6.2 - 6.4

• Salmonella spp.


  Gram negative bacterium with lipopolysaccharide endotoxins on outer membrane. Triggers release of proinflammatory cytokines. Acute inflammation results in diarrhoea. Releases endotoxins into host when bacterium dies.
• Staphylococcus spp.


  Secretes soluble proteins called exotoxins e.g. barrel-shaped proteins that embed in host cell membrane so contents leak, protease toxins, superantigens that trigger 20% of T cells (usual 0.001%) so can cause toxic shock.
• How Mycobacterium tuberculosis causes disease

  1. Triggers inflammatory response by infecting phagocytes in lungs.

  2. Infected phagocytes are sealed in waxy-coated tubercles so bacteria remain dormant. First infection has no symptoms.

  3. If another factor weakens immune system, bacteria become active & destroy lung tissue.
• How does penicillin work?

  Beta-lactam bactericidal antibiotic. Prevents formation of peptidoglycan cross-links in bacterial cell wall, causing osmotic lysis.
• How does tetracycline work?

  Bacteriostatic antibiotic. Prevents protein synthesis by binding to small subunit of ribosome so tRNA cannot attach. Therefore inhibits growth & division.
• What causes antibiotic resistance

  1. Random genetic mutation, often on plasmid, confers resistance e.g. antigen shape changes.

  2. These bacteria have selective advantage in the presence of antibiotics, reproduce & pass allele for resistance to offspring.

  3. Directional selection results in resistant strain.
• What causes antigen variability

  1. Random genetic mutation changes DNA base sequence.

  2. Results in different sequence of codons on mRNA

  3. Different primary structure of antigen = H-bonds, ionic bonds & disulfide bridges form in different places in tertiary structure.

  4. Different shape of antigen.
• Antigen variability affects the incidence of disease

  • Memory cells no longer complementary to antigen = individual not immune = can catch the disease more than once / cannot recognise pathogen e.g. HIV.
  • Many varieties of a pathogen = difficult to develop vaccine containing all antigen types.
• How hospitals minimise the spread of antibiotic resistant bacteria

  1. Screening & quarantine of affected patients.

  2. Hygiene code of practice e.g. alcohol-based antibacterial gels.

  3. Antibiotics prescribed only when necessary & course completed to minimise selection pressure.
• Horizontal conjugation transfers plasmids with resistance allele from one bacterium to another rapidly, making antibiotic-resistant bacteria difficult to control.