Microbiology of Dental Caries

• What things in the oral environment are relevant to caries?

  Tooth surface

  ~700 species of bacteria

  Saliva

  Gingival crevicular fluid

  Periodic availability of nutrients from host diet
• What features of

  S. mutansmakes it easier for it to survive?It's acidogenic & aciduric- produces high levels of acid & can survive & flourish in low pH conditions

  
• What virulence factors are present in

  S. mutansthat may enable it to contribute to the caries process?-Produces Antigen I/II so adherence to surfaces

  
  

  -Glucosyl transferases- produce glucan polymers from sucrose

  
  

  -Glucan binding proteins- attachment of cells to glucans
• Have any vaccines been developed for these virulence factors?

  Yes, there have been active and passive vaccines produced against all 3 factors but no commercial vaccine is yet available
• What kind of organism is lactobacillus?

  It appears to be an opportunist organism that requires a low pH habitat so lactobacilli don't initiate caries but will colonise existing lesions.

  
  

  Doesn't only appear in the mouth but also in the body and fermented food e.g yogurt

  
• Describe the importance of Veillonella in caries.

  - Gram negative anaerobic cocci that is associated with caries.

  
  

  -Requires lactate as substrate for growth

  
  

  -Lactate is produced by both streptococci and lactobacilli

  
  

  -May be present in carious lesions because high levels of lactate are present.

  
  

  -It has been suggested that veillonella may be beneficial to caries by reducing lactate levels but little evidence to support this.

  
• What was the consensus reached about the microbiology of caries from recent culture & DNA-based studies?

  -There's a complex bacterial community in carious lesions

  -S. mutansis frequently not present

  -Some samples from caries-free sites have high levels ofS.mutans

  Other acidogenic species are significantly associated with carious lesions, includingScardovia wiggsiae, lactobacilli etc.
• What laboratory experiment did Bradshaw & Marsh carry out which forms the basis of much of our current thinking about the role of bacteria in caries?

  -9 oral bacteria species growing together

  
  

  -Glucose pulsed into them for 10 days.

  
  

  -Fall in pH seen

  
  

  -Increase in s.mutans and lactobacillus

  
  

  -If pH was held at 7, no change in microbiota

  
  

  Therefore, change in microbiota was in response to change in environment and not its cause

  
• What is the extended ecological plaque hypothesis (Takahashi & Nyved 2008)?

  Dynamic stability stage: associated with health; only mild or infrequent acidification

  Acidogenic stage:arises if there is frequent/moderate acidification

  Aciduric stage:so much carbohydrate is being consumed so regularly that the pH of the whole mouth falls

  
• What was the background information that Takahashi & Nyved used to form their plaque hypothesis?

  -S. mutans and lactobacilli are highly aciduric

  -Mutans streptococci only a minor component of healthy plaque

  
• Describe the dynamic stability stage.

  -Acid produced from sugars by a range of bacteria causing pH to be lowered.

  
  

  -Alkalisation phase occurs

  
  

  -production of alkali via :

  Ureolysis and arginine deiminase

  
• Describe the acidogenic stage.

  -Repeated, raised levels of sugar intake

  -Reduced salivary flow

  -poor oral hygiene

  
  

  Microbiota is typically dominated by non-mutans streptococci and actinomycetes

  
• Describe the aciduric stage.

  -Occurs after a prolonged acidogenic stage

  
  

  -Buffering capacity of saliva is lost

  
  

  -Bacterial community changes and consists mainly of s.mutans, lactobacilli and bifidobacterium.

  
  

  -presence of aciduric bacteria compounds the diseased state.

  
• What is a significant problem since more older people are retaining their teeth?

  Caries affecting the exposed roots

  
  

  Oral hygiene is key because there's a strong correlation with the amount of plaque on the root surface and root caries

  
• What did microbial analysis find in relation to root caries lesions?

  S. mutansand lactobacilli found, together withActinomycesspecies & bifidobacteria
• What does alpha amylase do?

  Hydrolyses starch to glucose & maltose, thus making it available for fermentation

  It also binds with high affinity toStreptococciand retains activity when bound,

  so it's involved in bacterial adherence to the pellicle
• In what ways can sugar become available to oral bacteria for fermentation?

  Can be present in diet & directly obtained from that route

  
  

  Can be broken down from starch by amylase

  
  

  Can be derived from salivary glycoproteins by bacteria themselves
• How are the sugars then transported to the interior of the cell?

  PTS (phosphotransferase system) & other systems
• What happens once the sugars have entered the cell?

  They enter glycolysis at different stages of the pathway, but all sugars eventually give rise to pyruvate which is a key metabolic intermediate
• Where does the ATP for the PTS system to work come from?

  The conversion of phosphoenolpyruvate to pyruvate

  
• What are the major pathways of glucose fermentation?

  
• What does the bacteria in plaque do to sucrose?

  Metabolises it to synthesise extracellular polysaccharides (EPS) which constitute the plaque matrix

  Some bacteria e.g.S. mutansalso form intracellular polysaccharides
• What are EPS (extracellular polysaccharides) and what do they contribute to?

  "Sticky" polymers & they contribute to the adherence of bacteria to the pellicle and plaque

  
  

  They also make bacteria more resistant to being washed away by dietary food & drink & salivary flow
• What can EPS be used as a source of?

  It can be used as a source of sugars for metabolism & growth when nutrients are limited
• What are the enzymes that form EPS called?

  Glucosyl transferases
• How many glucosyl transferases does

  S. mutansproduce?3: GTFB, GTFC & GTFD (B,C,D form)

  
  

  GTFB & GTFC (B and C) make water-insoluble glucans, rich in alpha-1,3-glucosidic linkages

  
  

  GTFD makes water-soluble glucans rich in alpha-1,6-glucosidic linkages
• Why are insoluble glucans more problematic?

  Because they, in particular, give plaque bulk & form a sticky mass which aids adhesion
• What do glucan binding proteins (GBPs) mediate?

  The specific attachment of bacterial cells to glucan
• Describe the process of glucan & fructan synthesis.

  
• Describe the process of glucan & fructan hydrolysis.

  
• What is enolase?

  A key regulatory enzyme of the glycolytic pathway

  
  

  mediating the conversion of 2-phosphoglycerate to phosphoenolpyruvate

  
• What inhibits the action of enolase?

  Fluoride

  
• What are the effects of fluoride inhibition of enolase?

  Reduced sugar transport because PEP is a key component of PTS system

  
  

  Reduced acid production because pyruvate production is reduced
• What is xylitol?

  Sweetening agent
• Describe the study carried out with xylitol and the findings.

  
• What is the primary mode of action of xylitol thought to be?

  Passive; in that if the diet is thought to be rich in xylitol, it's likely to be lower in sugar
• What direct effect does xylitol have on

  S. mutans?It's transported intoS. mutanscells by fructose-PTS system but enters "futile cycle" of phosphorylation, dephosphorylation & export from cell

  The futile cycle uses energy but produces none for the cell
• Is xylitol of high cariogenicity?

  No, it's of low cariogenicity & inhibits glycosyl transferase in cariogenic bacteria
• What is the aim of replacement therapy for caries?

  
• Summary of lecture content.