Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Microbial flora of the oral cavity, dental caries

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the oldest life forms on Earth. They evolved long before the

presence of photosynthetic green plants, and so existed in an

oxygen-free world.

In the rumen, the methane-producing bacteria occupy a

central role in regulating the anaerobic breakdown (fermenta-

tion) of food. The bacteria remove hydrogen gas through the se

of the gas in the reduction of carbon dioxide to form methane.

By producing methane, the concentration of hydrogen is kept

at a low level that allows other bacterial species to grow. This

microbial diversity makes fermentation more efficient.

The bacterial production of methane is of economic

importance. “Biogas” obtained from digesters can be a com-

mercial and domestic energy source, although more economic

sources of energy currently limit this use. In large-scale live-

stock operations, the use of methane producing bacteria is

being increasing popular as a means of odor-control.

As on Earth, methane producing bacteria may be one of

the earliest forms of life on other planets. Experiments that

duplicate the atmosphere of the planet Mars have been suc-

cessful in growing methane producing bacteria. Aside from its

fundamental scientific importance, the discovery might be

exploited in future manned missions to Mars. Methane is

described as being a greenhouse gas, which means it can warm

the surface atmosphere. On a small-scale, methane production

might create a more hospitable atmosphere on the surface of

Mars. Additionally, the combustible nature of methane, uti-

lized on Earth as a biogas, could someday provide rocket fuel

for spacecraft.

See also Biogeochemical cycles; Chemoautotrophic and

chemolithitrophic bacteria; Extremophiles

MICRO ARRAYS• seeDNACHIPS AND MICROARRAYS

MICROBIAL FLORA OF THE ORAL

CAVITYMicrobial flora of the oral cavity, dental caries, DENTAL CARIES

The microbial flora of the oral cavity are rich and extremely

diverse. This reflects the abundant nutrients and moisture, and

hospitable temperature, and the availability of surfaces on

which bacterial populations can develop. The presence of a

myriad of microorganismsis a natural part of proper oral

health. However, an imbalance in the microbial flora can lead

to the production of acidic compounds by some microorgan-

isms that can damage the teeth and gums. Damage to the teeth

is referred to a dental caries.

Microbes can adhere to surfaces throughout the oral

cavity. These include the tongue, epithelial cells lining the roof

of the mouth and the cheeks, and the hard enamel of the teeth.

In particular, the microbial communities that exist on the sur-

face of the teeth are known as dental plaque. The adherent

communities also represent a biofilm. Oral biofilms develop

over time into exceedingly complex communities. Hundreds

of species of bacteriahave been identified in such biofilms.

Development of the adherent populations of microor-

ganisms in the oral cavity begins with the association and irre-

versible adhesion of certain bacteria to the tooth surface.

Components of the host oral cavity, such as proteins and gly-

coproteins from the saliva, also adhere. This early coating is

referred to as the conditioning film. The conditioning film

alters the chemistry of the tooth surface, encouraging the adhe-

sion of other microbial species. Over time, as the biofilm thick-

ens, gradients develop within the biofilm. For example, oxygen

may be relatively plentiful at the outer extremity of the biofilm,

with the core of the biofilm being essentially oxygen-free. Such

environmental alterations promote the development of differ-

ent types of bacteria in different regions of the biofilm.

This changing pattern represents what is termed bacter-

ial succession. Examples of some bacteria that are typically

present as primary colonizers include Streptococcus,

Actinomyces, Neisseria, and Veillonella. Examples of second-

ary colonizers include Fusobacterium nucleatum, Prevotella

intermedia, and Capnocytophagaspecies. With further time,

another group of bacteria can become associated with the

adherent community. Examples of these bacteria include

Campylobacter rectus, Eikenella corrodens, Actinobacillus

actinomycetemcomitans, and the oral spirochetesof the genus

Treponema.

Under normal circumstances, the microbial flora in the

oral cavity reaches equilibrium, where the chemical by-prod-

ucts of growth of some microbes are utilized by other

microbes for their growth. Furthermore, the metabolic activi-

ties of some bacteria can use up oxygen, creating conditions

that are favorable for the growth of those bacteria that require

oxygen-free conditions.

This equilibrium can break down. An example is when

the diet is high in sugars that can be readily used by bacteria.

The pHin the adherent community is lowered, which selects

for the predominance of acid-loving bacteria, principally

Streptococcus mutans and Lactobacillus species. These

species can produce acidic products. The resulting condition is

termed dental caries. Dental caries is the second most common

of all maladies in humans, next only to the common cold. It is

the most important cause of tooth loss in people under 10

years of age.

Dental caries typically proceeds in stages. Discoloration

and loosening of the hard enamel covering of the tooth precedes

the formation of a microscopic hole in the enamel. The hole

subsequently widens and damage to the interior of the tooth

usually results. If damage occurs to the core of the tooth, a

region containing what is termed pulp, and the roots anchoring

the tooth to the jaw, the tooth is usually beyond saving.

Removal of the tooth is necessary to prevent accumulation of

bacterial products that could pose further adverse health effects.

Dental caries can be lessened or even prevented by coat-

ing the surface of the tooth with a protective sealant. This is

usually done as soon as a child acquires the second set of

teeth. Another strategy to thwart the development of dental

caries is the inclusion of a chemical called fluoride in drinking

water. Evidence supports the use of fluoride to lessen the pre-

dominance of acid-producing bacteria in the oral cavity.

Finally, good oral hygieneis of paramount importance in den-

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