Dental plaque is a biofilm, usually a pale yellow, that develops naturally on the teeth.
Like any biofilm, dental plaque is formed by colonizing bacteria trying to attach themselves
to the tooth's smooth surface. It has been speculated that plaque forms part of the defense
systems of the host by helping to prevent colonization of microorganisms that may be
pathogenic. The oral cavity contains the only known anatomical
aspect of the human body that does not have a regulated system of shedding surfaces: the
teeth. This allows microorganisms to adhere to the surface of teeth for long periods of
time. These multiple species of bacteria become dental biofilm. Dental biofilm, more commonly
referred to as dental plaque, is composed of about a thousand species of bacteria that
take part in the complex ecosystems of the mouth. The natural, non-frequent regulation
of tooth shedding plays a large role in making dental biofilm the most diverse biofilm in
the human body despite the relatively small size of the teeth.
The human oral cavity is also called the human oral microbiome. This is because the human
oral cavity can contain several environments at a given moment that could vary from tooth
to tooth. Additionally it has been estimated that 25,000 species of bacteria reside in
the mouth. This is in contrast to the previously estimated 700+ species. Studies have found
that out of the 25,000 species that exist in the oral cavity, about 1,000 species can
exist as part of the dental biofilm ecosystem. This is also in contrast to the previous estimate
of more than 500 species as part of the dental biofilm. These 1,000 species have the ability
to change their environment through a series of biotic relationships.
At first, the biofilm is soft enough to come off by using the fingernail. However, it starts
to harden within 48 hours, and in about 10 days the plaque becomes dental calculus, which
is hard and difficult to remove. Dental plaque can give rise to dental caries—the
localised destruction of the tissues of the tooth by acid produced from the bacterial
degradation of fermentable sugars—and periodontal problems such as gingivitis and chronic periodontitis.
Basic plaque formation
The mechanisms of plaque formation include: Adsorption of proteins and bacteria to form
a film on the tooth surface. The effect of van der Waals and electrostatic
forces between microbial surfaces and the film to create reversible adhesion to the
teeth. Irreversible adhesion due to intermolecular
interactions between cell surfaces and the pellicle.
Secondary colonisers attach to primary colonisers by intermolecular interaction.
The cells divide and generate a biofilm. Microorganisms
As previously mentioned there are about 1,000 out of the 25,000 species of bacteria that
are involved with the formation of dental biofilm. Due to this fairly large number there
is fierce competition among the bacteria present on dental biofilm for nutrients present in
the mouth. Only about fifty percent of the 1,000 species have been cultured for study.
Scientists at the Forsyth Institute in Cambridge, Massachusetts have begun the Human Oral Microbiome
Project to identify bacteria and study the complex ecosystems of the mouth. On their
website, one can find information about the identified organisms as well as the method
being utilized to identify the organisms. Environment
The ecological factors provided by the environment of the mouth cavity are directly proportional
to the species richness and species biodiversity of the microorganisms that reside on the teeth.
The main ecological factors are pH, saliva, temperature and redox reactions. The majority
of microbial organisms prefer neutral pH levels. Saliva acts as a buffer, maintaining the pH
in the mouth between 6.75 and 7.25. In addition to acting as a buffer, saliva is also a main
source of nutrients for the thousands of bacteria. A two degree change has been shown to drastically
shift the dominant species in the plaque. The normal temperature of the mouth ranges
between 35–36 °C. Redox reactions are carried out by aerobic bacteria. This keeps
the oxygen levels in the mouth at a semi-stable homeostatic condition. This allows other bacteria
to survive, which will be discussed in the next section.
Biotic relationships The microorganisms in the oral cavity live
with one another in commensal or mutualistic symbiotic relationships. Typically, anaerobic
bacteria would succumb to high levels of oxygen, but with the redox reactions discussed in
the previous section they are able to survive. This commensal relationship allows a mixture
of aerobic and anaerobic bacteria to live in the same area. The formation begins by
the adsorption of early colonizers onto an acquired pellicle through chemical processes.
An acquired pellicle is a layer of saliva that is composed of mainly glycoproteins and
forms shortly after cleaning of the teeth or exposure of new teeth. These bound early
colonizers manipulate the environment for the immediate benefit of other bacteria. Once
the environment has been manipulated other bacterial 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 process called quorum
sensing. Quorum sensing virtually allows all the bacteria to benefit from one another.
This ability can allow a bacterium to feel the presence of other bacteria around it.
Due to this communication, bacteria have the ability to change their genotype as a result
of population concentration and/or environmental changes to remain as competent competitors.
These relationships tend to exhibit homeostasis until there is some type of disruption in
the ecosystem. The most common reasons for ecosystem disruption
are the ecological factors that were discussed in the environment section. The bacteria that
exhibits the most fit plasticity for the change in environment dominates the given environment.
Often, this could lead to opportunistic pathogens that lead to dental caries and periodontal
disease. Pathogens that have the potential to cause dental caries flourish in acidic
environments. Pathogenic bacteria that have the potential to cause periodontal disease
flourish in a slightly alkaline environment. Components of plaque
Plaque consists of microorganisms and extracellular matrix.
The microorganisms that form the biofilm are mainly Streptococcus mutans and anaerobes,
with the composition varying by location in the mouth. Examples of such anaerobes include
fusobacterium and actinobacteria. The extracellular matrix contains proteins,
long chain polysaccharides and lipids. The microorganisms present in dental plaque
are all naturally present in the oral cavity, 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 sucrose;
it is in this state that they start to produce acids.
Acids released from dental plaque lead to demineralization of the adjacent tooth surface,
and consequently to dental caries. Saliva is also unable to penetrate the build-up of
plaque and thus cannot act to neutralize the acid produced by the bacteria and remineralize
the tooth surface. They also cause irritation of the gums around
the teeth that could lead to gingivitis, periodontal disease and tooth loss.
Plaque build up can also become mineralized and form calculus.
Consequence of plaque build up Plaque is a haven for oral micro-organisms
and continues to build up in the oral cavity until it can mineralize into calculus or cause
plaque associated gum disease. Detection of plaque build up
Plaque detection is usually detected clinically by plaque disclosing agents. Disclosing agents
contain dye which turns bright red to indicate plaque buildup.
See also
Flossing Gingiva
Dental disease Oral hygiene
Oral microbiology References
External links A Biofilm Primer