MICROBIOLOGY OF DENTAL 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 Flourappatite is much less likely to be degraded

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 acid- tolerant 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

(Glucan) n + n-fructose glucosyltransferase n-sucrose (Glucan) n + n-fructose

(Fructan) n + n-glucose 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

Formation of end products of metabolism by 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

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

Low pH tends to reduce the concentration of DEMINERALIZATION Low pH tends to reduce the concentration of tribasic phosphate by adding H+ to phosphate 6PO43- + H+ ----------> 6HPO42- + H+ ----------> 6H2PO41- 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

Interfere with bacterial membrane ion permeability 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 Know the general mechanism for this.

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