L7 Microbiology of Periodontal Disease

Question 1

What difficulties are faced when identifying which specific bacteria are involved in the periodontal disease process?

Answer 1

• Diverse oral microflora of at least 800 bacterial species
• It is an endogenous infection (present in the normal microflora) meaning some of the bacteria associated are also present in low number in health
• Random bursts of disease activity cannot be predicted, making it more difficult to study

Question 2

What are Koch’s postulates (modified for endogenous infection)?

Answer 2

1. Microbe present in sufficient numbers to cause disease
2. Microbe generates high levels of sepcific antibody
3. Microbe produces relevant virulence factors
4. Microbe causes disease in animal model
5. Elimination of the microbe causes clinical improvement

Question 3

What type of conditions do most bacteria involved in periodontal disease favour?

Answer 3

Anaerobic conditions

Question 4

What is the specific plaque hypothesis?

Answer 4

States that specific bacterial species are responsible for periodontal disease.
E.g. P.gingivalis associated with chronic periodontitis, aggregatibacter associated with aggressive periodontitis.

• However, this hypothesis does not explain all cases, there will be patients with periodontitis without presence of P. gingivalis. Or patients who do have P. gingivalis and no periodontitis.

Question 5

What is the non-specific plaque hypothesis?

Answer 5

Suggests that species composition of plaque is irrelevant to disease, but it is the amount of plaque that is important. E.g. population studies found high plaque levels = more disease, low levels of plaque = less disease.

• However, there are cases where patients have very little plaque and extensive destruction. Or patients with extensive levels of plaque and minimal destruction.

Question 6

What is the ecological plaque hypothesis?

Answer 6

States that ecological conditions determine healthy plaque vs disease producing plaque.

• External influences shift the balance towards health or disease (dysbiosis)
• E.g. plaque accumulation increases inflammation, creates anaerobic environment favouring growth of certain bactera, increased pH
• Change from gram positive bacteria to predominantly gram negative bacteria
• Believed that there are key bacteria which are very good at being pathogenic- called keystone pathogens- key in changing conditions and tipping towards a disease condition

Question 7

Describe the composition of dental plaque.

Answer 7

• Mushroom like structures
• Anaerobic bacteria in the centre, protected from the toxic effects of oxygen
• Formal structure, not disorganised
• Channels exist to bring nutrients to bacterial colonies and remove metabolites

Question 8

Describe the biofilm composition in the oral cavity in health.

Answer 8

• Mainly facultatively anaerboic gram positive cocci

- Streptococcus mitis group, Veillonella spp, Actinomyces naeslundii, Actinomyces viscous

Question 9

Describe the biofilm composition in the oral cavity in patients with chronic marginal gingivitis.

Answer 9

• Approx. 45% gram negative bacteria
• Streptoccus mitis group, Streptococcus anginosus group, Actinomyces israelii, Actinomyces naeslundii, Veillonella spp, Prevotella intermedia, Fusobacterium nucleatum

Question 10

Describe the biofilm composition in the oral cavity in patients with chronic periodontitis.

Answer 10

• Approx. 75% gram negative bacteria, 90% of which are strict anaerobes
• Motile rods and spirochaetes are prominent
• Porphyromonas gingivalis, Prevotella intermedia, Fusobacterium nucleatum, Tannerella forsythia, Selenomonas spp

Question 11

How do bacteria cause periodontal disease?

Answer 11

• Adhere to tooth/biofilm via adhesins and fimbriae
• Evade host defences, e.g. some produce toxins to damage leukocytes (leucoidins), produces enzymes to breakdown antibodies, produce proteases, have a protective capsule
• Cause tissue damage e.g. eznymes (collagenases, hyaluronidase, proteases), bone resorbing factors, cytotoxins (ammonia, butyric acid)

Question 12

How can cytokine production be beneficial and also harmful?

Answer 12

• Immune cell recruitment

- Can cause osteoclast differentiation

Question 13

How can macrophage production be beneficial and also harmful?

Answer 13

• Phagocytose bacteria

- Release proteases

Question 14

How can immunoglobulin production be beneficial and also harmful?

Answer 14

• Enhance phagocytosis

- Ag/Ab complexes can cause hypersensitivity reactions

Question 15

Describe the stages of periodontal disease progression.

Answer 15

• Plaque present, GCF exudes at increased rate which provides novel substrates for bacterial metabolism
• Bacteria release enzymes such as collagenase and hyaluronidase and cytoxins which damage tissue
• Tissue becomes inflamed and reddened
• pH of pocket increases as numbers of proteolytic bacteria increase
• Host immune response stimulated by bacterial surface molecules and products
• Macrophages, T and B lymphocytes infiltrate

Question 16

What are the effects of the host immune response on periodontal disease?

Answer 16

• Macrophages phagocytose bacteria, causes release of proteases and ROS
• Cytokines induce host cells to release proteases
• IgG on B-cells binds to bacterial antigens
• T helper cells bind to IgG
• Antibodies influence many cell types to release inflammatory mediators (e.g. prostaglandins) which stimulate bone resorption