Oral Microflora

-nicole lee

What is Microflora

Microflora refers to the community of microorganisms coexisting in a particular site on the human body. Oral microflora refers to the various different strains of bacteria which have the oral cavity as its primary habitat. (Hill & Marsh, 1990, p.2) These strains of bacteria colonize the various different surfaces present in the oral cavity, and communicate between each other through complex cell signaling processes. The body’s own defenses also play a role in maintaining a balance and ensuring a healthy oral environment. (Marsh & Martin, 1999, p.1) Microflora activity in the oral cavity gives rise to the development of caries (Marsh & Nyvad, 2008, p.164) as well as other dental diseases such as infections of the gingiva and periodontum. (Hill & Marsh, 1990, p.2)


The origin of Microflora


The microflora present in our mouths is acquired from birth, (Marsh & Martin, 1999, p.1) as newborn babies have no bacteria in their mouths. Over time, the oral cavity becomes colonized by the strains of bacteria that eventually form the resident microflora, which establish a permanent population in the mouth. (Hill & Marsh, 1990, p.4) Over time as the individual is further exposed to external sources of bacteria, the biodiversity of the oral cavity increases, to a point where stability is reached. This is termed the climax community. (
Marsh & Nyvad,, 2008, p.165)

The source of these microorganisms mainly comes from saliva, although it also includes the food and water consumed by the individual. This process takes place within the first few hours of life. Studies have shown that majority of children obtain their resident microflora from their mothers, as they often possess identical strains of bacteria. This is known as vertical transmission. Horizontal transmission also takes place as children interact with their peers, and later in life between spouses and partners. (
Marsh & Nyvad, 2008, p.164)


Function of Microflora

While microflora consists of cells of non-mammalian origin, they are usually non-pathogenic and form an integral part of the host. The presence of friendly resident microorganisms on oral surfaces contributes to the body’s defense against foreign pathogens, which are generally transient and can give rise to harmful infections. This is known as colonization resistance. (
Marsh & Nyvad, 2008, p.164)

Colonization resistance is firstly achieved through the saturation of oral surfaces with preexisting resident bacteria, which reduces the available sites left for the attachment of exogenous organisms. Also, essential nutrients derived from the saliva and various proteins in the oral cavity are also more effectively utilized by the resident microflora, which inhibits infections via competition for resources. The conditions of the mouth are also modified by the presence of its resident microflora, which both maintains unfavorable conditions for invading microbes, as well as producing substances such as hydrogen peroxide that actively inhibits pathogen growth. (
Marsh & Nyvad, 2008, p.164)

As a result, the harmony between the oral microflora and the body’s natural defenses acts as an effective deterrent to infections, and thus maintains the health of the host. (Marsh & Martin, 1999, p.1)


Variations in Microflora

While everyone has microflora, it varies between people depending on the condition of their oral cavity, as well as their exposure to particular strains of microbes. Not all individuals would have the same microflora profile and the same strains of particular types of bacteria, and likewise not all bacteria is ubiquitous in all individuals. (Hill & Marsh, 1990, p.3)


Oral surfaces
The oral cavity contains various different forms of tissue which have different properties. As such, rather than being homogeneous, the microflora in the mouth is distributed according to the different types of surfaces present, (Marsh & Nyvad, 2008, p.165) which provide each particular species with its optimal growing environment. Certain breeds of bacteria are dependent on specific surfaces to adhere to. (Hill & Marsh, 1990, p.3)

Mucosal Surfaces:
These include the palate, cheek and tongue, which have cells which are constantly replaced due to the normal wear-and-tear of the mouth. The constant shedding of these epithelial cells poses a challenge for the colonizing bacteria, which also shed as well. (Hill & Marsh, 1990, p.19) Nonetheless, the different mucosal surfaces also have different properties which contribute to the presence of different types of bacteria. For example, areas such as the palate contain keratinized squamous epithelium, unlike the softer non-keratinized areas such as the cheeks. As a result, the microorganisms found on these surfaces can be expected to be different. (Marsh & Martin, 1999, p.7)

Likewise, the tongue is a unique surface for the adhesion of microflora. The surface of the tongue is covered with papillary structures, which provides ample crevices for a wide variety of different strains of bacteria to reside. (Marsh & Nyvad, 2008, p.165) The presence of these crypts allows for bacteria to be protected from the normal shedding and removal by saliva flow, and hence species not found elsewhere, such as obligate anaerobes, can be found on the dorsum of the tongue. Some of these species, such as the gram-negative anaerobes, have been implicated in the formation of caries. (Marsh & Martin, 1999, p.7)

Gingival crevicular fluid:
At the gingival margin, serum flows from the inside the gingiva into the mouth. The flow of this fluid removes foreign microbes which do not adhere to these surfaces. For the resident population, the flow of crevicular fluid provides these organisms with a source of nutrition, as they have specialised enzymes which can break down these proteins to provide them with the nutrients necessary for growth.(Marsh & Martin, 1999, p.9) As such, the gingival margins are hosts to specific types of bacteria as well.

