Antibacterial Modes of Action

• What is an antimicrobial?

  • any chemicals which kill or inhibit microbial growth in or on a body surface
• What is an antibacterial?

  • any chemical which specifically targets bacteria and kills or inhibits its growth
• What is an antibiotic?

  • an antimicrobial agent which is synthesised by a microorganism and selectively inhibits or kills other microorganisms
  • drug used to treat bacterial infections
• What is an example of an antibiotic?

  • penicillin (Penicillin notataum)
  • it is produced by a mould
• What are 3 sources of antibiotics?

  1. microorganism = e.g. penicillin
  2. synthesis = chloramphenicol
  3. semi-synthesis (most common) = amoxicillin
• What are actions of antibacterials?

  • only kill the actively growing bacteria
  • hard to treat dormant infections
• What does bactericidal mean?

  • kills bacteria
• What does bacteriostatic mean?

  • slows down or stalls bacterial growth
• What is selective toxicity?

  • selectively kills or inhibits the target organism whilst causing no or minimal harm to the host
• What are ideal properties of antibacterials?

  • specific - ability of a drug/chemical agent to interact with a defined target
  • selective - less selectivity means more adverse effects
  • bactericidal
  • minimal emergence of resistance to the drug
• What are ideal pharmacological activities of antibacterials?

  • non toxic to host
  • long plasma half life
  • good tissue distribution
  • low plasma protein binding
  • oral and parenteral
  • no interference with other drugs
• Describe the cell wall as a target of antibacterials

  • major target
  • many drugs
  • unique to prokaryotes
  • provides a selective target
  • mainly bactericidal
• Describe protein synthesis as a target of antibacterials

  • many drugs
  • exploit differences between prokaryotic and eukaryotic ribosomes (which are a selective target)
  • mainly bacteriostatic (e.g. tetracycline)
• Describe nucleic acid sythesis as a target of antibacterials

  • many drugs
  • there are different 'cellular machinery'
  • selective target
  • bacteria lack a nucleus
  • mainly bactericidal
• Describe the cell membrane as a target of antibacterials

  • few drugs target the cell membrane
  • have similarity to eukaryote cell membranes
  • few targets are selective for bacteria
  • mainly bactericidal
• Give examples of inhibitors of cell wall synthesis

  • β-lactams (e.g. penicillins, cephalosporins, carbapenems, monobactams)
  • glycopeptides (e.g. vancomycin, teicoplanin)
  • fosfomycin
  • cycloserine
  • bacitracin
• Describe peptidoglycan biosynthesis

  1. peptidoglycan monomer is built up inside the cell
  2. monomer is transported across the membrane (to the exterior)
  3. glycosylases join the monomer onto the existing peptidoglycan (linking the glycan backbone)
  4. transpeptidases (penicillin binding proteins - PBP) link PG amino acid side chains
• Describe β-lactams

  • all contain a β-lactam ring
  • there are differences in structure of the ring attached to the β-lactam and the side chains (R group)
• What are the 3 modes of action of β-lactams?

  direct modes (inhibit cross-linking of the peptide side chains)

  1. bind to inhibit the action of transpeptidases (usually known as PBPs) = prevents cross linking, prevents stable formation of peptidoglycan, results in cell lysis
  2. get incorporated into the peptide side chain = prevents cross linking, prevents stable formation of peptidoglycan, results in cell lysis

  indirect

  1. stimulate bacteria to produce autolysins, break down cell wall, leads to cell lysis
• Describe the relationship between β-lactams and gram negative bacteria

  • only some β-lactams can pass through porin channels
  • means that only some β-lactams can treat some gram negative bacteria
• Describe glycopeptides

  • e.g. vancomycin, teicoplanin
  • acts on gram positive bacteria only
  • target : binds to the terminal amino acids on the peptide side chain of the monomer
  • prevents the cross linking of the peptide side chains
  • prevents glycosylase enzyme from adding PG monomer onto PG chain
  • large molecules cannot penetrate the outer membrane of the gram negative cells
• What are examples of inhibitors that act on the cell membrane?

  • polymyxins (e.g. polymyxins E/ colistin) = fell out of use for its severe nephrotoxicity, now used for severe gram negative infections
  • lipopeptides (e.g. daptomycin/Cubicin)
• Describe the action of polymyxins on the plasma membrane

  1. binds to lipid A
  2. distorts the membrane
  3. penetrates the cell wall
  4. secondary effect = disrupts the membrane integrity
  5. allows the leakage of cytoplasmic contents
• What are 4 actions of antibacterials acting on nucleic acid synthesis?

  1. metabolic inhibition of NA synthesis (e.g. sulphonamides, trimethoprim)
  2. affects DNA replication (e.g. fluoroguinolones - ciprofloxacin, levoflaxacin)
  3. affect RNA polymerase (e.g. rifamycins - rifampicin)
  4. affect DNA (e.g. nitroimidazoles - metronidazole)
• Describe DNA replication

  1. starts at the origin
  2. dsDNA is unwound by helicase to expose template DNA strands
  3. unwinding generates supercoils which are then removed by DNA gyrase (aka. topoisomerase II)
  4. zone of unwound DNA is known as the replication fork
  5. DNA polymerase syntheses complimentary DNA strands at the replication fork
  6. replication is bidirectional
  7. two replication forks collide at the opposite sides of the chromosome
  8. 2 circular chromosomes are linked together
  9. topoisomerase IV separates DNA
  10. chromosomes are partitioned into each daughter cells
• Describes fluoroquinolones

  • e.g. ciprofloxacin and levofloxacin
  • bind to inhibit DNA gyrase/topoisomerase IV when complexed with bacterial DNA
  • this inhibits DNA replication and the packaging of DNA within the bacterial cell
  • leads to bacterial cell lysis
• Describe the difference between bacterial and mammalian ribosomes

  differences between the structures provide selective inhibition of bacterial protein synthesis

  mammalian

  • 60S subunit: 26S rRNA, 5.8S rRNA, 5S rRNA and proteins
  • 40S subunit: 18S rRNA and proteins

  bacterial

  • 50S subunit: 23S rRNA, 5S rRNA and proteins
  • 30S subunit: 16S rRNA and proteins
• Describe protein synthesis

  ribosomes contain 3 sites (Exit, Peptidyl, Acceptor)

  1. initiation = ribosome assembles around the template mRNA, first tRNA is attached at the start codon
  2. elongation = tRNA transfers an amino acid to the mRNA corresponding to the next codon, ribosome translocates to the next mRNA codon to continue the process (creating an amino acid chain)
  3. termination = when a stop codon is reached the ribosome will release the polypeptide
• Describe the action of tetracyclines

  • e.g. doxycycline
  • binds reversibly to the A site on 16S rRNA in the 30S subunit
  • inhibits the binding of the tRNA to the A site (inhibiting protein synthesis)
  • selectivity occurs through better binding to the bacterial ribosome and better accumalation of the antibiotic inside the bacterial cell
  • effects are bacteriostatic
• Describe the action of aminoglycosides

  4 modes of action

  1. prevent the formation of the initiation complex
  2. inhibit the binding of tRNA to A site
  3. cause misreading of the codons (resulting in dysfunctional proteins)
  4. increase bacterial membrane permeability

  • e.g. gentamicin, tobramycin
  • bind irreversibly to the A site on 16S rRNA in the 30S subunit
  • effects are bactericidal