Topic 6.2-4 Bacteria as pathogens, action of antibiotics and antibiotic resistance Flashcards
Why are bacteria described as agents as infection?
- Produce exotoxins
- Endotoxins on surface trigger immune response
- Invade and destroy host tissues
How does salmonella spp. cause disease?
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.
How does Staphylococcus spp. cause disease?
Secretes soluble proteins called toxins e.g.:
* Barrel-shaped proteins embed in host cell membrane so contents leak
* Protease toxins
* Superantigens trigger 20% of T cells (usual 0.001% so can cause toxic shock
How does Mycobacterium tuberculosis cause disease?
- Triggers inflammatory response by infecting phogocytes in lungs
- Infected phagocytes are sealed in waxy-coated tubercules so bacteria remain dormant. First infection has no symptoms.
- If another factor weakens immune system, bacteria become active and destroy lung tissue
How does penicillin work?
Beta-lactam bacteriacidal 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 and division.
NB: bacteriosatic antibiotics may also inhibit nucleic acid formation
What causes antibiotic resistance?
- Random genetic mutation, often on plasmid, confers resistance e.g. antigen shape changes.
- These bacteria have selective advantage in the presense of antibiotics, reproduce and pass allele for resistance to offspring.
- Directional selection results in resistant strain
What causes antigen variability?
- Random genetic mutation changes DNA base sequence
- Results in different structure of codons on mRNA
- Different priimary structure of antigen = hydrogen bonds, ionic bonds and disulfide bridges from in different places in tertiary structure
- Different shape of antigen
Explain how 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 do hospitals minimise the spread of antiobiotic resistant bacteria?
- Screening and quarentine of affected patients
- Hygiene code of practise e.g. alcohol-based antibacterial gels
- Anitbiotics prescribed only when necessary and course completed to minimise selection pressure
Why is it so difficult to control the spread of antibiotic-resistant bacteria?
Horizintal conjugation transfers plasmids with resistance alleles from one bacterium to another rapidly