MICROBIOLOGY OF DENTAL CARIES Caries: Localized destruction of the tissues of the tooth by bacterial...
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Transcript of MICROBIOLOGY OF DENTAL CARIES Caries: Localized destruction of the tissues of the tooth by bacterial...
MICROBIOLOGY OF DENTAL CARIES
Caries:
Localized destruction of the tissues of thetooth by bacterial fermentation of dietarycarbohydrates
A multifactorial, plaque-related chronicinfection of the enamel, cementum or dentine
MICROBIOLOGY OF DENTAL CARIES
Key factors in the development of caries:
HostSusceptible tooth surfaceSaliva
Plaque bacteria
DietFermentable carbohydrates
MICROBIOLOGY OF DENTAL CARIES
Specific plaque hypothesis:
mutans streptococci are important in cariesinitiation
Non-specific plaque hypothesis:
Heterogeneous groups of bacteria are involvedin caries initiation
MICROBIOLOGY OF DENTAL CARIES
Ecological plaque hypothesis:
Cariogenic flora found in natural plaque are weaklycompetitive and comprise only a minority of thetotal community
Increase in fermentable carbohydrates results inprolonged low pH, promoting the growth of acid-tolerant bacteria and initiating demineralization
MICROBIOLOGY OF DENTAL CARIES
Ecological plaque hypothesis:
The balance in the plaque community turns in favorof mutans streptococci and lactobacilli
There is a dynamic relationship between the bacteriaand the host, and changes in major host factors suchas salivary flow can affect plaque development
MICROBIOLOGY OF DENTAL CARIES
Properties of cariogenic flora that correlate withtheir pathogenicity:
Ability to rapidly metabolize sugars to acids(acidogenicity)
Survival and growth under low pH conditions(aciduricity)
Ability to synthesize extracellular and intracellularpolysaccharides
CARIOGENICITY OF STREPTOCCUS MUTANS
Significant correlation between S. mutans counts insaliva & plaque with the prevalence and incidenceof caries
Prevalence: The number of cases of a disease presentin a specified population at a given time
Incidence: The frequency of occurrence of anydisease over a period of time in relation to thepopulation in which it occurs
S. mutans can be isolated from precise sites on thetooth surface before the development of caries
CARIOGENICITY OF STREPTOCCUS MUTANS
Correlation between the progression of cariouslesions and S. mutans counts
Produces extracellular polysaccharides from sucrosewhich facilitates microbial colonization
Most effective Streptococcus in experimental cariesin animals (rodents & non-human primates)
Ability to initiate and maintain growth and continueacid production in sites with a low pH
Formation of end products of metabolism bymutans streptococci
Glucose-6-phosphate Fructose-1,6-diphosphate
Phosphoenolpyruvate Glyceraldehyde-3-phosphate
CARIOGENICITY OF LACTOBACILLUS SPECIES
Present in increased numbers in most cariouscavities 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 lowNumbers in dental plaque in early carious lesions
are usually lowTheir population size is a poor predictor of the
number of future plaquesTheir numbers in saliva increase only after
caries develop
CARIOGENICITY OF LACTOBACILLUS SPECIES
Present consensus:
Lactobacilli are not involved in the initiation ofdental caries
They are involved in the progression of thelesion deep into enamel and dentine
They are pioneer organisms in the advancingcarious process
DEMINERALIZATION
Low pH causes demineralization by reducingthe concentration of the tribasic phosphate (PO4
3-)which is needed to form hydroxyapatite
10Ca2+ + 6PO43- + 2H2O ---> 2H+ + Ca10 (PO4)6(OH)2
hydroxyapatite
DEMINERALIZATION
Low pH tends to reduce the concentration oftribasic phosphate by adding H+ to phosphate
6PO4
3- + H+ ----------> 6HPO42- + H+ ----------> 6H2PO4
1- pK= 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 immunizationAntibodies 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 substitutesXylitol 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 therapyLow virulence mutants of mutans streptococci deficient in GTF or lactate dehydrogenase activityMore competitive S. salivarius that can displace S. mutans
MICROBIOLOGY OF DENTAL CARIES
Strategies to control or prevent caries:
Antimicrobials Chlorhexidine
Inhibits sugar transport in streptococciInhibits amino acid uptake and
catabolism in S. sanguisInhibits a protease of P. gingivalisAffects 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 streptococciInhibits a protease of P. gingivalisEnhanced 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 withextensive or recurrent caries, prior to deliveringdental care (e.g. extensive crown and bridgetreatment)
High salivary counts of mutans streptococci(> 106/mL) and lactobacilli (> 104/mL) indicatehigh risk of disease