Mechanisms of Action of Antibiotics (Antibacterial Agents)

• Types of antibiotics:

  • Bacteriocidal (drugs that kill bacterial cells)
  • Bacteriostatic (drugs that inhibit/retard bacterial growth and multiplication)
  • Minimum inhibitory concentration - above this is inhibits bacterial growth and replication
  • Minimum bacteriocidal concentration - above this the bacterial cells are killed
• Gram-positive:

  • Single cell membrane
  • Thick bacterial cell wall - made up mostly of peptidoglycan

  Gram-negative:

  • 2 bacterial cell membranes (inner and outer)
  • Thinner cell wall between them made up of less peptidoglycan

  
• Mechanisms of action:

  • Inhibit cell wall synthesis (because it's different in bacterial cells than in human cells)
  • Alteration of cell membrane (same point as above)
  • Inhibit protein synthesis
  • Interfere with bacterial nucleic acid (stop genes being able to function)
  • Anti-metabolic activity
• Inhibition of cell wall synthesis:

  • Cell wall made up of peptidoglycan - unique to bacterial cells - therefore can target this with antibiotics; doesn't affect human cells
  • Peptidoglycan monomer is transported across cell membrane by lipid carrier
  • Once it has crossed onto the outside of the cell membrane, it's then cross-linked into a peptidoglycan polymer - a process that is catalysed by an enzyme (penicillin binding protein)

  
• Sites of action for inhibition of cell wall synthesis:

  • Interference with enzyme catalysing cross-linkage (penicillin binding proteins)
  • Interference with peptidoglycan monomer
  • Interference with lipid carrier

  All this leads to a poorly formed cell wall, so the bacterial cell will undergo cell lysis.
• Inhibition of cell wall synthesis:

  • Beta-lactams (penicillins, cephalosporins, carbapenems)
  • Inhibition of peptidoglycan synthesis, by binding enzyme required for cross-linking (penicillin binding protein)
  • Vancomycin
  • Disrupts peptidoglycan cross-linkage by binding to peptidoglycan monomers - stops them becoming cross-linked
  • Bacitracin
  • Disrupts lipid carrier required for glycan transport across the bacterial cell membrane

  
• Alteration of cell membrane:

  • Bacterial cell can't maintain potential across membrane (depolarises cell, so causes leakage of cell contents
  • Disruption cross-membrane potential
  • Polymyxins (B and E) - disrupt the lipid cell membrane
  • Daptomycin - disrupts cell membrane by inserting itself into it
• Inhibition of protein synthesis:

  • 30S ribosome site
  • Aminoglycosides (Streptomycin, Gentamycin)
  • Tetracyclines (Doxycycline, minocycline, tetracycline)
  • 50S ribosome site
  • Macrolides (erythromycin, azithromycin, clarithromycin)
  • Chloramphenicol
  • Clindamycin

  Prokaryotic ribosomes in bacteria are different to the eukaryotic ribosomes found in human cells - therefore potential target for antibiotics
• Protein synthesis:

  • mRNA attaches to 30S subunit of ribosome
  • tRNA brings amino acid to A site on ribosome
  • Transpeptidation of amino acid to growing peptide from P site

  
• Translocation:

  • Ejection of tRNA from P site
  • Translocation of tRNA from A site to P site with growing peptide
  • Ribosome moves one codon on mRNA
  • New tRNA attaches to A site

  
• Inhibition of protein synthesis - tetracycline (antibacterial):

  • Site of action: 30S unit
  • Action inhibited: Entry of incoming acetyl tRNA
• Inhibition of protein synthesis - gentamycin (antibacterial):

  • Site of action: 30S unit
  • Action inhibited: Correct reading of mRNA
• Inhibition of protein synthesis - erythromycin (antibacterial):

  • Site of action: 50S unit
  • Action inhibited: Translocation
• Inhibition of protein synthesis - clindamycin (antibacterial):

  • Site of action: 50S unit
  • Action inhibited: Translocation
• Inhibition of protein synthesis - chloramphenicol (antibacterial):

  • Site of action: 50S unit
  • Action inhibited: Transpeptidation
• Inhibition of nucleid acid synthesis:

  • DNA effects
  • Quinolones (ciprofloxacin, moxifloxacin, ofloxacin)
  • Metronidazole
  • RNA effects (transcription)
  • Rifampicin
• Anti-metabolic activity - folic acid and DNA production:

  • Synthesis of folic acid is required for the synthesis of DNA - so folic acid, thymine and purines are potential targets for antibiotics
  • Trimethoprim acts on dihydrofolate reductase, inhibiting it
  • Sulfamethoxazole and dapsone act on dihydropteroate synthetase, inhibiting it

  
• Antimicrobial resistance:

  • Misuse of antibiotics leads to increased resistance
  • Misuse includes:
  • Incorrect dose
  • Incorrect duration
  • Inappropriate choice of antibiotic
  • Use in unwarranted clinical situations
• Antimicrobial resistance:

  • Natural
  • Inherent resistance (eg don't have the pathway that the antibiotic inhibits)
  • Acquired
  • Modification/acquisition of new genetic material
• Antimicrobial resistance:

  • Efflux of antibiotic - antibiotic is pumped out before it can take its effect - bacteria acquire genes that allow these to be built
  • Enzyme modification - enzymes that tag something on to the antibiotic to stop it from working the way it normally would
  • Enzyme degradation - enzymes that degrade the antibiotic to stop it working
  • Multi-drug resistant organisms
  • Major threat to humanity
• MRSA:

  • Methicillin Resistant Staphylococcus aureus
  • Result of increasing resistance to bacteria to antibiotics
  • Treated with:
  • Vancomycin
  • Teicoplanin
  • Linezolid
• Acquisition and spread of antibiotic resistance:

  • Vertical gene transfer
  • Spontaneous mutation
  • Horizontal gene transfer
  • Conjugation
  • Transformation
  • Transduction