Bacteria and biofilms

How many microorganisms are in one gram of plaque?

4 × 10^10 microorganism

How many organisms are in 1 ml of saliva

10^8 organisms/ml

What are some oral habitats?

Lips, cheeks, palate

• shedding surface

• Keratinised

Tongue

• highly papillated

• Reservoir

Teeth

• number of surfaces

• Gingival crevice

• Non-shedding surface

• Plaque buildup

What are biofilms?

Free-floating bacteria attach to a solid surface

Multiple species

Embedded in extracellular slime layer

Antibiotic resistance

Latent state (nutritional needs)

E.g. plaque on teeth, dental implants, contaminated waterlines, indwelling prosthetic devices, ear infections, contact lenses

Dental unit water lines and biofilms

Form where water is left to sit e.g. tubing

DUWL contamination with biofilm poses two major issues

• infection risk (e.g. pseudomonas)

• Equipment breakdown from occluding the lines and also corroding metal compounds

The pellicle

Pellicle: Salivary glycoproteins that attach to tooth enamel

Adherence is essential for survival of bacteria

Pellicle formation occurs in seconds on cleaned enamel and reaches the maximum in 90-120min

Within a few mins pioneer bacteria will attach to pellicle

Initial adhesion involves salivary components adsorbed to the oral cavity surface (pellicle)

What are examples of pioneer species?

Strep. sanguinis, strep. Gordonii, strep. mutans

What are the stages in plaque development?

Attachment, initial colonisation, secondary colonisation, mature biofilm

What is calculus?

Calcified plaque

Calcium and phosphate salts in saliva - hard water areas

Rough porous surface - more plaque can form

Has to be removed by scaling instruments

What factors lead to caries?

• mutans streptococci

• Lactobacilli

• Other acidogenic/aciduric streptococci

• Frequent sugar/ low pH challenges low saliva flow

What factors lead to periodontal disease?

Gram-negative anaerobes

• treponema denticola

• Porphyromonas gingivalis

• Tannarella forsythia

• Aggregatibacter actinomycetemcomitans

Inflammation

Immune suppression

Increased gingival crevicular flow

Dental caries

Breakdown of the hard tissues of the teeth

• acid environment → demineralisation → erodes enamel or dentin

Enamel caries

• common in young children

• Fissures most common site

• Mutans streptococci, lactobacillus

Root surface caries

• more common in older people

• Thought to possibly be polymicrobial

Transmissible

What factors are needed for caries?

Tooth, bacteria, diet, time

How does bacterial metabolism contribute to the development of caries?

Cariogenic bacteria produce acids

• glycolysis/fermentation

• Dissolves some of the minerals in the adjacent enamel surface - calcium phosphate and carbonate salts are lost from hydroxyapatite crystals - demineralisation

Saliva can neutralise the acid

• can enable repair of the enamel - remineralisation

The more fermentable carbohydrates are eaten, the more acid is produced, and the risk of caries increases

Ecological plaque hypothesis for caries

^^

How do lesions form?

S. Mutans attaches to the pellicle

Uses GTFs to produce glucans from sucrose

• consolidates biofilm

Up-regulates CHO metabolism genes

• ferments CHOs to lactic acid, acetate

• Drops pH

Acid tolerance

• expresses FoF1 ATPase to pump out protons

Lesion develops

What is saliva?

Complex secretion

• 99% water, 1% organic and inorganic molecules

• Provides molecules for pellicle

Secreted from salivary glands located in the mouth

Secrete 500-700ml/day

• average volume in mouth is 1.1 ml

• Rate is 0.25-0.35 ml/min at rest; 1.5 ml/min when stimulated

Quality and quantity are important in maintaining oral health

What is the role of saliva?

Dilutes and eliminates sugars and other dietary components, microorganisms

• patients with low flow rates (xerostomia) have increased risk of caries

Buffers pH changes produced by ingestion of foods with a low pH e.g. carbonated drinks, citrus fruit

Balances demineralisation/remineralisation

• saturated with Ca²+, OH- and PO4

Periodontal diseases

Diseases where the supporting tissues of the teeth are attacked

Leading cause of tooth loss

Plaque accumulation + inflammatory response

Increase in gingival crevicular fluid

Periodontal pockets (anaerobic)

Selection for proteolytic, gram-negative bacteria

Germ-free animals

Ecological plaque hypothesis for periodontal disease

^^

Microorganisms involved in the oral cavity

^^

Gingivitis

Early stages of periodontal disease

Inflammation of the gingiva in response to dental plaque

Host responses lead to destruction of gingival tissues

Periodontitis/periodontal disease

Fifth most prevalent health problem among Australians

Inflammation of the periodontium

Occurs when the gingivitis is untreated and inflammation spreads into ligaments and bones that support the teeth

Shift from gram-positive aerobes to gram-negative anaerobes (strong inflammatory response)

Periodontal pockets form creating an environment that is highly anaerobic, pH shifts to 7.7-7.8

• proteilytic bacteria flourish

The common signs of periodontal disease (both gingivitis and periodontitis)are:

• red, swollen or tender gingiva

• Bleeding of the gingiva whilst brushing or flossing

• Pus around the teeth or gingival tissues

As it progresses you will find

• loose or separating teeth

• Pain or pressure when chewing

Adherence to host surfaces

Adhesins and receptors usually interact in a complementary and fashion

Similar to interactions between enzymes/substrates

Usually many bonds form over area of contact

Oral cavity ligands

Pellicle contains mucins, salivary glycoproteins, minerals and immunoglobulins

• proline-rich proteins (PRPs), statherin bind to enamel

• Mucins, and agglutinins (e.g. gp340 glycoprotein) bind to enamel and epithelial surfaces

Attachment

Bacteria also attach to each other using adhesins

Some bacteria can’t attach directly to surface, need to attach to another species

• can influence community structure

• E.g. Porphyromonas gingivalis binding to streptococcus gordonii

What are the factors that affect adherence?

Host factors (ligands), saliva (suspending medium) and bacterial factors (adhesins)

Plaque maturation

As plaque matures on the tooth it increases in mass and thickness and the microbial composition also changes

After 7 days streptococci are still the main organisms on the tooth surface

After 14 days there is a shift to anaerobic rods

Climax community = situation where you have bacteria sloughing off and other bacteria attaching to the plaque, an equilibrium

Mature plaque is potentially more pathogenic

Subgingival plaque

Tooth attached plaque (detrimental to periodontal tissues)

Epithelial attached plaque (less risky)

Unattached plaque (bacteria not part of biofilm)

Calculus

Supragingival

• above the gingival margin

• Yellowish-white

Subgingival

• tooth root surfaces below the gingival margin

• Periodontal pocket

• Black/ dark green

Mutans streptococci

Seven different species and 8 serotypes (a-h)

S. mutans serotypes c/e/f, S. sobrinus serotypes d/g most common in humans

Produces extracellular polysaccharides (glucans) from sucrose that help in colonisation

Can initiate and maintain growth, metabolism, acid production at low pH

Efficient and rapid metabolism of sugars to lactic acid

Lactobacillus spp.

Cariogenic qualities

• increased numbers in most carious cavities affecting enamel enamel and tooth surfaces

• Able to initiate and maintain growth at low pH

• Produce lactic acid below pH 5

• Some strains produce extracellular polysaccharides

Not involved in initiation but in progression of lesion deeper into enamel and dentine

Porphyromonas gingivalis

Obligate anaerobic Gram-negative bacillus

Found mostly in Subgingival plaque

Part of the red complex

Keystone species

• present in low numbers but has huge influence on microbial community structure