Antibiotic Therapy

Key definitions

  • Antibiotics: a drug used to treat or prevent infection caused by microorganisms
  • Bacteriostatic: inhibit growth of bacteria
  • Bactericidal: kill bacteria

Antibiotics that act on the bacterial cell wall

  • Inhibit cell wall synthesis by preventing cross linking of peptidoglycan - bactericidal

Penicillins e.g. flucloxacillin, amoxicillin

  • β lactam
  • Very few side effects
  • Range from narrow to broad spectrum
  • Excreted (rapidly) via kidneys
  • Safe in pregnancy

Cephalosporins e.g. cefaclor, cefadroxil and cefalexin

  • Modified β lactam
  • Excreted via kidneys and urine
  • Safe in pregnancy

Glycopeptides e.g. vancomycin

  • Not absorbed when given orally - have to be administered IV
  • Side effects: vancomycin damages kidneys, occasionally causes ‘red man syndrome’ (allergy)

Antibiotics that inhibit protein synthesis

  • Attach to bacterial ribosomes
  • Usually protein synthesis can resume when antibiotic is removed - bacteriostatic
    • Exception: aminoglycosides - binding to ribosome is lethal

Macrolides e.g. erythromycin, azithromycin and clarithromycin

  • Excreted via liver, biliary tract and into the gut
  • Lipophilic - easily pass through cell membranes
    • Useful for infections where bacteria ‘hide’ in host cells
  • Erythromycin is safe in pregnancy (others have not been trialed)

Aminoglycosides e.g. gentamicin

  • Must be given IV
  • Active mainly against Gram-negative aerobic organisms
  • Excreted in urine
  • Side effects: kidney damage, deafness/dizziness

Others

  • Clindamycin - 2nd line against serious Staphylococcal and Streptococcal infection, particularly in penicillin-allergic patients
    • Also active against ‘true’ anaerobes
  • Chloramphenicol - topical eye infections
  • Tetracyclines
    • Side effects - permanent staining of teeth (and bones) in children under 12
  • All excreted via liver and biliary system

Antibiotics that act on bacterial DNA

Metronidazole

  • Oral, IV
  • Break strands of bacterial DNA
  • Used for infections by ‘true’ anaerobes and some infections by protozoa
  • Interacts with alcohol

Trimethoprim

  • Inhibits bacterial folic acid synthesis

Fluoroquinolones e.g. ciprofloxacin, levofloxacin

  • Interact with topoisomerases – bacteria can no longer replicate
  • Bactericidal
  • Use is severely restricted - risk of C. difficile
  • Only antibiotics that can be given orally to treat Pseudomonas
  • Side effects: weakens tendons, may cause joint damage in young children, may cause seizures