Biology 103 - Microbes and You

Lecture 22 Outline

Tooth decay - cavities, dentifrices, gingivitis


Diagram of biofilm formation

Biofilm on a catheter

A biofilm in your gut

Biofilm on a prosthetic surface

Diagram of tooth brush missing gingival crevice

Diagram of plaque

Diagram of early gingival pocket development

Stages of carie development

Large dental carie

Another large dental carie

Rampant caries

More rampant caries

Cavity and gingivitis

Severe calculus

Movie showing dental probing

Movie showing dental probing

Advanced gingivitis in an AIDS patient

Gingival calculus

Ulcerative gingivitis

Advanced periodontal disease


The Mouth
the mouth is the beginning of the digestive system
chewing (masticating) not only grinds foods but degrades it with enzymes in the saliva
saliva is a complex mixture of salts, carbohydrates, and enzymes
some the enzymes (amylases) break down carbohydrates into sugars
the mouth is teeming with bacteria that eat these sugars
although there are many bacteria in the mouth there are really only a few species of bacteria
each bacterial species has a unique habitat in the mouth
mainly Streptococci are found in the mouth
Streptococcus salivarius lives on the upper tongue
Streptococcus sanguis lives on the teeth
Streptoccocus mitis lives on the inner cheek
Streptococcus mutans live on the teeth
The Streptococci are gram positive (and have a sticky cell wall) and facultative anaerobes
facultative organisms can live in both aerobic and anaerobic environments
Lactobacilli and Actinomyces are also important oral microflora

most microbes prefer to live in biofilms, rather than be free-living (planktonic)
biofilms are films on surfaces that contain bacteria held together with a matrix (glue)
the matrix is extracellular polysaccharides exuded by bacteria
biofilms concentrate nutrients, protect the bacteria from predators, and protect the bacteria from antibiotics and disinfectants
biofilms begin to form on clean surfaces when bacteria first adhere to the surface
adherence can be enhanced by surface proteins on the bacterium
once adhered, the bacterium produced sticky matrix material that glues it to the surface
the bacterium then multiplies and a biofilm is formed
other bacteria (that may not make much glue themselves) colonize the developing biofilm

Plaque and Caries
dental plaque is a film that forms on teeth composed of saliva, bacteria, bacterial extracellular polysaccharide, and food particles
first, aerobic bacteria adhere to the surface of a tooth
the aerobes actively eat sugars and respire, thus using lots of oxygen
the microenvironment becomes more and more anaerobic
Streptococcus mutans colonizes the tooth, begins growing, and produces large quantities of extracellular polysaccharide matrix
the matrix forms a biofilm with other microbes joining in
The bacteria start performing anaerobic metabolism - fermentation
this produces acids, in particular, lactic acid
this promotes the growth of Lactobacilli which eat lactic acid
eventually the filamentous bacteria, the Actinomyces, colonize the biofilm
The acids produced by plaque bacteria dissolves the calcium phosphate minerals of the tooth
there is an equilibrium between the calcium in the tooth and the calcium in the saliva
acids make the calcium in the tooth more likely to dissolve and be carried away by the saliva
the tooth enamel (outer layer of crown) is very hard and doesn't dissolve easily even with acids
but the underlying dentin dissolves easily with lactic acid
eventually a cavity is formed in the dentin below the enamel
when the cavity is large enough, the enamel will crack, exposing the dentin
bacteria find the dentin a rich environment for growth and quickly eat through the dentin into the underlying pulp (where the blood vessels and nerves are)
this is now a rampant carie and the tooth will likely be removed
fluoride makes the tooth mineral harder to dissolve
this is why fluoride treatment is effective in preventing dental caries

Gingivitis and periodontal disease
where the teeth meet the gums there is a crevice called the gingival crevice
bacteria can colonize this crevice fed by food sugars
as the bacteria grow, the gingiva (gums) becomes inflamed, swells, and may bleed
this disease is called gingivitis
the bacteria involved are Bacteroides, Porphyromonas, and Fusobacterium
the plaque (biofilm) that forms in the gingival crevice can eventually get calcified forming calculi
as the disease progresses, the gingival space deepens into pockets
healthy gingival pockets are 1-2 mm deep
once the pockets reach 4-6 mm deep, the risk of developing serious problems increases
eventually ulcers (open sores) can form in the gums
as the pocket deepens, it eventually reaches an area where the teeth are held in the mouth by fibers
when these fibers are degraded, the patient has periodontal disease
this can make the teeth loosen and fall out
in addition, as the pockets deepen, the bacteria may begin to attack the bone found in the gums and eventually the jaw bone
the same acid that dissolve the teeth also dissolve the bone
this can lead to tooth loss and other problems
also, bacteria in deep gingival pockets can attack lower portions of the tooth where there is no enamel
the bacteria easily find their way to the pulp and destroy the living portions of the tooth (without even causing a common cavity)
repair of this type of lesion will require root canal work (ouch!)
in Trench Mouth, the gingival tissue is attacked by spirochetes
this leads to acute necrotizing ulcerative gingivitis

Dentifrices (toothpaste) and Mouthwash
Brushing and flossing can physically remove bacteria, plaque and calculi from the teeth
soaps in dentifrices can also help to dislodge bacteria
antibacterial compounds are also commonly included in toothpastes
chlorhexidine - a cation (positively charged) that disrupts membranes and bacterial adherence
bacteria are negatively charged, teeth are positively charged
cations bind to bacteria and mask their negative charges so that they can no longer adhere to teeth
surfactants, like sodium lauryl sulfate, are anions (negatively charged) that also block bacterial adherence by masking charges on teeth
fluoride not only strengthens teeth, but also acts as an anion
Listerine - a phenolic that is membrane active, killing bacteria
triclosan, found in Colgate Total, is another membrane active antiseptic
some chewing gums, like Wrigley's Extra, also prevent dental caries and gingivitis
these products contain xylitol and sorbitol
oral bacteria tend to eat glucose and sucrose
when Streptococcus mutans eats sucrose, it makes lots of extracellular polysaccharide
this leads to biofilms formation, plaque formation, and calculi development
sugar alcohols like xylitol compete with sucrose and glucose
the bacteria eat xylitol and don't make as much glue
they also don't make as much lactic acid
this lowers the incidence of gingivitis and dental caries

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