Transcript Antibiotic Selection

Antibiotics
Biotechnology II
Antibiotics Disrupt Cell Wall Synthesis,
Protein Synthesis, Nucleic Acid Synthesis
and Metabolism
Univ S. Carolina
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.
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.
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
Antibiotic Susceptibility Testing-MIC
Disk Diffusion Test
Determination of MIC
Str
Tet
8
4
2
1
0
Tetracycline (g/ml)
MIC = 2 g/ml
Ery
Chl
Amp
Size of zone of inhibition depends on sensitivity, solubility, rate of diffusion.
Compare results to MIC tables generated using standards.
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.
Mechanisms of Antibiotics
•Inhibition of Protein Synthesis
• Mostly bacteriostatic
• Selectivity due to differences in prokaryotic and
eukaryotic ribosomes
• Some toxicity - 70S ribosomes eukaryotic in mitochondria
•Inhibitors of INITATION
• 30S Ribosomal Subunit (Aminoglycosides, Tetracyclines,
Spectinomycin)
• 50S Ribosomal Subunit (Chloramphenicol, Macrolides)
•Inhibitors of ELONGATION
• Elongation Factor G (Fusidic acid)
Mechanisms of Antibiotics
• Inhibitors of Nucleic Acid Synthesis
• Inhibitors of RNA Synthesis: Selectivity due to
differences between prokaryotic and
eukaryotic RNA polymerase: (Rifampcin)
• Inhibitors of DNA Synthesis: Selectivity due to
differences between prokaryotic and
eukaryotic enzymes: (Quinolones)
• Inhibitors of Folic Acid Synthesis :bacteria make
it, we get it from diet (Sulfonamides)
Antimicrobial Drug Resistance
Principles and Definitions

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
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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.
Antimicrobial Drug Resistance
Mechanisms

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)

Inactivation of the antibiotic
b-lactamase
Chloramphenicol Acetyl Transferase

Mutation in the target site.
– Penicillin binding proteins (penicillins)
– RNA polymerase (rifampin)
– 30S ribosome (streptomycin)
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Replacement of a sensitive enzyme with a resistant
enzyme
– Plasmid mediated acquisition of a resistant enzyme (sulfonamides,
trimethoprim)