Microbiology Overview

• Infections are the most common cause of death in low income countries
• where are microorganisms found in healthy people?

  • skin
  • mouth
  • stomach
  • colon
  • upper respiratory tract
• the mouth, large intestine and upper respiratory tract are densely populated with normal flora
• the skin and stomach are moderately populated with normal flora
• the llower respiratory tract and bladder have few microbes in healthy people
• sterile sites in healthy people

  • blood
  • tissues
  • joint fluids
  • CSF
• There is usually high species diversity at each normal flora site
• Staphyloccocus is found on the skin
• Streptococcus is found in the mouth, nose and throat
• Microorganisms that can cause severe infections can also be found as normal flora
• Normal flora in the GI tract

  • lactobacillus
  • bacteroides
  • enterobacteriacae
• Why is normal flora important?

  • protection from pathogen colonisation
  • nutritional capability e.g. provision of vitamins and amino acids
  • development of immune function
• If the normal flora is disrupted it can lead to overgrowth of a pathogen
• Bacteria causing meningitis

  • neisseria mengitidis
  • streptococcus pneumonia
  • haemophilus influenza
• Lyme disease is a systemic infection caused by Borrelia burgdorferi, a microorganism abundant in ticks that can be transferred to a human if the tick bites
• Guillain-Barre syndrome often follows a Campylobacter jejuni infection
• In a prokaryotic cell DNA is not membrane bound.
• Prokaryotic ribosomes are 70s - 30s and 50s subunits
• Peptidoglycan - long polysaccharide chains and short peptide side chains which crosslink
• For bacterial growth bonds must be cut to add more peptidoglycan and new crosslinks must form
• The Gram stain is a test to divide bacteria into two groups on the basis of their cell wall
• Gram stain

  • fixation
  • crystal violet
  • iodine treatment
  • decolourisation
  • counter stain (safranin)
• Gram positive cocci

  
• Gram positive spiracetes

  
• Gram negative cocci

  
• Gram positive cocci

  
• Gram negative rod

  
• Gram positive rod

  
• Bacteria replicate outside the host if they have appropriate nutrients, temperature, oxygen levels, moisture and pH
• inflammation in the brain means the blood brain barrier starts to break down, the gaps get bigger and things that should not be present are able to pass through the walls and enter CSF e.g. proteins and neutrophils
• less glucose in CSF than blood suggests an organism such as bacteria is feeding on glucose stores in the brain
• Streptococcus pneumoniae - gram positive cocci
• Neisseria meningitidis - gram negative cocci
• Haemophilus influenzae - gram positive rods