Teeth and artificial hard surfaces:
Having non-shedding surfaces, microbes can accumulate on teeth as well as other hard areas such as artificially-inserted crowns, bridges, fillings and dentures. This generally forms a dense layer of bioflim known as dental plaque, which has been attributed to the formation of mineralized calculus, as well as the growth of diseases such as caries. (Hill & Marsh, 1990, p.22)

Fissures and deep grooves in the teeth, especially with regard to the occlusal surfaces of posterior teeth, provide ideal conditions for the formation and build-up of dental plaque, as these areas are seldom in direct contact and mechanical friction, which also removes bioflim from these surfaces. (Hill & Marsh, 1990, p.22)

It must be noted, however, that most of the microbe species that have been found colonizing hard surfaces such as teeth, are generally exclusive to tissues in the oral cavity with such properties. Hence, most of the bacteria found in dental plaque is absent in individuals without teeth, such as babies or adults such as the elderly who have lost all their teeth. (Hill & Marsh, 1990, p.22)

Oral Microflora - Cariology
(Hill & Marsh, 1990, p.3)


Age profiles
Over the course of an individual's life, the conditions in the oral cavity changes. Likewise, the microflora of the mouth in a person changes accordingly. As previously mentioned, babies are born with a sterile oral environment which is soon colonized by bacteria through exposure to the environment and other people. This process takes place within the first few hours of birth. (
Marsh & Nyvad, 2008, p.165)

In the development of the individual, bacteria continues to be acquired. It has also been postulated that specific strains of bacteria are more active in colonization in particular time frames, such as that of mutans streptococci at around 19-31 months of age.(
Marsh & Nyvad, 2008, p.165) Colonization by new strains of bacteria is also present in the onset of tooth eruption, and the presence of hard tooth surfaces allows for the adhesion of previously absent strains of bacteria.

The microflora of the individual also changes with aging, as the immunity of the host decreases naturally with age. This could lead to a direct increase in the levels of non-oral bacteria that resides in the mouth, and a change in the balance of microflora present. Indirectly, due to the changes in lifestyle and other associated health problems, the microflora also changes as a person ages. This could be due to the loss of teeth and increased usage of dentures, which promotes the growth of yeast, as well as the usage of long term medications which have side effects such as reduced secretion of saliva. As such, these changes alter the endogenous bacteria and provide foreign microbes with the opportunity to colonize greater areas of the mouth. (
Marsh & Nyvad, 2008, p.165)

Saliva
Saliva plays a vital role in the maintenance of a healthy oral environment, and to regulate the growth of specific strains of microflora present in the mouth. By acting as a buffer, saliva maintains the pH of the mouth, ensuring the optimal growth of the resident colonies. (
Marsh & Nyvad, 2008, p.165) Most of the naturally-occuring microbes present in the mouth also utilize the glycoproteins and proteins in the saliva as their main source of nutrition. (Marsh & Martin, 1999, p.8)

The presence of saliva and its constituent proteins and glycoproteins is also responsible for the formation of the pellicle, a film on the surface of the tooth enamel where the microorganisms of the microflora can adhere to.

The constant flow of saliva is also responsible for the removal of non-endogenous bacteria which is unable to adhere to specific sites in the mouth. (Hill & Marsh, 1990, p.22) Saliva thus acts a collector of these cells and facilitates their removal. (Marsh & Martin, 1999, p.8) This contributes to the way different bacteria dominate different oral surfaces, as other bacteria not adapted to adhere is quickly washed away by the saliva and swallowed. (
Marsh & Nyvad, 2008, p.166) As a result, areas which receive markedly less saliva flow, such as deep gingival crevices, proximal spaces and occlusal fissures, tend to have significantly higher levels of bacterial buildup. (Hill & Marsh, 1990, p.22)

Lastly, saliva also plays a role in carrying bacteria. Being the main source of microbial transmission, the passage of saliva between individuals has been credited with the main source of microflora in the colonization of the oral cavity. (
Marsh & Nyvad, 2008, p.164) Saliva also circulates bacteria within the oral cavity, resulting in re-colonization of oral surfaces where the microflora might be removed via mechanical forces such as cleaning. Most of the bacteria carried in the saliva come from the dorsum of the tongue. (Hill & Marsh, 1990, pp. 10, 22)

