacterium to feel the presence(存在) of other bacteria around it. Due to this communication, bacteria have the ability to change their genotype(and thus their phenotype) as a result of population concentration(浓度) and/or environmental(环境的) changes to remain as competent(胜任的) competitors.^[11] These relationships tend(照料) toexhibit(展览) homeostasis(体内平衡) until there is some type of disruption(破坏) in theecosystem(生态系统).

The most common reasons for ecosystem disruption are the ecological(生态的) factors(因素) that were discussed in the environment section. The bacteria that exhibits the most fit plasticity for the change in environment dominates(控制) the given environment. Often, this could lead toopportunistic(机会主义的) pathogens that lead to dental caries and periodontal disease. Pathogensthat have the potential(潜能) to cause dental caries flourish(兴旺) in acidic(酸的) environments.Pathogenic bacteria that have the potential(潜能) to cause periodontal disease flourish(兴旺) in a slightly alkaline(碱性的) environment.^[12]

Components of plaque

Plaque(匾) consists of microorganisms(微生物) and extracellular(细胞外的) matrix(矩阵).

The microorganisms that form the biofilm(生物膜) are mainly Streptococcus mutans and anaerobes, with the composition varying(变化) by location in the mouth. Examples of such anaerobes(厌氧性生物) include fusobacterium and actinobacteria.

The extracellular(细胞外的) matrix(矩阵) contains proteins(蛋白质), long chain polysaccharides(多糖) and lipids(脂质).

The microorganisms(微生物) present in dental(牙科的) plaque(匾) are all naturally present in the oral cavity(腔), and are normally harmless. However, failure to remove plaque by regular tooth brushing means that they are allowed to build up in a thick layer. Those microorganisms nearest the tooth surface ferment dietary(规定的食物) sucrose(蔗糖); it is in this state that they start to produce acids(酸).

• Acids released from dental plaque lead to demineralization of the adjacent(邻近的) tooth surface, and consequently(因此) to dental caries. Saliva is also unable to penetrate(渗透)the build-up of plaque and thus cannot act to neutralize(抵销) the acid produced by the bacteria and remineralize the tooth surface.
• They also cause irritation(刺激) of the gums(口香糖) around the teeth that could lead togingivitis, periodontal disease and tooth loss.
• Plaque(匾) build up can also become mineralized(矿物化) and form calculus (tartar).

Consequence(结果) of plaque build up

Plaque is a haven(港口) for oral micro-organisms(微生物) and continues to build up in the oralcavity(腔) until it can mineralize in to calculus(结石) (also known as tartar) or cause plaqueassociated(关联的) gum disease.

Detection(侦查) of plaque build up

Plaque detection is usually detected(察觉) clinically(临床地) by plaque disclosing(公开) agents. Disclosing agents contains dye which turns bright red to indicate(表明) plaque buildup(增强).