Oral microbiology

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Normal Flora.


Commensal flora= 10x more microbes than human cells.  These are the natural bacteria that live on and also in a halthy person.  The number is around 10^4 bacteria living in the human gut alone, they are considered as part of the commensal flora however are not commensalist.  Most have mutagenic or even pathogenic relationships with their human host.  


Major areas of colonisation are the skin, mouth, upper respiratory tract, lower GI (Gastro-intestinal) tract and the urogenital tract.


Commensalism is a symbiotic relationship whwere the symbiont benefits, but the host it neither helped or harmed.  Commensalists are actually the least studied bacteria.


On the skin majority of microbes are Gram +VE aerobes, they are aerobes as they are able to colonise in the presence of air.  Most common are: Staphylococcus epidermidis, Mycobacteria, corynebacteri & Propionibacterium. 


The oral cavity/mouth houses both aerobes and anaerobes, because it has both aerobic and anaerobic conditions.  Many of the microbes in the mouth aid the formation plaque and dental cavitites and so are hard to classify as commensal. 


In the upper respiratory tract , which is the nostrals, nasal cavities and throat, the most common bacteria are: Stapylococcus aureus and Streptococcus pneumonea, with the latter being pathogenic.


Most of the bacteria attach onto a surface.  

They also have a climax community, whereby through ecological succession have reached a steady state.  The climax community (Equilibrium) occurs because the community is composed of species best adapted to average conditions in that specific area.


Some pathogens are oportunistic, and take advantage of certain situations, eg. a compromised immune system presents a good opportunity for growth and infection, whereas a healthy immune system does not.  


Endogenous infection: this is when an infection is caused by an infectious agent that is already present in the body, but has previously been dormant.  



pH of 6.75 caused by buffers, and contains ions such as Calcium and also Urea.  It is the transport fluid for microbes (approximately 10^8cfu/ml) and also acts as a lubricant (amylase and mucin).  The saliva is secreted through 3 glands: Partoid, submandibular and sublingual glands.


Mucins are a family of high molecular wight proteins produced by most epithelial cells, they have the ability to form gels. Whereas amylase is an enzyme that catalyses the breakdown of starch into sugars.  It begins the process of digestion in the mouth (saliva).


The buffer is an ionic compound which resists change in the concentration of an ion in solution.  They work by temporarily binding to an ion thus removing it from solution.  pH buffers resist change in pH by attaching to or releasing H+ ions.  it is important for the pH of the mouth to remain constant because otherwise plaque and enamel erosion can occur.  If pH was to go too low, then this is when erosion of enamel will occur.  pH will never fall much below 6.3 and this is due to the presence of bicarbonate.  


Many parts, like the tonsils and intestines have what are called crypts, and these are fissures in the cell surface, the crypts are lined with epithelium and increase surface area.  The crypt greatly increases the contact surface between environmental influences and lymphoid tissue.  


Biofilm: is a microbial community enveloped by the extracellular biopolymer which these microbial cells produce.  It helps them adhere to the interface of a liquid or surface. It is such a process which causes tooth decay, when there is a biofilm on the tooth, aerobic conditions occur and pH changes due to high CO2, biofilm protects bacteria and allows a build up. more build up means plaque or enamel damage (or both).



  • Salivary pellicle : this is a protein film that forms on the surface of the enamel by selective binding of glycoproteins for the saliva that prevent the deposition of salivary calcium phosphate.  It forms in seconds after a tooth is cleaned.  It protects teeth from acids produced by oral microbes after consuming the available carbohydrates.  
  • Pioneer species (streptococci) stick to biofilm
  • Form micro-colonies
  • coaggregation occurs: Cell-Cell recognition of genetically distinct cell types.
  • Replication of attached microbes

Coaggregation:  Specific interactions between pioneers and receptors in pellicle.  More interactions between Pioneers and secondary colonisers.  Fusobacterium bridge between early and mature plaque organisms.


Fusobacterium bridge: this is a genus of filmamentous, anaerobic Gram -VE bacteroides.  They contribute to skin ulsers and several other human diseases such as Lemierre's syndrome and periodontal diseases.  These bacteriodes have a potent lipopolysaccharide ---> these are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond.  They are found in  the outer membrane of Gram -VE bacteria and act as endotoxins and elicit strong immunal responses in animals.  Endotoxins ---> this is just a toxin that is a structural molecule of the bacteria and is recognised by the immune system.  


Plaque can be from 3-20 cells thick, horizontal and vertical layering.  As plaque matures, its diversity increases and population increases to both Gram +VE and -VE.  Facultative anaerobes and obligate anaerobes too. 


Facultative anaerobes = an organism that makes ATP by aerobic respiration if oxygen is present but it also capable of switching to fermentation.  

Obligate anaerobes = microbes that live and grow in the absense of molecular oxygen and are often killed if they come into contact with oxygen due to the lack of the superoxide dismutase and catalase enzymes which would convert the lethal superoxide formed in theiir cells due to presence of oxygen.  


Approx 95% of adults suffer gum disease to some extent, and infections are normally caused by one or more (polymicrobial) microbe.







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