Resident and transient microflora
The microflora in the oral cavity has not been exhaustively studied and classified, and it may vary across individuals, in which some microbes might be present or absent. (Hill & Marsh, 1990, p.4) Despite this, more than 700 strains of bacteria have been identified in the oral cavity. (Aas et al., 2005, p.
It must be taken into account, however, that the data largely depends on the sensitivity of the methods used for detection. This is especially so for species which occur in lower numbers in smaller, more specific areas. (Hill & Marsh, 1990, p.4)

As a result of these studies, it has been deduced that while the microflora in each person’s mouth is very different, certain species of bacteria are more prolific across the demographic, and can be found commonly in all human mouths in general. These include species under the Streptococcus (spherical gram positive bacteria) and Actinomyces (rod-shaped gram positive bacteria) genii. The success of these organisms lies in their ability to colonize a wide variety of different oral surfaces and exist prolifically. This has given rise to the concept of a “basic plaque” microflora, which suggests that there are some core species which are universal to all humans. (Hill & Marsh, 1990, p.4) S. mitis, S. sanguis, S. mitior and S. oralis are species of streptococcus that are commonly identified as the main constituents of the basic plaque microflora. (Hill & Marsh, 1990, p.17) Table 3 below lists some of the most commonly recognised microbes considered to be members of the human resident oral microflora.

Conversely, some microorganisms that do not naturally belong in the oral microflora may be found in the mouth. These species might be due to migration of the microflora from other sites on the body, such as the skin, pharynx or intestinal tract, or might have entered the mouth due to contact with the environment when contaminated food or water is ingested. Generally, they cause no symptoms or problems due to the protective effects of the native oral microflora, but pathogens such as HIV, hepatitis B, tuberculosis, herpes simplex viruses may still be spread through the saliva, blood, or other secretions between individuals. (Hill & Marsh, 1990, p.5) These pathogens may manifest in opportunistic infections when the mucosa surface is breached, or if a person’s immunity is compromised due to physiological stresses or drug usage.


Oral Microflora - CariologyOral Microflora - Cariology
(Hill & Marsh, 1990, p.16)



Diseases due to Microflora


The colonization of microflora on the various oral surfaces may result in diseases, most notably in the incidence of caries, as well as stomatitis in dentures, where the surrounding gingiva becomes inflamed as a result of exposure to the bacteria and yeasts present on dentures. Such pathological states are generally due to an imbalance in the conditions of the oral cavity, such as the prolonged exposure to a high sugar concentration as a result of diet, or due to the reduction in saliva, as previously discussed.

These endogenous factors makes an individual more prone to the development of caries. Streptococcus mutans is the main strain of oral bacteria associated with caries, as they have been more commonly detected in higher numbers in individuals with such problems. Likewise, for periodontal diseases, bacteria such as Bacteroides Gingivalis and various other strains are isolated in greater numbers in such individuals. (Hill & Marsh, 1990, p.5)

Likewise, opportunistic infections can occur due to compromised oral structures, which expose vulnerable inner tooth structure to the bacteria in the mouth. This is especially so in the elderly, as gingival pockets generally deepen as people age and their gums recede. As a result, the elderly is more susceptible to root caries as the exposed cementum gets colonized. (Marsh & Martin, 1999, p.2) This is also the reason why the presence of caries in the enamel usually spreads inwards towards the dentine if left unchecked, as more susceptible tooth structure gets exposed.

The bacteria in the microflora may also bring about infections in other parts of the body such as the liver and the brain, where they may cause abscesses, or infective endocarditis where the heart membrane gets inflamed. (Hill & Marsh, 1990, p.17) Patients who do not practice proper oral hygiene, or have suppressed immune systems are more susceptible to imbalances in microflora, and opportunistic infections as a result.
Table 1 below lists the most common diseases which occur as a result of an infection by the microorganisms in the oral microflora.

Oral Microflora - Cariology
(Hill & Marsh, 1990, p.5)


References:

Aas, J. A., Paster, B. J., Stokes, L. N., Olsen, I., Dewhirst, F. E. (2005). Defining the normal bacteria flora of the oral cavity. J Clin Microbiol. 2005 Nov;43(11):5721-32.

Hill, M. J. & Marsh, P. D. (1990). Factors controlling the Microflora of the Healthy Mouth. In M.J. Hill & P.D. Marsh (Eds.) Human Microbial Ecology (1st Ed) (pp. 2- 47). Boca Raton, Florida: CRC Press.

Marsh, P. D. & Martin, M. V. (1999). Oral Microbiology (4th Ed)(p.1-2). Woburn, Massachusetts, U.S.A.:Butterworth-Heinemann Medical.

Marsh, P. D. & Nyvad, P. D. B. (2008). The oral microflora and biofilms on teeth. In O. Fejerskov and E. Kidd (Eds.). Dental Caries: the Disease and its Clinical Management (2nd Ed). Oxford: Blackwell Munksgaard Ltd.


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