Lecture

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Lecture 37Dr. Buckhaults
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Antibiotics Disrupt Cell Wall Synthesis, Protein
Synthesis, Nucleic Acid Synthesis and Metabolism
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Principles and Definitions

• Selectivity
• Selectivity vs toxicity
• Therapeutic index
• Toxic dose/ Effective dose
• Categories of antibiotics
• Bacteriostatic
• Reversibly inhibit growth
• Duration of treatment sufficient for host
  defenses to eradicate infection
• Bactericidal-
• Kill bacteria
• Usually antibiotic of choice for infections in
  sites such as endocardium or the meninges where
  host defenses are ineffective.

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Principles and Definitions

• Selectivity
• Therapeutic index

• Categories of antibiotics
• Use of bacteriostatic vs bactericidal antibiotic
• Therapeutic index better for bacteriostatic
  antibiotic
• Resistance to bactericidal antibiotic
• Protein toxin mediates disease use
  bacteriostatic protein synthesis inhibitor to
  immediately block synthesis of toxin.

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Principles and Definitions

• Antibiotic susceptibility testing (in vitro)
• Bacteriostatic Antibiotics
• Minimum inhibitory concentration (MIC)
• Lowest concentration that results in inhibition
  of visible growth (colonies on a plate or
  turbidity of liquid culture)
• Bactericidal Antibiotics
• Minimum bactericidal concentration (MBC)
• Lowest concentration that kills 99.9 of the
  original inoculum

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Antibiotic Susceptibility Testing-MIC
Size of zone of inhibition depends on
sensitivity, solubility, rate of diffusion.
Compare results to MIC tables generated using
standards.
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Zone Diameter Standards for Disk Diffusion Tests
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Principles and Definitions

• Combination therapy
• Prevent emergence of resistant strains
• Temporary treatment until diagnosis is made
• Take advantage of antibiotic synergism
• Penicillins and aminoglycosides inhibit cell wall
  synthesis and allow aminoglycosides to enter the
  bacterium and inhibit protein synthesis.
• CAUTION Antibiotic antagonism
• Penicillins and bacteriostatic antibiotics. Cell
  wall synthesis is not occurring in cells that are
  not growing.
• Antibiotics vs chemotherapeutic agents vs
  antimicrobials
• Antibiotics-naturally occurring materials
• Chemotherapeutic-synthesized in the lab (most
  antibiotics are now synthesized and are therefore
  actually chemotherapeutic agents.

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Antibiotics that Inhibit Protein Synthesis

• Inhibitors of INITATION
• 30S Ribosomal Subunit (Aminoglycosides,
  Tetracyclines, Spectinomycin)
• 50S Ribosomal Subunit (Chloramphenicol,
  Macrolides)
• Inhibitors of ELONGATION
• Elongation Factor G (Fusidic acid)

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Review of Initiation of Protein Synthesis
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Review of Elongation of Protein Synthesis
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Survey of Antibiotics

• Discuss one prototype for each category
• Mode of Action
• Spectrum of Activity
• Resistance
• Synergy or Adverse Effects

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Protein Synthesis Inhibitors

• Mostly bacteriostatic
• Selectivity due to differences in prokaryotic and
  eukaryotic ribosomes
• Some toxicity - 70S ribosomes eukaryotic in
  mitochondria

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Antimicrobials that Bind to the 30S Ribosomal
Subunit
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Aminoglycosides (only bactericidal protein
synthesis inhibitor)streptomycin, kanamycin,
gentamicin, tobramycin, amikacin, netilmicin,
neomycin (topical)

• Modes of action -
• Irreversibly bind to the 16S ribosomal RNA and
  freeze the 30S initiation complex (30S-mRNA-tRNA)
  and prevents initiation of translation.
• Increase the affinity of the A site for t-RNA
  regardless of the anticodon specificity. Induces
  misreading of the mRNA for proteins already being
  synthesized.
• Destabilize microbial membranes
• Multiple modes of action is the reason this
  protein synthesis inhibitor is bactericidal.

