L4 Microbiology of Caries

Question 1

What host factors with regards to enamel susceptibility influence caries presence?

Answer 1

• Extent of mineralisation
• Fluoride content
• Host genetics

Question 2

What host factors with regards to saliva influence caries?

Answer 2

• Antibodies e.g. secretory IgA inhibits adherence of bacteria to tooth
• Lysozyme causes hydrolysis of peptidoglycan in gram positive bacteria
• Lactoferrin binds iron to make it inaccessible to bacteria
• pH and buffering, neutralises acid
• Peroxidase systems, anti-microbial actions
• Salivary flow allows some mechanical cleaning
• Remineralisation due to presence of calcium, phosphate and fluoride

Question 3

What theory predicted the correct mechanism of caries action?

Answer 3

Miller’s chemo-parasitic theory of dental caries:

• CHO broken down by bacteria
• Organic acids produced
• Acids cause enamel dissolution

Question 4

Which 3 hypotheses exist relating to which bacteria cause caries?

Answer 4

1. Specific plaque hypothesis: only a few species active in disease
2. Non-specific plaque hypothesis: disease is the outcome of the overall activity of total plaque bacteria, plaque is polymicrobial but some bacteria dominate
3. Ecological plaque hypothesis

Question 5

Describe the ecological plaque hypothesis.

Answer 5

• Combines two earlier hypotheses
• Organisms associated with disease may be present
  at sound sites, but at levels too low to be clinically relevant
• Changes in local environment may alter the
  microbiota balance (dysbiosis) e.g. sugar, low pH, poor OHI, poor salivary flow
• Mutualistic symbiosis becomes parastitic symbiosis

Question 6

How do we know that bacteria is involved in caries development?

Answer 6

• Gnotobiotic animal studies
• Immunisation against certain bacterial species reduces caries incidence
• Humans on long-term broad spectrum antibiotics have reduced caries incidence

Question 7

Is dental caries an infectious disease?

Answer 7

• Yes, in gnotobiotic animal studies they found that sharing cages with infected animals caused both animal groups to develop caries

Question 8

What are Koch’s postulates?

Answer 8

A set of laws/conditions that allow you to identify whether a specific microorganism causes disease.

Question 9

What are Koch’s modified postulates for oral infection?

Answer 9

1. The microbe should be present in sufficient numbers
   to initiate disease
2. A high level of antibodies to the microbe should be
   present
3. Relevant virulence factors should be produced
4. The microbe should cause disease in an appropriate
   animal model
5. Elimination of the microbe should result in clinical improvement

Applies to caries and perio disease.

Question 10

What proportions of microorganisms involved in caries can be cultured or not cultured?

Answer 10

• 70% culturable in lab

- 30% unculturable

Question 11

What are the main virulence traits of cariogenic bacteria?

Answer 11

• Ability to adhere to teeth (adhesins)
• Tolerate low pH (aciduric)
• Produce acid in conditions

Question 12

Name the features of an ideal cariogenic bacterium.

Answer 12

• Adhere to enamel (adhesins)
• Efficient uptake of sugars
• Glucan formation from sucrose
• Acid formation
• Ability to tolerate low pH
• Ability to produce acid even at low pH

Question 13

What bacteria are implicated in dental caries?

Answer 13

• S.mutans, S.sobrinucs, S.criceti, S.ratti, S.salivarius, S.sanguinis
• Lactobacillus species
• Actinomyces species

Question 14

Describe S.mutans and its prevalence in caries.

Answer 14

• Rapidly produces acid at low pH
• Animals immunised against the bacteria did not develop caries
• Produces extraceullar polysaccharides from sucrose which aids colonisation
• Produces intracellular polysaccharide which acts as a glycogen store when extracellular CHO levels are low
• Some S.mutans strains are more cariogenic than others

Question 15

Describe bacterial interactions with S.mutans.

Answer 15

• S.sanguinis produce hydrogen peroxide to reduce S.mutans growth
• S.mutans produces glutathione synthetase to neutralise hydrogen peroxide
• S.mutans produces bacteriocins (e.g. mutacin V and IV) to inhibit growth of other bacteria

Question 16

What is the window of infectivity for S.mutans?

Answer 16

Children aged 18-24 months. If S.mutans does not colonise in this period it is very difficult to do so in later years.

Question 17

How does S.mutans adhere to enamel?

Answer 17

S.mutans can bind to antigen I and II to aid adherence to tooth tissue.
AgI/II may interact with components of the pellicle to aid binding to the tooth.

Question 18

Describe lactobacillus species and its prevalence in dental caries.

Answer 18

• Facultative anaerobic bacteria (can grow in both aerobic and anaerobic conditions)
• Gram positive
• Extremely aciduric
• Acidogenic, produce lactic acid at extremely low pH

Question 19

What are the 2 lactobacillus species?

Answer 19

- Homofermentative species:
L. acidophilus 
L. casei
- Heterofermentative species:
L. fermentum

Question 20

What is homolactic fermentation?

Answer 20

2 molecules of lactate produced from 1 glucose molecule.

Question 21

What is heterolactic fermentation?

Answer 21

1 molecule of lactate produced from 1 glucose substrate. Theoretically less cariogenic than homolactic fermenters.

Question 22

Describe actinomyces species and its prevalence in dental caries.

Answer 22

• Gram positive anaerboic bacilli
• Associated with root surface and dentinal caries
• Prevalent in middle aged and elderly people
• E.g. A.odontolyticus, A.naeslundii

Question 23

Describe veillonella species and its prevalence in dental caries.

Answer 23

• Gram negative anaerboic cocci
• Present in significant numbers in supragingival plaque
• Potential protective effect: utilises lactate and metabolises for energy, coverts lactic acid to weaker acids e.g. acetic and propionic acid

Question 24

Which bacteria are involved in organic matrix destruction rather than enamel dissolution?

Answer 24

• Clostridium
• Eubacterium
• Propionbacterium
• Bifidobacterium

Question 25

What test can help to determine an individual’s susceptibility to caries?

Answer 25

• Test for bacteria numbers in saliva
• Culture on MBS (Mitis Salivarius Bacitracin) agar plate
• 10^6 S.mutans and 10^4 Lactobacillus per ml of stimulated saliva considered significant risk for future caries

Question 26

Why are vaccines against caries not currently available?

Answer 26

Issues with currently produced vaccines e.g. cross reactivity with cardiac tissues

Question 27

What are the main preventative mechanisms against dental caries?

Answer 27

• Sugar substitutes e.g. xylitol, saccharin, lycasin, starches (bacteria find it more difficult to metabolise these)
• Mechanical cleansing
• Antimicrobial agents e.g. chlorhexidine
• Fissure sealants
• Fluoride: Fluorapatite reduced solubility. Also antimicrobial effect (enolase inhibitor).