Mouth as a microbial habitat

• The Mouth

  A varied and dense microbial population, with 100's of species of bacteria, 108 - 109 cfu/ml of saliva
• Primary ecological niche

  An organism might be found in only one location in the mouth
• Ecosystems

  Distinct areas of the mouth, such as buccal epithelium, dorsum of tongue, supra and sub gingival tooth surface, crevicular epithelial surfaces

  
• The healthy mouth is one of the best examples of a host supporting enormous numbers of microorganisms with no harmful effect
• Changes in the balance between host and microbes can result in pathogenesis, which may be caused by alterations in host defense or by changes in the microbiota
• Research into oral microbial ecology and disease

  • Complexity of oral microflora
  • Fastidious nature of many oral organisms
  • Composition can vary at one site over time
• Direct Microscopic Examination

  Limited value
• Microbial Culturing

  Usually require specialised sampling and culturing techniques, e.g. for subgingival anaerobes
• Selective Culture Media

  Contain antimicrobial substances that suppress the growth of all species other than the one of interest, but none are perfect
• Isolate Identification

  Conventional taxonomic methods – e.g. Gram stain, colony morphology, still requires further tests, slow

  
• Biochemical characterisation

  Metabolic reactions, e.g. S. mutans ferments sorbitol
• Molecular techniques

  1. Monoclonal Antibodies

  2. DNA probes (16S RNA)

  3. Maldi-TOF- Biotyper
• NextGen DNA sequencing of the Microbiome

  Extracts all the DNA in a sample and matches against database, used to identify oral bacteria in ancient calculus
• Oral Microbial Habitats

  Not all microbes that enter the mouth are able to colonise, distinct habitats (micro environments) each able to support characteristic species
• Lips, cheek, palate

  • Biomass restricted - desquamation, each surface has specialised host cell type
• Tongue

  • Highly papillated, may act as reservoir for anaerobes
• Teeth


  • Non-shedding surface, allows large numbers of microbes, distinct surfaces allow for distinct communities

  
• Properties of habitats change over time

  1. Newborn mucosal surfaces uncolonised

  2. Tooth emergence

  3. Change from 10 to 20 dentition

  4. Dental treatment – e.g. scaling and polishing

  5. Disease

  6. Tooth loss

  7. Prostheses

  8. Diet- exogenous nutrients

  9. Saliva flow not constant over 24hrs- affected by Age & disease

  10. Antibiotics- may produce significant shift in microbiota (Candida-yeasts which are not bacteria)
• Teeth

  • Large #s of microbes, mostly bacteria (dental plaque), found in health and disease, shift in dominant plaque species – disease, ecological diversity provided by a number of different surfaces
• Protected tooth surfaces

  • Interproximal, gingival crevice, pits & fissures, can support larger numbers of microbes & diversity

  
• Smooth tooth surfaces

  • Relatively exposed, smaller numbers and less diverse, some variety – buccal and lingual

  
• Dorsum of tongue


  • Papillary surfaces, protection from mastication and salivary flow, oxygen levels Redox low (Redox) - anaerobes
• Palate

  • Keratinised squamous epithelium – non-keratinised
• Saliva

  Circadian rhythms – composition & flow vary, stimulated vs non-stimulated, low in [CHO], salivary proteins part of pellicle, nutrient supply for microbes, aggregation of microbes – swallowed, inhibits growth of exogenous organisms – protective, antibodies, protective peptides - histatins
• Gingival Crevicular Fluid (GCF)


  Serum like, exudes from junctional epithelium (slight positive pressure removes non-adherent bacteria), healthy sites – flow ~ 0.3 ml/tooth/hour (inflammation), flow removes non-adherent cells, nutrition (proteins, proteolytic microbes, hemin), defence (immunoglobins, leukocytes)
• Saliva and GCF component concentrations
• Environmental Factors Influencing Microbial Growth

  Organisms which are best adapted to the environment will grow best - will be selected from a mixed population
• Temperature

  • All have characteristic maximum, minimum and optimum temperatures for growth, organisms which inhabit the human body grow most rapidly within the range 20oC to 45oC, with growth optima between 35oC and 40oC, oral cavity temp ranges 35-36oC, inflamed periodontal pocket ~ 40oC, elevated temp have been shown to alter gene regulation (e.g. Heat shock proteins)
• pH

  • Most bacteria have an optimum pH for growth in the range 6.5 - 7.5 with limits somewhere between 5 and 9, acidophilic bacteria can grow at a low pH, important in caries, diseased periodontal pocket may be pH 8
• Oxygen


  Strict Aerobes, Facultative Anaerobes, Microaerophilic, Strict Anaerobes
• Redox Potential (Eh)

  Oral Bacteria vary widely in their requirements for oxygen, aerobic bacteria use oxygen in respiration, obligate aerobes have an absolute requirement for oxygen to grow, anaerobiosis is a variable term – Eh +200
• this produces alkaline end products
• Diseased periodontal pocket may be pH 8
• Physiochemical Factors Affecting Growth
• Types of bacteria based on oxygen requirements

  • Strict Aerobes
  • Facultative Anaerobes
  • Microaerophilic
  • Strict Anaerobes
• Strict Aerobes

  Only multiply in presence of Oxygen, respiration is their only way of making energy
• Facultative Anaerobes

  Ability to shift their metabolism from respiration to fermentation, grow in presence or absence of air
• Microaerophilic

  Live in the presence of low O2 tensions, High O2 tensions are inhibitory- enzyme sensitivity
• Strict Anaerobes

  Only multiply in the complete absence of Oxygen, cellular enzymes need to be in a reduced state to function
• Redox Potential (Eh): Oral Bacteria vary widely in their requirements for oxygen