MICROBIOLOGY OF DENTAL CARIES
Caries:
Localized destruction of the tissues of the
tooth by bacterial fermentation of dietary
carbohydrates
A multifactorial, plaque-related chronic
infection of the enamel, cementum or dentine
Traces of plaque
and
decaying enamel
Enamel
penetrated
by bacteria
Early enamel
caries seen by
polarized light
microscopy
MICROBIOLOGY OF DENTAL CARIES
Key factors in the development of caries:
Host
Susceptible tooth surface
Saliva
Plaque bacteria
Diet
Fermentable carbohydrates
Interplay of major etiologic factors in dental caries
MICROBIOLOGY OF DENTAL CARIES
Specific plaque hypothesis:
mutans streptococci are important in caries
initiation
Non-specific plaque hypothesis:
Heterogeneous groups of bacteria are involved
in caries initiation
MICROBIOLOGY OF DENTAL CARIES
Ecological plaque hypothesis:
Cariogenic flora found in natural plaque are weakly
competitive and comprise only a minority of the
total community
Increase in fermentable carbohydrates results in
prolonged low pH, promoting the growth of acidtolerant bacteria and initiating demineralization
MICROBIOLOGY OF DENTAL CARIES
Ecological plaque hypothesis:
The balance in the plaque community turns in favor
of mutans streptococci and lactobacilli
There is a dynamic relationship between the bacteria
and the host, and changes in major host factors such
as salivary flow can affect plaque development
Ecological plaque hypothesis
MICROBIOLOGY OF DENTAL CARIES
Properties of cariogenic flora that correlate with
their pathogenicity:
Ability to rapidly metabolize sugars to acids
(acidogenicity)
Survival and growth under low pH conditions
(aciduricity)
Ability to synthesize extracellular and intracellular
polysaccharides
glucosyltransferase
n-sucrose
(Glucan) n + n-fructose
fructosyltransferase
n-sucrose
(Fructan) n + n-glucose
“Primary enemy of
the teeth”
Lennart Nilsson
The Body Victorious
Streptococcus
mutans
CARIOGENICITY OF STREPTOCCUS MUTANS
Significant correlation between S. mutans counts in
saliva & plaque with the prevalence and incidence
of caries
Prevalence: The number of cases of a disease present
in a specified population at a given time
Incidence: The frequency of occurrence of any
disease over a period of time in relation to the
population in which it occurs
S. mutans can be isolated from precise sites on the
tooth surface before the development of caries
CARIOGENICITY OF STREPTOCCUS MUTANS
Correlation between the progression of carious
lesions and S. mutans counts
Produces extracellular polysaccharides from sucrose
which facilitates microbial colonization
Most effective Streptococcus in experimental caries
in animals (rodents & non-human primates)
Ability to initiate and maintain growth and continue
acid production in sites with a low pH
Glucose-6-phosphate
Fructose-1,6-diphosphate
Phosphoenolpyruvate
Glyceraldehyde
-3-phosphate
Formation of end products of metabolism by
mutans streptococci
CARIOGENICITY OF LACTOBACILLUS SPECIES
Present in increased numbers in most carious
cavities affecting enamel & root surfaces
Numbers in saliva correlate with caries activity
Some strains produce caries in gnotobiotic rats
Initiate and maintain growth at low pH (aciduric)
CARIOGENICITY OF LACTOBACILLUS SPECIES
Produce lactic acid in conditions below pH 5
(acidogenic)
However:
Affinity for the tooth surface is low
Numbers in dental plaque in early carious lesions
are usually low
Their population size is a poor predictor of the
number of future plaques
Their numbers in saliva increase only after
caries develop
CARIOGENICITY OF LACTOBACILLUS SPECIES
Present consensus:
Lactobacilli are not involved in the initiation of
dental caries
They are involved in the progression of the
lesion deep into enamel and dentine
They are pioneer organisms in the advancing
carious process
DEMINERALIZATION
Low pH causes demineralization by reducing
the concentration of the tribasic phosphate (PO43-)
which is needed to form hydroxyapatite
10Ca2+ + 6PO43- + 2H2O ---> 2H+ + Ca10 (PO4)6(OH)2
hydroxyapatite
DEMINERALIZATION
Low pH tends to reduce the concentration of
tribasic phosphate by adding H+ to phosphate
6PO43- + H+ ----------> 6HPO42- + H+ ----------> 6H2PO41pK= 7.0
pK= 4.0
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
• Sugar substitutes
• Fluoridation (to increase enamel hardness)
• Fissure sealants
• Control of cariogenic flora
Antimicrobials
Passive immunization?
Replacement therapy?
Vaccines??
Fluoride ions
Substitute for the hydroxyl
groups in hydroxyapatite
(Fluoroapatite less soluble in
acid)
Promote remineralization of
early carious lesions
Fluoride ions
Interfere with bacterial
membrane ion
permeability
Reduce glycolysis
(inhibition of enolase: phosphoglycerate ->
phosphoenolpyruvate)
Inactivate key metabolic
enzymes by acidifying
bacterial cell interior
Inhibit synthesis of
polysaccharides
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
Passive immunization
Antibodies against antigen I/II of mutans
streptococci inhibit recolonization after
chlorhexidine treatment
Monoclonal antibodies produced in
transgenic plants prevented recolonization
for 4 months
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
Sugar substitutes
Xylitol inhibits sugar metabolism of mutans
streptococci as well as glycolysis
pH is maintained at 7, vs reduction to 5 by
sucrose
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
Replacement therapy
Low virulence mutants of mutans streptococci
deficient in GTF or lactate dehydrogenase
activity
More competitive S. salivarius that can displace
S. mutans
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
Antimicrobials
Chlorhexidine
Inhibits sugar transport in streptococci
Inhibits amino acid uptake and catabolism
in S. sanguis
Inhibits a protease of P. gingivalis
Affects membrane functions, such as ATP
synthase and maintenance of ion
gradients in streptococci
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
Antimicrobials
Triclosan
Inhibits acid production by streptococci
Inhibits a protease of P. gingivalis
Enhanced by co-polymer or zinc citrate
“Substantive” : binds effectively to oral
surfaces, like chlorhexidine
MICROBIOLOGY OF DENTAL CARIES
Microbiological tests:
To identify caries risk factors in patients with
extensive or recurrent caries, prior to delivering
dental care (e.g. extensive crown and bridge
treatment)
High salivary counts of mutans streptococci
(> 106/mL) and lactobacilli (> 104/mL) indicate
high risk of disease
Culture slide test to detect mutans streptococci
in saliva