NORMAL HUMAN MICROBIOTA

• "normal flora"


  normal, usual, or indigenous flora
• Normal human microbiota


  The population of microorganisms that inhabit the skin and mucous membranes of healthy normal persons
• Role of normal human microbiota

  • Provide a first line of defense against microbial pathogens
  • Assist in digestion
  • Play a role in toxin degradation
  • Contribute to maturation of the immune system
• Two groups of microbiota

  • Resident flora
  • Transient flora
• Resident flora


  • Fixed types of microorganisms regularly found in a given area at a given age, if disturbed, it promptly reestablishes itself
  • Colonize the area for months or years
• Transient flora


  • Consists of non-pathogenic or potentially pathogenic microorganisms that inhabit the skin and mucous membranes for hours, days or weeks
  • Derived from the environment
  • Does not produce disease
  • Does not establish permanently in the surface
• Transient flora are generally of little significance so long as the normal resident flora remains intact
• If the resident flora is disturbed

  Transient microorganisms may colonize, proliferate and produce disease
• "Carriers"


  Hosts capable of transmitting infection
• "Carrier state"


  Can be acute, short lived or transient, or chronic (lasting for months, years or permanently)
• Commensals


  Organisms that are constantly present on the body surfaces
• Factors that influence the flourishing of commensals in a given area

  • Temperature
  • Moisture
  • Presence of certain nutrients
  • Inhibitory substances
• Role of resident microbiota

  • Synthesize vitamin K and aid in the absorption of nutrients in the intestinal tract
  • Prevent colonization by pathogens and possible disease through "bacterial interference" on mucous membranes and skin
• Normal resident microbiota are harmless and may be beneficial in their normal location in the host and in the absent of coincident abnormalities
• If normal resident microbiota are forcefully removed from the restrictions of their environment and introduced into the bloodstream or tissues

  They become pathogenic
• Normal resident microbiota that can become pathogenic

  • Streptococci of the viridans group (most common resident microorganism in the upper respiratory tract) - if large numbers are introduced into the bloodstream (following a tooth extraction or oral surgery), may settle on deformed or prosthetic heart valve and produce infective endocarditis
• Normal microbiota of the skin

  • Staphylococcus epidermidis
  • Staphylococcus aureus (in small numbers)
  • Micrococcus species
  • Alpha hemolytic and non-hemolytic streptococci (e.g. Streptococcus mitis)
  • Corynebacterium species
  • Propionibacterium species
  • Peptostreptococcus species
  • Acinetobacter species
  • Small numbers of other organisms (Candida species, Pseudomonas aeruginosa, etc.)
  • Fungi and yeasts (skin folds)
  • Acid fast, nonpathogenic Mycobacteria (areas rich in sebaceous secretions like genitalia, external ear)
• Factors to eliminate non-resident microorganisms

  • Low pH
  • Fatty acids in sebaceous secretions
  • Presence of lysozyme