Medical microbiology

• types of pathogens

  • Obligate pathogens
  • Opportunistic
  • Accidental pathogens
• Symptoms and signs of infection


  • Inflammation
  • Pain
  • Pyrexia
  • Tachycardia
  • Rigors
  • Increased white blood cell count
  • Increased C reactive protein
• what is C reactive protein

  Produced in the liver and has immediate response against infection
• Infection


  Can be latent or sub-clinical
• Distinguishing between pathogens and commensals is not always clear
• Host defence


  • Pathogen which has capacity to cause disease but only when hosts defences are weakened
• how do we understand disease

  • Koch's postulates
  • criteria to establish a relationship between an organism and disease
• Pathogenicity

  Capacity of a microorganism to cause an infection
• Requirements for pathogenicity

  • Infectivity
  • Virulence
• Infectivity


  Ability to become established
• Factors conferring infectivity


  • Attachment
  • Acid resistance
• Virulence


  Capacity to cause harmful effects once established
• Virulence factors


  • Invasiveness
  • Toxic production
  • Evasion to immune systems
• Virulence factors are genetically determined microbial components
• Virulence factors are specific to strains not species
• Examples of invasiveness


  • Necrotising fasciitis
  • Cellulitis
  • Connective tissue breakdown
  • Fibrinolysis
• Types of toxins


  • Exotoxins
  • Enterotoxins
  • Endotoxin
• Exotoxins


  Released extracellularly by the micro-organism
• Endotoxin


  Structurally part of the gram negative cell wall
• Cholera enterotoxin


  1. Increases cAMP levels

  2. Inhibits uptake of sodium and chloride ions

  3. Stimulates secretion of chloride and bicarbonate ions

  4. Causes massive outflow of water
• Cholera causes death by dehydration due to diarrhoea
• Cholera is treated by rehydration
• Superantigens


  • Exotoxins of strep pyrones and staph aureus
  • Able to stimulate the division of T cells in the absence of a specific antigen
  • Overwhelming cytokine production causes toxic shock
• Components of endotoxin


  • Lipid A
  • Oligosaccharide core
  • Specific polysaccharide chain
• E.coli and other gram negative bacteria induce a severe uncontrolled host response to endotoxin
• Host response to endotoxin


  • Cytokine production
  • Fever
  • Rigors
  • Hypotension
  • Tachycardia
  • Collapse
• Gram stain


  • Simple framework to classify clinically important organisms
  • Different species have varying capacity to cause disease
  • Different classes of antibiotics are effective against gram positive/gram negative bacteria
  • Gram stain can give early indication of the type of bacteria which may be causing infection
• Gram positive cocci


  • Staphylococci
  • Streptococci
• Coagulase test


  Used to distinguish between staphylococci and streptococci
• Staphylococcus aureus is commonly resistant to penicillin due to beta-lactamase
• Some strains of Staphylococcus aureus are methicillin resistant (MRSA)
• MRSA poses major problems in infection control within hospitals
• The first description of Staphylococcus causing post op wound infection was in Aberdeen, 1870
• Coagulase-negative staphylococci


  • Can be pathogenic in the presence of foreign bodies/prostheses
• Examples of foreign bodies/prostheses

  • Prosthetic heart valves
  • Prosthetic hip joints
  • Pacemaker wires
• Types of haemolysis


  • Alpha-haemolytic
  • Beta-haemolytic
• Alpha-haemolytic


  Partial damage of the red blood cells which reduces haemoglobin to methaemoglobin
• Beta-haemolytic


  Complete haemolysis > complete lysis or red cells in media
• Alpha-haemolytic streptococci


  • Streptococcus pneumoniae
  • Viridians streptococci
• Streptococcus pneumoniae


  • A gram positive bacteria a alpha-haemolytic member of streptococcus
  • Can cause pneumonia, meningitis and septicaemia