Bacti 3

• Antibiotic classes based on site of action

  • Cell wall
  • Ribosome
  • Bacterial TOPO II enzyme
  • Cell membrane
  • Bacterial RNA Polymerase
  • Folic acid synthesis pathway
• Cell wall antibiotics


  Interrupt peptide cross-bridging step needed to give strength to cell wall, causing cells to burst due to osmotic pressure
• Penicillins and cephalosporins


  • Use beta-lactam ring to irreversibly bind to transpeptidase enzyme
  • Resistance through beta-lactamase enzyme which cleaves beta-lactam ring
• Cephalosporins


  • Different generations have different abilities
• Glycopeptide antibiotics


  Bind to end of peptide chain to stop cross-bridging, resistance occurs if microbe alters peptide chain
• Aminoglycosides


  Bind to 30S ribosomal subunit, valuable but have significant toxicity
• Tetracyclines


  Bind to 30S ribosomal subunit, less important than they used to be but still valuable
• Macrolides and ketolides


  Bind to 50S ribosomal subunit, excellent broad spectrum but bacteriostatic
• Oxazolidinones


  Linezolid binds to 50S subunit and stops subunits pairing, also bacteriostatic
• Chloramphenicol


  Only drug in its class, rarely used today due to significant toxicity
• Streptogramins


  Quinupristin and dalfopristin attack 50S subunit at different sites, used together synergistically
• Lincosamides


  Clindamycin binds to 50S subunit
• Fluoroquinolones


  Affect bacterial TOPO II enzyme
• Daptomycin


  Attacks cell membranes of Gram positive microbes, making them leaky, useful for specific infections
• Rifamycins


  Inhibit bacterial RNA Polymerase
• Polymixins


  Cause leakage of outer membrane in Gram negative cells, used for difficult CRE bacteria
• Trimethoprim and sulfa drugs


  Target bacterial folic acid synthesis pathway
• Methotrexate


  Also affects folic acid synthesis pathway, used against anaerobic bacteria, parasites, autoimmune diseases, and chemotherapy
• If you have a new antibiotic, you will sooner or later find microbes with resistance to it
• Antibiotics are made by some microorganisms in order to compete with other microorganisms, and so the others have developed resistance genes to fight back
• Resistance genes are gradually acquired by pathogens and then the pressure of having to fight the antibiotic frequently means that the resistance genes are selected for
• Methicillin resistance genes could be traced back to 1945, many years before the drug was actually used
• Ways bacteria can acquire DNA

  • Transformation
  • Transduction
  • Conjugation
• Transformation


  Free DNA in the environment is taken into the cell, usually short sequences and not gene-sized fragments
• Transduction


  Bacteriophage infects a cell and mistakenly packages a piece of host DNA while building new phage particles, rare occurrence
• Promiscuous plasmids, transposons, and integrons aid resistance gene transfer by conjugation
• Resistance mechanisms

  • Prevent antibiotic entry
  • Pump antibiotic out
  • Destroy or inactivate antibiotic
  • Change antibiotic target
• Prevent antibiotic entry


  Mutation in porin protein to stop antibiotic from entering with other materials
• Pump antibiotic out


  Bacteria have efflux pumps to remove antibiotics, tetracycline resistance pump is best studied example
• Destroy or inactivate antibiotic


  e.g. chloramphenicol resistance by acetylation, aminoglycosides by phosphorylation, beta-lactams by beta-lactamase
• Change antibiotic target


  Small mutations can change target so antibiotic no longer binds, selected for by antibiotic pressure
• Antibiotic resistance screening

  1. Disk diffusion assay

  2. Spectroscopic methods

  3. Molecular methods using PCR
• Disk diffusion assay measures zone of inhibition around antibiotic-impregnated disk to categorize bacteria as resistant, intermediate, or susceptible