Week 3:Antibiotics and Mode of Action

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  1. Antimicrobial agents

    - definition
    Chemical substances that kill or inhibit the growth of microbes
  2. Name 3 groups of antimicrobial agents
    • 1. Natural agent:
    • –A natural substance produced by certain groups of microorganisms;

    - commonly called antibiotic

    • 2. Semi-synthetic agent:
    • –A hybrid substance, a molecular version produced by the microbe is subsequently modified by the chemist to achieve desired properties (to improve effectiveness, etc.) 

    • 3. Synthetic agent:
    • –synthesized entirely by chemical means
  3. Effect of antimicrobial agents on bacterial growth:

    - bactericidal
    - bacteriostatic
    bactericidal- kills bacteria

    Bacteriostatic– suppressing the growth of bacteria

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  4. Name 5 Basic sites of antibiotic activity
    • 1. Cell wall synthesis
    • 2. cell membrane integrity
    • 3. nucleic acid synthesis
    • 4. Protein Synthesis
    • 5. biosynthesis pathways
  5. Name some Inhibitors of cell wall synthesis

    -  name what type of cell wall inhibitors they are
    • Peptidoglycan Inhibitors
    • • b-lactam antibiotics (b-lactams)
    • –share a common β-lactam ring structure


    •Bacitracin (topical use)

    • Inhibit Acid fast cell wall
    • - Isoniazid
    • - ethionamide
    • - ethambutol
    • - cycloserine (for mycobacteria)
  6. 4 Common β-lactam ring in b-lactam antibiotics
    • 1. Penicillin
    • 2. Cephalosporins
    • 3. Monobactams
    • 4. Carbapenems
  7. b-lactam antibiotic mode of action

    - targets?
    - bacericidal? bacteriostatic?
    •Blocking peptidoglycan synthesis by binding and inhibiting penicillin-binding proteins (PBPs), transpeptidase and transglycosylase

    • Weakening cell wall  -> lysis of bacterial cell

    • Generally bactericidal
  8. Vancomycin

    - origin?
    - targets?
    - prevents what?
    - inactive against?
    - Originally obtained from Streptomyces orientalis

    - inhibits bacterial cell wall synthesis by binding to the peptidoglycan precursor at the Ala terminus

    • - prevents formation of:
    • glycosidic bonds
    • pentaglycine peptide bonds

    - Inactive against gram-negative cells

    - Last line of defense against MRSA (methicillin resistant S. aureus)
  9. Peptidoglycan (murein)
    unique to bacteria (present both in Gram + and -)

    •principle compound of cell wall

    •Subunit of peptidoglycan is composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), and tetrapeptide attached to NAM

    •Peptidoglycan layer is a repeating subunit of peptidoglycan, forming a mesh-like layer outside the cell membrane.
  10. Vancomyosin Mode of Action
    •Inhibits formation of (via binding to Ala of tetrapeptide):

    - glycosidic bonds between the sugars by transglycosylase

    - peptide cross-links by transpeptidase

    •Weakens cell wall -> lysis of bacterial cell

    •Generally bactericidal
  11. Bacitracin

    - produced by?
    - prevents what?
    •cell wall synthesis inhibitor

    •produced by Bacillus species

    • •prevents the peptidoglycan transportation across the cell membrane by inhibiting bactoprenol (the conveyer of peptidoglycan
    • monomers). 

    As a result, no building blocks are available for peptidoglycan synthesis.
  12. Isoniazid

    inhibitor of?
    - Inhibitors of acid-fast cell wall synthesis

    –blocks the incorporation of mycolic acid into acid-fast cell walls
  13. ethambutol

    inhibitor of?
    blocks what?
    - Inhibitors of acid-fast cell wall synthesis

    – ethambutol interferes with the incorporation of arabinoglactan.
  14. Polymyxins

    - produced by?
    - binds to?
    - causes what?
    - effective on gram (-)/ (+)?
    - bactericidal? bacteriostatic?
    –produced by Bacillus polymyxis

    –binds to membrane phospholipids

    - causes cleavages (openings) in cell membrane, resulting in loss of cell contents

    –effective mainly against gram-negative bacteria;

    - cannot effectively diffuse through the thick peptidoglycan layer in gram-positives

