The Normal Bacterial Flora of Humans (page 5)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Dental Caries,
Gingivitis and Periodontal Disease
The most frequent and economically-important condition in humans
resulting from interactions with our normal flora is probably
dental caries. Dental plaque,
dental caries, gingivitis and periodontal disease result from actions
initiated and
carried out by the normal bacterial flora.
Dental plaque,
which is material
adhering to the teeth, consists of bacterial cells (60-70% the volume
of
the plaque), salivary polymers, and bacterial extracellular products.
Plaque
is a naturally-constructed biofilm, in which the consortia of bacteria
may reach a thickness of 300-500 cells on the surfaces of the teeth.
These
accumulations subject the teeth and gingival tissues to high
concentrations
of bacterial metabolites, which result in dental disease.
The dominant bacterial species in dental plaque are Streptococcus
sanguis and Streptococcus mutans, both of which are
considered
responsible for plaque.
Streptococcus
mutans.
Gram stain. CDC.
Plaque formation is
initiated by a weak attachment of the
streptococcal
cells to salivary glycoproteins forming a pellicle on the surface of
the
teeth. This is followed by a stronger attachment by means of
extracellular
sticky polymers of glucose (glucans) which are synthesized by the
bacteria
from dietary sugars (principally sucrose). An enzyme on the cell
surface
of Streptococcus mutans,
glycosyl transferase, is involved
in
initial attachment of the bacterial cells to the tooth surface and in
the
conversion of sucrose to dextran polymers (glucans) which
form plaque.
Dental
plaque, scanning electron micrograph illustrating the diversity of
microbes in plaque.
Dental Caries is the destruction
of the enamel, dentin or cementum of teeth due to bacterial activities.
Caries are initiated by direct demineralization of the enamel of teeth
due to lactic acid and other organic acids which accumulate in dental
plaque.
Lactic acid bacteria in the plaque produce lactic acid from the
fermentation
of sugars and other carbohydrates in the diet of the host.
Streptococcus
mutans and Streptococcus
sanguis are most consistently been associated with the
initiation of
dental
caries, but other lactic acid bacteria are probably involved as well.
These
organisms normally colonize the occlusal fissures and contact points
between
the teeth, and this correlates with the incidence of decay on these
surfaces.
Cross section of a
tooth illustrating the various structural regions susceptible to
colonization or attack by microbes.
Streptococcus mutans in particular has a number of
physiological and
biochemical
properties which implicate it in the initiation of dental caries.
1. It is a regular component of the normal oral flora of humans which
occurs in relatively large numbers. It readily colonizes tooth
surfaces:
salivary components (mucins, which are glycoproteins) form a thin film
on the tooth called the enamel pellicle. The adsorbed mucins are
thought
to serve as molecular receptors for ligands on the bacterial cell
surface.
2. It contains a cell-bound protein, glycosyl transferase, that
serves
an adhesin for attachment to the tooth, and as an enzyme that
polymerizes dietary sugars into glucans that leads to the formation
of
plaque.
3. It produces lactic acid from the utilization of dietary
carbohydrate
which demineralizes tooth enamel. S.
mutans produces more lactic acid
and
is more acid-tolerant than most other streptococci.
4. It stores polysaccharides made from dietary sugars which can be
utilized
as reserve carbon and energy sources for production of lactic acid. The
extracellular glucans formed by S.
mutans are, in fact, bacterial
capsular
polysaccharides that function as carbohydrate reserves. The organisms
can
also form intracellular polysaccharides from sugars which are stored in
cells and then metabolized to lactic acid.
Streptococcus mutans appears to be important in the initiation
of
dental caries because its activities lead to colonization of the tooth
surfaces, plaque formation, and localized demineralization of tooth
enamel.
It is not however, the only cause of dental decay. After initial
weakening
of the enamel, various oral bacteria gain access to interior regions of
the tooth. Lactobacilli, Actinomyces, and various proteolytic
bacteria
are commonly found in human carious dentin and cementum, which suggests
that they are secondary invaders that contribute to the progression of
the lesions.
Actinomyces israelii
Periodontal
Diseases are
bacterial
infections that affect the supporting structures of the teeth (gingiva,
cementum, periodontal membrane and alveolar bone). The most common
form,
gingivitis,
is an inflammatory condition of the gums. It is associated with
accumulations
of bacterial plaque in the area. Increased populations of Actinomyces
have been found, and they have been suggested as the cause.
Diseases that are confined to the gum usually do not lead to
loss of
teeth, but there are other more serious forms of periodontal disease
that
affect periodontal membrane and alveolar bone resulting in tooth loss.
Bacteria in these lesions are very complex populations consisting of
Gram-positive
organisms (including Actinomyces and streptococci) and
Gram-negative
organisms (including spirochetes and Bacteroides). The
mechanisms
of tissue destruction in periodontal disease are not clearly defined
but
hydrolytic enzymes, endotoxins, and other toxic bacterial metabolites
seem
to be involved.
