Antimicrobial Agents

• Antibiotics


  Substances produced by microorganisms that are capable of destroying microorganisms
• Natural Sources of antibiotics

  • Fungi
  • Bacteria
• Classification of Antibiotics according to Mechanism of Action

  • Agents that Interfere with Cell Wall Synthesis
  • Agents that Alter the Function or Permeability of the Cell Membrane
  • Agents that Inhibit Protein Synthesis
  • Agents that Act on Nucleic Acid
• Agents that Interfere with Cell Wall Synthesis

  • Act by inhibiting the different stages of peptidoglycan synthesis or by destroying already formed peptidoglycan
  • Examples: beta-lactams (interferes with immature to mature peptidoglycan) and glycopeptides (interferes with the completion of the synthesis of the cell wall)
• Agents that Alter the Function or Permeability of the Cell Membrane

  • Interfere by disrupting the outer membrane of the cell and inhibit metabolic processes
  • Antifungal drugs alter the permeability of cell membrane
  • Azoles inhibit the synthesis of ergosterol, a component of the fungal cell membrane
• Agents that Inhibit Protein Synthesis

  • Bind to either the 30S or the 50S subunit of a bacterial ribosome
  • 30S inhibitors primarily meddle with the initiation process
  • 50S inhibitors interfere with the elongation process (Macrolides, Chlorampenicol)
• Agents that Act on Nucleic Acid

  • Agents that act on DNA Topoisomerase- Topoisomerase are essential for the synthesis of DNA (quinolones)
  • Agents that Inhibit RNA Synthesis- Interferes with the beta subunit of an RNA polyisomerase that is needed for RNA synthesis
• Mechanisms of Drug Resistance

  • Intrinsic resistance- Stable genetic property that is encoded into the genome of the bacteria
  • Acquired resistance- Ability of a species to be resistant to a thing that they were susceptible to
• Resistance via genetic exchange

  1. Transformation- free DNA inserts itself into a member of the same species (simplest form of exchange)

  2. Transduction- Transfer of genetic material via a bacteriophage

  3. Conjugation- Transfer via the sex pili
• Drug Modification or Inactivation

  • Hydrolysis=Drug Inactivation
  • Gene codes for enzymes that can metabolize the antibiotic
• Prevention of influx/cellular intake

  • Gram-negative bacteria (some) have developed the ability to modify the lipid composition of their cell membranes, preventing the medication from getting in
  • Bacteria that have an efflux pump prevent the antibiotic from accumulating inside the cell
• Modification of Target Site

  • Antibiotics target specific sites for it to be effective. Modification of these sites can render a drug ineffective.
• Examples of Target Site Modification

  • Peptide subunits of peptidoglycan (Glycopeptides)
  • Ribosome subunits (Macrolides, Tetracyclines, Aminoglycosides)
  • Metabolic Enzymes (Sulfa drugs, Sulfones, trimethoprim)
  • Lipopolysaccharide Structure (Polymyxins)
  • DNA Gyrase (Fluororquinolones)
  • RNA Polymerase (Rifampin)
• Overproduction or Bypass of Target Enzyme


  • If the bacteria overproduces the target enzyme, the efficacy of the antibiotic will be lessened
• Target Mimicry


  • Involves the bacteria making very similar copies of the target enzyme to confuse the antibiotic
• Antibiotics are drugs that kill or inhibit the growth of bacteria.
• Bacteriostatic agents prevent bacterial multiplication, while bactericidal agents destroy bacteria.
• The spectrum of activity refers to the range of microorganisms affected by an antimicrobial agent.