HC12: Introduction to infectious diseases

The earth's history

Bacteria form an essential part in the development of higher organisms:

1. 4 billion years ago: the first bacteria (prokaryotes)
2. 2,5 billion years ago: the first 1-cellular organisms (eukaryotes)
3. 1,5 billion years ago: the first multicellular eukaryotes
4. 0,5 billion years ago: the first fish

If the age of the earth was 1 year, bacteria would've come in February and humans only would have existed in the last minute of December 31st.

Determination of bacteria

Bacteria can be classified based on the structure of the cell wall

• No cell wall → mycoplasma
• Gram-negative → escherichia coli
• Gram-positive → streptococcus pyogenes
• Acid-fast → mycobacterium tuberculosis

Gram negative and positive bacteria can be distinguished with gram stains:

• Gram-negative
  • Curved → vibrio campylobacter
  • Rods → escherichia and salmonella
  • Cocci → neisseria
    • Spherical
• Gram-positive
  • Rods
    • Spore forming → clostridium
    • Non spore forming
  • Cocci
    • Groups → staphylococci
    • Chains → streptococci

Interaction

Virulence factors describe what the bacterium does with the host (the human). Defense mechanisms describe how the host reacts to the bacterium.

S. pyogenes:

Streptococcus pyogenes is a gram-positive bacteria. They are called extracellular bacteria → cannot survive in a cell. Virulence factors of S. pyogenes are:

• Structural → leads to acute inflammation
  • Peptidoglycan
  • Lipoteichoic acid
• Products
• Characteristics → acute inflammation

S. pyogenes can enter the body when the barrier function is impaired, for example when the granulocyte function doesn't work due to alcoholism. Defense mechanisms of S. pyogenes are made up of the innate immune system.

Laboratory tests

There are 2 blood tests which can be used to test for inflammation:

• C-reactive protein (CRP)
  • Acute phase protein
    • There are lots of cytokines → liver responds by exciting lots of proteins (also CRP) → acute phase response
  • Rises within hours
  • Disappears in days
• Erythrocyte sedimentation rate (ESR)
  • Measure of the amount of proteins in the blood
    • When there are a lot of proteins, they glue together as "stacks"
      • Many proteins can do this
  • Dependent on the concentration of plasma proteins versus red blood cells
    • Fibrinogen
    • Immunoglobulins
  • Nonspecific
  • Rises and disappears more slowly

Both are blood tests that aren't specific for a specific bacterium. CRP is an acute marker, ESR is a chronic marker.

Entamoeba histolytica

Entamoeba histolytica is a human pathogen. It lives in the bowel, where it may make a person ill. It is excreted with the feces in 2 forms:

• Cyst form: survives in the environment → bacteria in this form can be transmitted to other humans

Trophozoites: doesn't survive in the environment → dies within seconds/minutes