IDStewardship

Ampicillin

Ampicillin oralampicillin injection

KEY POINTS

  • Ampicillin is an amino-penicillin and beta-lactam antibiotic that works by inhibiting the bacterial cell wall in susceptible organisms
  • Available as an oral or injectable, but amoxicillin is the more commonly employed oral amino-penicillin
  • Has activity versus Gram positives, Gram negatives and anaerobes
    • Does not work against penicillin-resistant Staphylococcus aureus
      • Most MSSA is penicillin-resistant
    • A good drug for beta-hemolytic Streptococci (e.g., Strep. pyogenes & Strep. agalactiae)
    • Beware resistance with alpha-hemolytic Streptococci (e.g., Strep. pneumoniae & Viridans Streptococci)
    • Aminopenicillins are widely considered the drugs of choice for susceptible Enterococci and Listeria monocytogenes
      • Common to combine ampicillin IV with ceftriaxone IV when treating Enterococcal infective endocarditis that is ampicillin-sesntive
    • Many Gram negative anaerobes are resistant to ampicillin alone, add beta-lactamase inhibitor sulbactam to expand this coverage and hit susceptible Bacteroides fragilis
      • Note ampicillin-sulbactam is brand name Unasyn
      • Amp-sulb has activity versus MSSA, but ampicillin alone only has activity against penicillin-sensitive Staphylococcus aureus
  • Dose range is typically 1gm every 6 hours to 2gm every 4 hours
    • Requires dosage adjustment for renal insufficiency
    • Short stability of the IV formulation can be problematic, limiting options for extended or continuous infusion strategies
  • Beware hypersensitivity reaction and anaphylaxis
    • Penicillin skin testing can assist in ruling out most unclear penicillin allergy labels

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