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

Traces of plaque

and

decaying enamel

Enamelpenetrated by bacteria

Early enamelcaries seen by polarized light microscopy

MICROBIOLOGY OF DENTAL CARIES

Key factors in the development of caries:

HostSusceptible tooth surfaceSaliva

Plaque bacteria

DietFermentable carbohydrates

Interplay of major etiologic factors in dental caries

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

Ecological plaque hypothesis

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

n-sucrose glucosyltransferase

(Glucan) n + n-fructose

n-sucrose fructosyltransferase

(Fructan) n + n-glucose

“Primary enemy of the teeth” Lennart NilssonThe Body Victorious

Streptococcus mutans

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

Culture slide test to detect mutans streptococci in saliva