Preventive Dentistry
Role of bacteria in caries development
Lec.11 Dr.Jihan Abdulhussein
Role of mutans streptococci in dental caries
Includes the following correlations of mutans streptococci counts in saliva and
plaque with the prevalence and incidence of caries
Mutans streptococci can be often isolated from the tooth surface
immediately before development of caries.
Positive correlation between the progression of carious lesions and S.
mutans counts
Production of extracellular polysaccharides from sucrose (which help to
cement the plaque organisms together and to the tooth surface)
Most effective streptococcus in caries studies in animals have ability to
initiate and maintain microbial growth and to continue acid production at
low pH values
Rapid metabolism of sugars to lactic and other organic acids
Ability to attain the critical pH for enamel demineralization more rapidly
than other common plaque bacteria
Ability to produce intracellular polysaccharides (IPS) as glycogen, which
may act as a food store for use when dietary carbohydrates are low
Immunization of animals with specific S. mutans serotypes significantly
reduces the incidence of caries.
The role of lactobacilli:
Previously believed to be the causative agents of dental caries. They were
candidate organisms for caries because of:
Their high numbers in most carious lesions affecting enamel (many studies
have now shown its high prevalence in root surface caries too)
The positive correlation between their numbers in plaque and saliva and
caries activity
Their ability to grow in low-pH environments (below pH 5) and to produce
lactic acid.
Their
ability
to
synthesize
both
extracellular
and
intracellular
polysaccharides from sucrose
The ability of some strains to produce caries in gnotobiotic (germ-free) rats
The fact that their numbers in dental plaque derived from healthy sites are
usually low.
On the negative side, however, lactobacilli are rarely isolated from plaque before
the development of caries and they are often absent from incipient lesions.
Although the role of lactobacilli in the carious process is not well defined, it is
believed that:
They are involved more in the progression of the deep enamel lesion
(rather than the initiation)
They are the pioneer organisms in the advancing front of the carious
process, especially in dentine.
The role of Actinomyces spp
Root lesions differ from enamel caries in that the calcified tissues are softened
without obvious cavitation. The evidence for the involvement of (A. viscosus) in
root surface caries is based on:
Association studies in vivo.
In vitro experimental work with pure cultures.
Experimental work in gnotobiotic rodents.
Even though Actinomyces spp. (especially A. viscosus) predominates in most
plaque samples taken from root surface lesions, some studies have reported both
mutans streptococci and Lactobacillus spp. in these lesions. Furthermore, the sites
from which these organisms were isolated appeared to have a higher risk of
developing root surface caries than other sites. The role of Actinomyces spp. in
caries is therefore not clear.
The role of VeillonellaVeillonella
is a Gram-negative anaerobic coccus that is present in significant numbers in
most supragingival plaque samples.
As Veillonella spp. require lactate for growth, but are unable to metabolize
normal dietary carbohydrates, they use lactate produced by other microorganisms
and convert it into a range of weaker and probably less cariogenic organic acids,
e.g. propionic acid. Hence this organism may have a beneficial effect on dental
caries. This protective effect has been demonstrated in vitro and in animal
experiments, but not in humans.
Patient evaluation
1- Microbiological analysis
2- Assessment of dietary habits
3- Determination of salivary flow rate and buffering capacity
1-Microbiological tests in caries assessment .Saliva samples can be used to
establish the numbers of Streptococcus mutans and Lactobacillus spp. in the oral
cavity, as follows:
A. A paraffin wax-stimulated sample of mixed saliva is collected.
B. In the laboratory the saliva is appropriately diluted and cultured on selective
media (mitis salivarius bacitracin agar for S. mutans; Rogosa SL agar for
Lactobacillus spp.)
C. The number of typical colonies (colony-forming units or CFU) is then
quantified and extrapolated to obtain the count per millilitre of saliva:
• high caries activity: >
10
6
/ml S. mutans and /or
>100 000/ml Lactobacillus spp.
• low caries activity:
<100 000/ml S. mutans and
<10 000/ml Lactobacillus spp.
2-Assessment of dietary habits
The major approaches to prevention of caries are:
stopping or reducing between-meal consumption of carbohydrates, or
substituting non-cariogenic artificial sweeteners (sugar substitutes), e.g.
sorbitol, xylitol or Iycasin
making the tooth structure less soluble to acid attack by using fluorides
using sealants to protect susceptible areas of the tooth (e.g. pits and
fissures) that cannot easily be kept plaque free by routine oral hygiene
measures
reducing cariogenic flora so that even in the presence of sucrose, acid
production will be minimal (e.g. oral hygiene aids, antimicrobial agents and
possibly immunization)
replacement of cariogenic bacteria by organisms with low or no cariogenic
potential.
3- Determination of salivary flow rate and buffering capacity
An appropriate flow of saliva is essential for the maintenance of oral health. It is
evident that the oral bacteria are subjected to several important salivary functions,
which affect their colonization, survival, and metabolism.
The most important mechanisms by which saliva can affect caries are:
a) Mechanical cleansing of debris and plaque bacteria
b) Antibacterial activity against the oral microflora, i.e., lysis and aggregation
c) Buffering and neutralization of plaque acids
d) Enhancement of remineralization.
The buffering capacity of saliva is important for the maintenance of normal pH
levels in saliva and plaque. A low secretion might indicate a low buffering effect
and a weak inverse relationship to caries has been noted by several inves tigators.
Both the saliva secretion rate and buffer capacity differ however at different parts
of the mouth. The composition and acidogenicity of plaque may be affected
differently when situated close to a salivary duct or hidden deep down in a
fissure. Nevertheless, unfavorable values of buffer capacity and salivary flow rate
should be considered as risk factors for the individual.
Control of cariogenic plaque flora: Control may be achieved by
• Mechanical cleansing,(e.g. tooth brushing)
• Antimicrobial therapy,(e.g. mouth rinse)
• Immunization.
• Replacement therapy.
• Probiotic therapy.
Active immunization against dental caries:
Using either cell wall-associated antigens (antigen I/ II) or glucosyltransferases
(extracellular enzymes) from mutans streptococci is effective in reducing
experimental dental caries in rats and monkeys. The vaccine may produce its
protective effect by:
• Inhibition of the microbial colonization of enamel by secretory
immunoglobulin A (IgA)
• Interference with bacterial metabolism
• Enhancement of phagocytic activity in the gingival crevice area due to the
opsonization of mutans streptococci with IgA or IgG antibodies.
Passive immunization:
Experimental studies indicate that when the natural levels of oral mutans
streptococci are suppressed by chlorhexidine, topical application of monoclonal
antibodies against antigenI/II of mutans streptococci prevents recolonization by
the organisms. Transgenic plants could be used to produce dimeric antibodies
with specificity to antigen I/II of streptococci that are stable in the mouth and
persist for longer periods than the monomeric antibody.
These new developments have heightened the hopes of an alternative caries
preventive strategy for the future.
Replacement therapy
Experimental studies indicate that genetically engineered, low-virulence mutants
of mutans streptococci that are deficient in glucosyl transferase or deficient in
lactate dehydrogenase activity can be 'seeded' into the oral environment
organisms can replace their more virulent counterparts and prevent their re-
emergence
Probiotic therapy
The term probiotic therapy or probiotics is now used for approaches where the
offending pathogen is replaced artificially by innocuous commensals that can
obtain a permanent foothold in the locale (e.g. oral cavity, intestines). It is
feasible that replacement therapy of this nature may be exploited to control
cariogenic flora in the future. However, assurances of the safety of these
replacement strains are needed by both the public and the authorities before these
methods are realized.