    –usually limited to topical usage

  15. inhibitors of protein synthesis

    - two groups
    • 30s subunit-binding antibiotics
    • –Aminoglycosides


    • 50s subunit-binding antibiotics
    • –Chloramphenical



  16. Aminoglycosides

    - binds what?
    - inhibits what?
    - bactericidal? bacteriostatic?
    • –Irreversibly bind to 30s ribosome, leading to:
    • 1) inhibition of protein synthesis
    • 2) production of aberrant protein synthesis

    –Penetration through the outer membrane requires O2 (anaerobes are resistant)

  17. Aminoglycosides mode of action
    - prevents elongation of polypeptide chain

    - interferes with proofreading process -> leads to misreading of codon or premature termination of protein synthesis
  18. Tetracyclines

    - binds what?
    - blocks?
    - bactericidal? bacteriostatic?
    - effective against what?
    - antibiotic that prevents protein synthesis

    –Bind reversibly to 30s subunit

    blocks binding of aminoacyl-tRNA to the 30S ribosome-mRNA complex


    –Effective against Chlamydia, Mycoplasma, Richettsia
  19. Tetracyclines mode of action

    - blocks what?
    - binds to what?
    - mode?
    block bacterial translation

    binds reversibly to the 30S subunit

    distorting it in such a way that the anticodons of the charged tRNAs cannot align properly with the codons of the mRNA
  20. 50S subunit-binding Antibiotics
    • Chloramphenicol
    • blocks aminoacyl-tRNA interaction w/ the peptidyltransferase center (A-site) in the 50s ribosome, inhibiting transpeptidation (‘connection’)

    • Clindamycin
    • - dissociation of the peptidyl-t-RNA complex

    Macrolides (e.g., Erythromycin, Azithromycin, Clarythromycin etc.)

    - prevent peptides elongation

    • Oxazolidinones (e.g., linezolid)
    • - inhibiting formation of the 70s initiation
    • complex
  21. Macrolides Mode of Action
    • 1. preventing peptidyltransferase, the enzyme that forms peptide bonds between the amino acids, from forming peptide bonds
    • between amino acids.

    2. prevent the transfer of the peptidyl tRNA from the A-site to the P-site.
  22. oxazolidinones mode of action
    bind to the 50S ribosomal subunit and interfere with its binding to the initiation complex.
  23. Inhibitors of nucleic acid synthesis

    - two groups
    • DNA replication inhibitors
    • Quinolones (Nalidixic Acid, Norfloxacin, fluoroquinolone, ciprofloxacin etc)

    –Bind to bacterial DNA gyrase (topoisomerase type II and IV)

    • RNA transcription inhibitors:
    • Rifamycins (Rifampicin, Rifabutin)

    –Bind to bacterial DNA-dependent RNA polymerase (β subunit), leading to the inhibition of  transcription initiation
  24. Inhibitors of biosynthesis pathways-- Folic Acid synthesis inhibitors
    • Folic Acid synthesis inhibitors (folic acid is a
    • essential Methyl-group source for thymidine)


  25. Sensitivity testing
    in vitro testing of antibiotics against isolated strain

    a.  disk diffusion method (Kirby-Bauer method)  

    b.  broth or agar dilution method (to measure MIC and MBC)

    c.  E-test (to measure MIC)
  26. Disk diffusion (Kirby-Bauer method)
    Agar + bacteria  = Bacterial lawn

    - Place disks containing antibiotic  on the plate and incubate it

    - examine zones of inhibition
  27. MIC vs. MBC
    MIC = "Minimum inhibitory concentration"

    - lowest [ ]of an antibiotic that inhibits the visible growth of a microorganism after overnight incubation

    MBC = "Minimum bactericidal concentration"

    - lowest [ ]of an antibiotic that completely kills microorganism
  28. E-Test
    - MIC of antibiotic is determined by where the growth of the bacteria starts

    - utilises a rectangular strip that has been impregnated w/ antibiotic

    - lawn of bacteria is spread and grown on an agar plate and the Etest strip is laid on top;

    - drug diffuses out into the agar, producing an exponential gradient of the drug to be tested
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Week 3:Antibiotics and Mode of Action
Micro Week 3 Lecture
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