Lecture 1 Week 7

Lecture 1 Week 7

1. Microbes

   • Bacteria, fungi, worms and protozoans, viruses, prions
   • Eukaryotes
     • Fungi, worms, protozoans
   • Prokaryotes
     • Bacteria
   • Acellular
     • Viruses, prions

2. Structure of bacteria

   • Prokaryotes
   • Simple cells, little physical compartmentalisation
   • Physical differences to eukaryotes
     • Major: Structural, presence of molecules (e.g. cell wall)
     • Minor: similarity (e.g. ribosomes)

3. Outside: capsule and other layers

   • hydrophobic extracellular polysaccharides or protein polymers
   • Polysaccharide = capsule (attached to cell wall) = glycocalyx
   • Slime layer (biofilm) = S layers
   • Protection from enviornment and host defences (e.g. S pneumoniae)
   • Adherance (e.g. Strep mutans; teeth)
   • Relate to virulence

4. Other outside structures

   • Flagella - movement
   • Fimbriae and pili
     • Adherance and conjugation
     • Fimbriae = short hairs
     • Pilli = long hairs

5. Structure of bacterial cell wall

   • Shape and rigidity, osmotic bursting, interactions e.g. adhesion
   • Major constituent = peptidoglycan = N-acetylmuramic acid + N-acetylglucosamine (NAM + NAG)
   • Gram positive = pentaglycine bridge
   • Gram negative = direct cross link between peptide chain

6. Gram positive cell wall (GPB)

   • Purple stain
   • peptidoglycan, thick, glycans chains joind by a short peptide
   • Provide ridigity
   • Protection from outside enviornment (prevent osmotic bursting)
   • Adhesion
   • Important in stimulating immune responses

7. Gram negative cell wall (GNB) and cell membrane

   • Pink gram stain, thin peptidoglycan layer
   • Outer membrane, linked to peptidoglycan by lipoprotein
   • Lipopolysaccaride (LPS) uniquely present in outer membrane instead of phospholipids
   • Porins (transport), OmpC
     • Important in limiting transport of some antibiotics

8. Unusual bacterial cell walls: mycobacterium spp

   • E,g, M tuberculosis
     • Mycolic acid linked to arabinogalactan
   • Target of specific anti-TB drugs
   • Complex lipids prevent Gram staining and decolourisation
   • Termed acid fast bacillus
   • Ziehl-Neelsen stain used for confirmative visualisation

9. Unusual bacteria: no cell walls: mycoplasma spp

   • Mycoplsma - e.g. Mycoplasma pneumoniae
   • Not rigid/protected structure - instracellular abcteria
   • Can be annoyance in laboratory
     • Contamination of mammalian cell culture

10. Bacterial genetics

    • Chromosomal (haploid)
    • Genes arranged in operons --> co-ordinated expression
    • Mobile elements
      • Transposons (Tn) and insertion sequences (IS)
      • Integrons - moves gene cassettes
      • Phage DNA (lysogenic insertions)
    • Extrachromosomal
      • plasmids
      • transferred by conjugation
    • Implications
      • Size - manipulable for whole genome analysis
      • responsive genome - adaptable gene expression
      • Transferrable genetic material - adaptable gene mutation and acquisition

11. Bacterial metabolism

    • Co-ordinated regulation of metabolic pathways
    • Via operons
    • Catabolite feedback systems
    • Anaerobic respiration - final electron acceptor is inorganic compound
    • Aerobic respiration - final electron acceptor is oxygen
    • Small molecule biosynthesis (e.g. amino acids)
    • Nitrogen and carbon fixation (fermentation)

12. Bacteria: growth characteristics

    • Growth on defined media: Supplements, indicators of biochemistry, oxygen
    • temperature

13. Bacterial growth: biofilms

    • Planktonic cells would be lost in environment
    • many bacteria grow as biofilms on surfaces
    • Requires: adhesion
      • Protective layer (e.g. EPS, extra polymeric substance)
      • Important cell-cell iinteractions

14. Significance of bacterial biofilms

    • Source of planktonic cells to re-colonise other areas
    • Resistant cells: drugs, disinfectants, physical removal, virulent cells

15. Bacteria: virulence

    • Virulence: relative ability of pathogen to cause disease
    • how well a bacteria can: colonise - adhesion and biofilm formation
    • Resist host defences
    • Be invasive and cause tissue damage

16. Examples: Surface virulence factors

    • Ability to colonise
      • Helicobacter pylori
      • Flagella is important to transverse mucus layer to stomach wall
    • Ability to adhere
      • Pseudomonas aeruginosa
      • Slime layer, particularly important in colonisation of lungs of cystic fibrosis patients
    • Ability to evade host repsonse
      • Streptococcus pneumonia
      • antiphagocytic capsule, production of sIgA protease

17. Examples: toxins as virulence factors

    • Exotoxins = proteins secreted from the cell as it grows
    • Enterotoxins: effect the intesitine
      • Vibrio cholerae = cholera toxin
    • Neurotoxin: acts on nerves
    • Tissue toxins: causing cell death or tissue damage
      • Strep pyogenes = variety of cytolysins
    • Endotoxins: proteins that are apart of the cell
      • Not secreted, released when the cell lyses
      • Fever, inflammation, inhibition of phagocytosis