Antimicrobial Agents

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Antimicrobial Agents

• Mechanisms of Action and Spectra
• of Activity

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OBJECTIVES

• Describe major classes of antimicrobials and
  their mechanisms of action
• Know the spectra of activity of commonly used
  antibiotics
• Understand the mechanisms of resistance to
  antibiotics
• Know the principle and rationale behind
  antibiotic susceptibility testing

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Overview of Antibiotics as Therapeutic Agents

• Selective Inhibition/Toxicity
• Due to the differences in structure and metabolic
  pathways
• Harm microorganisms, not the host
• Four major sites
• Cell wall
• Ribosomes
• DNA
• Cell membrane

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Spectrum of Activity

• Broad Spectrum Antibiotics
• Effective against many types
• Example Tetracycline
• Narrow Spectrum Antibiotics
• Effective against very few types
• Example Penicillin

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Bactericidal v. Bacteriostatic

• Bactericidal
• Kill bacteria
• Used when the host defense mechanisms are
  impaired
• Required in endocarditis, kidney infection

• Bacteriostatic
• Inhibit bacteria
• Used when the host defense mechanisms are intact
• Used in many infectious diseases

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MECHANISMS OF ACTION OF MAJOR GROUPS OF
ANTIBIOTICS
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STRUCTURE OF ?-LACTAM ANTIBIOTICS
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PENICILLIN HOME
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Penicillin Home

• Looks like a house with a new room added to the
  side
• Think of the R-group as of a funky antenna
• Changing antennae and or finishing the
  basement will create better homes
  (penicillins)

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Penicillin Home Improvement Project

• Adding a new antenna creates broad spectrum
  penicillins
• Example Ampicillin
• Adding additional antennae and finishing the
  basement creates cephalosporins
• Example 1st, 2nd, 3rd, 4th generation
  cephalosporins

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SPECTRUM OF ACTIVITY OF CEPHALOSPORINS
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MECHANISM OF ACTION OF ?-LACTAM ANTIBIOTICS
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?-lactam antibiotics are ineffective against
protoplasts and spheroplastsA. TRUEB. FALSE
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MECHANISMS OF ACTION OF ANTIRIBOSOMAL ANTIBIOTICS
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Inhibition of Protein Synthesis

• Anti-ribosomal antibiotics impair ribosomes by
  binding to either 50S or 30S ribosomal subunits
• Ribosomes are essential for translation of mRNA
  into proteins
• No translation ? No protein synthesis
• No protein synthesis ? No growth

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Ribosome Home Plate

• Baseball player slides into home
• The ball is fielded by the catcher who makes a
  CLEan TAG
• The word CLEean lies over the base these inhibit
  ? 50S
• The word TAG lies beneath the base these inhibit
  30S

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How are these drugs administered?
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MECHANISM OF ACTION OF SULFONAMIDES AND
TRIMETHOPRIM
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Mechanisms of Resistance

• Genetic Mechanisms
• Non-Genetic Mechanisms

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Genetic Mechanisms

• Chromosome-mediated
• Due to spontaneous mutation
• in the target molecule
• in the drug uptake system
• Plasmid-mediated
• Common in Gram-negative rods
• Transferred via conjugation
• Multidrug resistance
• Transposon-mediated

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Non-Genetic Mechanism

• Inaccessibility to drugs (e.g., abscess, TB
  lesion)
• Stationary phase (insusceptible to inhibitors of
  cell wall synthesis)
• Protoplasts and spheroplasts (insusceptible to
  inhibitors of cell wall synthesis)

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The End Result of Genetically Conferred Resistance

• Production of drug-inactivating enzymes
• Modification of target structures
• Alteration of membrane permeability

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Resistance to Beta-Lactams

• Gram
• ?-lactamase (Penicillinase)
• Alteration of the transpeptidase enzyme

• Gram -
• ?-Lactamase (Penicillinase)
• Alteration of porins

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How can we test for susceptibility/resistance
?
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Antibiotic Susceptibility Testing

• Dilution Method
• Disc Diffusion Method
• E-test
• High-Tech Methods

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Dilution Method

• Prepare two fold antibiotic dilutions
• Add 1/2 a million bacterial cells per tube
• Incubate overnight
• Check for turbidity
• Establish the MIC
• The lowest concentration of the drug that
  prevented the bacterial growth (no turbidity)

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Disc Diffusion Method

• Seed agar plates with bacteria in question
• Place antibiotic-discs over the seeded plate
• Incubate overnight
• Measure the inhibition zones
• Relate the results to the zones given in the
  interpretive chart
• There is an inverse relationship between the MICs
  and zone diameters

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Therapeutic Index Max. Safely
Achievable Level MIC
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Take Home Message

• Antibiotic Targets
• Bactericidal v. Bacteriostatic Drugs
• Spectrum of Activity
• Major Groups of Drugs
• Mechanisms of Resistance
• Susceptibility Tests