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Aminoglycosides (bactericidal)streptomycin,
kanamycin, gentamicin, tobramycin, amikacin,
netilmicin, neomycin (topical)

• Spectrum of Activity -Many gram-negative and some
  gram-positive bacteria Not useful for anaerobic
  (oxygen required for uptake of antibiotic) or
  intracellular bacteria.
• Resistance - Common
• Synergy - The aminoglycosides synergize with
  b-lactam antibiotics. The b-lactams inhibit cell
  wall synthesis and thereby increase the
  permeability of the membrane to aminoglycosides.

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Tetracyclines (bacteriostatic)tetracycline,
minocycline and doxycycline

• Mode of action - The tetracyclines reversibly
  bind to the 30S ribosome and inhibit binding of
  aminoacyl-t-RNA to the acceptor site on the 70S
  ribosome.
• Spectrum of activity - Broad spectrum Useful
  against intracellular bacteria
• Resistance - Common
• Adverse effects - Destruction of normal
  intestinal flora resulting in increased secondary
  infections staining and impairment of the
  structure of bone and teeth. Not used in children.

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Spectinomycin (bacteriostatic)

• Mode of action - Spectinomycin reversibly
  interferes with m-RNA interaction with the 30S
  ribosome. It is structurally similar to the
  aminoglycosides but does not cause misreading of
  mRNA. Does not destabilize membranes, and is
  therefore bacteriostatic
• Spectrum of activity - Used in the treatment of
  penicillin-resistant Neisseria gonorrhoeae
• Resistance - Rare in Neisseria gonorrhoeae

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Antimicrobials that Bind to the 50S Ribosomal
Subunit
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Chloramphenicol, Lincomycin, Clindamycin
(bacteriostatic)

• Mode of action - These antimicrobials bind to the
  50S ribosome and inhibit peptidyl transferase
  activity. No new peptide bonds formed.
• Spectrum of activity - Chloramphenicol - Broad
  range Lincomycin and clindamycin -
  Restricted range
• Resistance - Common
• Adverse effects - Chloramphenicol is toxic (bone
  marrow suppression) but is used in life
  threatening situations such as the treatment of
  bacterial meningitis.

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Macrolides (bacteriostatic)erythromycin,
clarithromycin, azithromycin, spiramycin

• Mode of action - The macrolides inhibit
  translocation of the ribosome.
• Spectrum of activity - Gram-positive bacteria,
  Mycoplasma, Legionella
• Resistance - Common

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Antimicrobials that Interfere with Elongation
Factors
Selectivity due to differences in prokaryotic and
eukaryotic elongation factors
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Fusidic acid (bacteriostatic)

• Mode of action - Fusidic acid binds to elongation
  factor G (EF-G) and inhibits release of EF-GDP
  from the EF-G/GDP complex. Cant reload EF-G with
  GTP.
• Spectrum of activity - Gram-positive cocci

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Inhibitors of Nucleic Acid Synthesis
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Inhibitors of RNA Synthesis
Selectivity due to differences between
prokaryotic and eukaryotic RNA polymerase
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Rifampin, Rifamycin, Rifampicin, Rifabutin
(bactericidal)

• Mode of action - These antimicrobials bind to
  DNA-dependent RNA polymerase and inhibit
  initiation of mRNA synthesis.
• Spectrum of activity - Broad spectrum but is used
  most commonly in the treatment of tuberculosis.
• Resistance - Common. Develops rapidly (RNA
  polymerase mutations)
• Combination therapy - Since resistance is
  common, rifampin is usually used in combination
  therapy to treat tuberculosis.

