Mod 5

• Antimicrobial agents


  Natural or synthetic substance that kills (bactericidal) or inhibits (bacteriostatic) growth of microorganisms
• Types of antimicrobial agents

  • Antibacterial/antibiotic
  • Antiviral
  • Antifungal
  • Antiparasitic
• Antibacterial agents classified by mechanism of action

  • Inhibitors of cell wall synthesis
  • Inhibitors of protein synthesis
  • Inhibitors of nucleic acid function or synthesis
  • Inhibitors of cell membrane function
  • Inhibitors of metabolism
• Bacteriostatic


  Inhibits bacterial growth
• Bactericidal


  Kills target organisms
• Examples of bacteriostatic antibacterial agents

  • Chloramphenicol
  • Erythromycin and other macrolides
  • Clindamycin
  • Sulfonamides
  • Trimethoprim
  • Tetracyclines
  • Tigecycline
  • Linezolid
  • Quinupristin/dalfopristin
• Examples of bactericidal antibacterial agents

  • Aminoglycosides
  • Beta-lactams
  • Vancomycin
  • Daptomycin
  • Teicoplanin
  • Quinolones
  • Rifampin
  • Metronidazole
• Classification of antibiotics by source

  • Natural drugs
  • Semi-synthetic drugs
  • Synthetic drugs
• Narrow-spectrum antibiotics


  Acting only on a single or a limited group of microorganisms
• Extended-spectrum antibiotics


  Effective against gram-positive and also against a significant number of gram-negative bacteria
• Broad-spectrum antibiotics


  Affect a wide variety of microbial species
• Inhibitors of cell wall synthesis

  Beta-lactams bind enzymes involved in peptidoglycan production (PBPs)

  Glycopeptides bind to precursors of cell wall synthesis (e.g. vancomycin, bacitracin, teicoplanin, daptomycin)
• Beta-lactam antibiotics


  • Penicillins (e.g. amoxicillin, ampicillin, dicloxacillin, indanyl, carbenicillin, methicillin, nafcillin, penicillin G, penicillin V, piperacillin, ticarcillin)

  Cephalosporins (1st gen: cefadroxil, cefazolin, cephalexin; 2nd gen: cefaclor, cefprozil, cefuroxime, cefoxitin; 3rd gen: cefdinir, cefixime, cefotaxime, ceftazidime, ceftibuten, ceftizoxime, ceftriaxone; 4th gen: cefepime)

  Carbapenems (ertapenem, imipenem, meropenem)

  Monobactams (aztreonam)
• Beta-lactamase inhibitors


  Given with a beta-lactam antibiotic to provide effective treatment, have structural similarity with the beta-lactam antibiotics and function as substrate of beta-lactamase, thus reducing harm on the beta-lactam antibiotic (e.g. clavulanic acid, sulbactam, tazobactam)
• Vancomycin


  Binds to D-Ala-D-Ala terminal of growing peptide chain, usually ineffective against gram-negative bacteria, used to treat methicillin-resistant Staphylococcus aureus (MRSA)
• Inhibitors of protein synthesis

  Aminoglycosides bind to 30S ribosomal subunit

  Macrolide-lincosamide-streptogramin (MLS) group bind to 50S ribosomal subunit

  Ketolides bind to 50S ribosomal subunit

  Oxazolidinones bind to 50S ribosomal subunit

  Chloramphenicol binds 50S ribosomal subunit

  Tetracyclines bind 30S ribosomal subunit

  Glycylglycines bind to 30S ribosomal subunit
• Abelardo B. Aguilar, a Filipino scientist, was "supposed to be" the discoverer of a powerful macrolide antibiotic erythromycin, but the pharmaceutical company Eli Lilly and Company filed for the patent and was granted in 1953
• Inhibitors of DNA/RNA synthesis

  Fluoroquinolones inhibit DNA synthesis by binding DNA gyrases

  Metronidazole causes breakage of DNA strands

  Rifampin inhibits RNA synthesis by binding DNA-dependent, RNA polymerase
• Inhibitors of other metabolic processes

  Nitrofurantoin may have several bacterial enzyme targets and directly damage DNA

  Polymyxins disrupt cell membrane

  Sulfonamides interfere with folic acid pathway by binding the enzyme dihydropteroate synthase

  Trimethoprim interferes with folic acid pathway by binding the enzyme dihydrofolate reductase
• Anti-tuberculosis drugs


  • Rifampicin
  • Isoniazid
  • Pyrazinamide
  • Ethambutol
  • Streptomycin
• Synergism


  Occurs when the antimicrobial activity of a combination of antimicrobial agents is greater than the activity of the individual agents alone
• Antimicrobial resistance


  Ability of a microbe to resist effects of medication, may be environmentally-mediated or microorganism-mediated (intrinsic or acquired)
• Biologic resistance


  Organism becomes less susceptible
• Clinical resistance


  Susceptibility has been lost, drug is no longer effective
• Impermeability


  Inability to penetrate the cell wall of bacteria, related to lipopolysaccharide and outer membrane proteins (porins)
• Biofilms


  Decreased penetration of antibiotics, decreased oxygen and nutrients, formation of persister cells
• Efflux


  Efflux pumps remove toxic substances from the cell
• Enzymatic inactivation


  Beta-lactamases hydrolyze beta-lactam antibiotics, extended-spectrum beta-lactamases (ESBLs) can hydrolyze even newly developed beta-lactams
• Target site modification


  Chromosomal mutations in penicillin-binding proteins (PBPs) or enzymatic alteration of ribosomal targets
• Acquisition of new targets

  Acquire cellular targets with reduced affinity for the antibiotic, e.g. methicillin-resistant Staphylococcus aureus (MRSA)
• Timeline of development of antibiotics vs. resistance

  
• Antimicrobial susceptibility testing


  Test performed on bacteria isolated from clinical specimens to determine which antimicrobial agents might be effective in treating infections
• Conventional susceptibility testing methods

  • Dilution tests (broth dilution, agar dilution)
  • Diffusion tests (disk agar diffusion, gradient diffusion)
• Broth dilution method


  Serial dilution of antibiotic is prepared, and to each tube a standard amount of inoculum is added, determines MIC and can determine MBC

  
• Minimum Inhibitory Concentration (MIC)

  Lowest concentration of antibiotic that inhibits growth
• Minimum Bactericidal Concentration (MBC)

  Lowest concentration of antibiotic that kills 99.9% of bacteria
• Susceptibility testing


  Also known as Antimicrobial Sensitivity Testing
• Purpose of susceptibility testing


  Test performed on bacteria isolated from clinical specimens to determine which antimicrobial agents might be effective in treating infections caused by the bacteria
• Conventional susceptibility testing methods

  • Dilution tests
  • Diffusion tests
• Broth dilution method


  Serial dilution of antibiotic is prepared, and to each tube a standard amount of inoculum is added