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Inhibitors of DNA Synthesis
Selectivity due to differences between
prokaryotic and eukaryotic enzymes
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Quinolones (bactericidal)nalidixic acid,
ciprofloxacin, ofloxacin, norfloxacin,
levofloxacin, lomefloxacin, sparfloxacin

• Mode of action - These antimicrobials bind to the
  alpha subunit of DNA gyrase (topoisomerase) and
  prevent supercoiling of DNA, thereby inhibiting
  DNA synthesis.
• Spectrum of activity - Gram-positive cocci and
  urinary tract infections
• Resistance - Common for nalidixic acid
  developing for ciprofloxacin

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Antimetabolite Antimicrobials
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Inhibitors of Folic Acid Synthesis

• Basis of Selectivity-Bacteria synthesize folic
  acid, humans do not. We get it from our diet.
• Review of Folic Acid Metabolism
• Tetrahydrofolate required for the methyl group on
  methionine, and for thymidine and purine
  synthesis.

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Sulfonamides, Sulfones (bacteriostatic)

• Mode of action - These antimicrobials are
  analogues of para-aminobenzoic acid and
  competitively inhibit pteridine synthetase, block
  the formation of dihydropteroic acid.
• Spectrum of activity - Broad range activity
  against gram-positive and gram-negative bacteria
  used primarily in urinary tract and Nocardia
  infections.
• Resistance - Common
• Combination therapy - The sulfonamides are used
  in combination with trimethoprim this
  combination blocks two distinct steps in folic
  acid metabolism and prevents the emergence of
  resistant strains.

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Trimethoprim, Methotrexate, Pyrimethamine
(bacteriostatic)

• Mode of action - These antimicrobials binds to
  dihydrofolate reductase and inhibit formation of
  tetrahydrofolic acid.
• Spectrum of activity - Broad range activity
  against gram-positive and gram-negative bacteria
  used primarily in urinary tract and Nocardia
  infections.
• Resistance - Common
• Combination therapy - These antimicrobials are
  used in combination with the sulfonamides this
  combination blocks two distinct steps in folic
  acid metabolism and prevents the emergence of
  resistant strains.

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Anti-Mycobacterial Antibiotics
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Para-aminosalicylic acid (PSA) (bacteriostatic)

• Mode of action - Similar to sulfonamides-
  competitively inhibit pteridine synthetase, block
  the formation of dihydropteroic acid
• Spectrum of activity - Specific for Mycobacterium
  tuberculosis

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Dapsone (bacteriostatic)

• Mode of action - Similar to sulfonamides-
  competitively inhibit pteridine synthetase, block
  the formation of dihydropteroic acid
• Spectrum of activity - Used in treatment of
  leprosy (Mycobacterium leprae)

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Isoniazid (INH) (bacteriostatic )

• Mode of action - Isoniazid inhibits synthesis of
  mycolic acids.
• Spectrum of activity - Used in treatment of
  tuberculosis
• Resistance - Has developed

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Antimicrobial Drug ResistancePrinciples and
Definitions

• Clinical resistance vs actual resistance
• Resistance can arise by new mutation or by gene
  transfer (e.g. acquisition of a plasmid)
• Resistance provides a selective advantage.
• Resistance can result from single or multiple
  steps
• Cross resistance vs multiple resistance
• Cross resistance -- Single mechanism-- closely
  related antibiotics are rendered ineffective
• Multiple resistance -- Multiple mechanisms --
  unrelated antibiotics. Acquire multiple plasmids.
  Big clinical problem.

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Antimicrobial Drug ResistanceMechanisms

• Altered permeability
• Altered influx
• Mutation in a transporter necessary to import
  antibiotic can lead to resistance.
• Altered efflux
• Acquire transporter gene that will pump the
  antibiotic out (Tetracycline)

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Antimicrobial Drug ResistanceMechanisms

• Inactivation of the antibiotic
• b-lactamase
• Chloramphenicol Acetyl Transferase

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Antimicrobial Drug ResistanceMechanisms

• Mutation in the target site.
• Penicillin binding proteins (penicillins)
• RNA polymerase (rifampin)
• 30S ribosome (streptomycin)

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Antimicrobial Drug ResistanceMechanisms

• Replacement of a sensitive enzyme with a
  resistant enzyme
• Plasmid mediated acquisition of a resistant
  enzyme (sulfonamides, trimethoprim)